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Overview
Comment:Merge the wal2 and begin-concurrent code. Both features work, but not at the same time.
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Timelines: family | ancestors | descendants | both | begin-concurrent-wal2
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SHA3-256:b7281a1caa574870a071bea3e96b1d8210c28c17f9094449b3ce1a42b311e6a1
User & Date: dan 2018-12-03 20:49:34
Wiki:begin-concurrent-wal2
Context
2018-12-04
19:41
First attempt at making features work together. Only the most minimal testing so far. check-in: fd707001 user: dan tags: begin-concurrent-wal2
2018-12-03
20:49
Merge the wal2 and begin-concurrent code. Both features work, but not at the same time. check-in: b7281a1c user: dan tags: begin-concurrent-wal2
20:38
Minor change to wal.c on this branch to make it more similar to trunk. check-in: 6a7af3ea user: dan tags: begin-concurrent
18:13
Increase a timeout in test file walprotocol2.test. To account for unix builds without HAVE_USLEEP. check-in: 480be916 user: dan tags: wal2
Changes
Hide Diffs Unified Diffs Ignore Whitespace Patch

Added ext/misc/bgckpt.c.









































































































































































































































































































































































































































































































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/*
** 2017-10-11
**
** The author disclaims copyright to this source code.  In place of
** a legal notice, here is a blessing:
**
**    May you do good and not evil.
**    May you find forgiveness for yourself and forgive others.
**    May you share freely, never taking more than you give.
**
******************************************************************************
**
*/

#if !defined(SQLITE_TEST) || defined(SQLITE_OS_UNIX)

#include "sqlite3.h"
#include <string.h>
#include <pthread.h>

/*
** API declarations.
*/
typedef struct Checkpointer Checkpointer;
int sqlite3_bgckpt_create(const char *zFilename, Checkpointer **pp);
int sqlite3_bgckpt_checkpoint(Checkpointer *p, int bBlock);
void sqlite3_bgckpt_destroy(Checkpointer *p);


struct Checkpointer {
  sqlite3 *db;                    /* Database handle */

  pthread_t thread;               /* Background thread */
  pthread_mutex_t mutex;
  pthread_cond_t cond;

  int rc;                         /* Error from "PRAGMA wal_checkpoint" */
  int bCkpt;                      /* True if checkpoint requested */
  int bExit;                      /* True if exit requested */
};

static void *bgckptThreadMain(void *pCtx){
  int rc = SQLITE_OK;
  Checkpointer *p = (Checkpointer*)pCtx;

  while( rc==SQLITE_OK ){
    int bExit;

    pthread_mutex_lock(&p->mutex);
    if( p->bCkpt==0 && p->bExit==0 ){
      pthread_cond_wait(&p->cond, &p->mutex);
    }
    p->bCkpt = 0;
    bExit = p->bExit;
    pthread_mutex_unlock(&p->mutex);

    if( bExit ) break;
    rc = sqlite3_exec(p->db, "PRAGMA wal_checkpoint", 0, 0, 0);
    if( rc==SQLITE_BUSY ){
      rc = SQLITE_OK;
    }
  }

  pthread_mutex_lock(&p->mutex);
  p->rc = rc;
  pthread_mutex_unlock(&p->mutex);
  return 0;
}

void sqlite3_bgckpt_destroy(Checkpointer *p){
  if( p ){
    void *ret = 0;

    /* Signal the background thread to exit */
    pthread_mutex_lock(&p->mutex);
    p->bExit = 1;
    pthread_cond_broadcast(&p->cond);
    pthread_mutex_unlock(&p->mutex);

    pthread_join(p->thread, &ret);
    sqlite3_close(p->db);
    sqlite3_free(p);
  }
}


int sqlite3_bgckpt_create(const char *zFilename, Checkpointer **pp){
  Checkpointer *pNew = 0;
  int rc;

  pNew = (Checkpointer*)sqlite3_malloc(sizeof(Checkpointer));
  if( pNew==0 ){
    rc = SQLITE_NOMEM;
  }else{
    memset(pNew, 0, sizeof(Checkpointer));
    rc = sqlite3_open(zFilename, &pNew->db);
  }

  if( rc==SQLITE_OK ){
    pthread_mutex_init(&pNew->mutex, 0);
    pthread_cond_init(&pNew->cond, 0);
    pthread_create(&pNew->thread, 0, bgckptThreadMain, (void*)pNew);
  }

  if( rc!=SQLITE_OK ){
    sqlite3_bgckpt_destroy(pNew);
    pNew = 0;
  }
  *pp = pNew;
  return rc;
}

int sqlite3_bgckpt_checkpoint(Checkpointer *p, int bBlock){
  int rc;
  pthread_mutex_lock(&p->mutex);
  rc = p->rc;
  if( rc==SQLITE_OK ){
    p->bCkpt = 1;
    pthread_cond_broadcast(&p->cond);
  }
  pthread_mutex_unlock(&p->mutex);
  return rc;
}

#ifdef SQLITE_TEST

#if defined(INCLUDE_SQLITE_TCL_H)
#  include "sqlite_tcl.h"
#else
#  include "tcl.h"
#  ifndef SQLITE_TCLAPI
#    define SQLITE_TCLAPI
#  endif
#endif

const char *sqlite3ErrName(int rc);

static void SQLITE_TCLAPI bgckpt_del(void * clientData){
  Checkpointer *pCkpt = (Checkpointer*)clientData;
  sqlite3_bgckpt_destroy(pCkpt);
}

/*
** Tclcmd: $ckpt SUBCMD ...
*/
static int SQLITE_TCLAPI bgckpt_obj_cmd(
  void * clientData,
  Tcl_Interp *interp,
  int objc,
  Tcl_Obj *CONST objv[]
){
  Checkpointer *pCkpt = (Checkpointer*)clientData;
  const char *aCmd[] = { "checkpoint", "destroy", 0 };
  int iCmd;

  if( objc<2 ){
    Tcl_WrongNumArgs(interp, 1, objv, "SUBCMD ...");
    return TCL_ERROR;
  }

  if( Tcl_GetIndexFromObj(interp, objv[1], aCmd, "sub-command", 0, &iCmd) ){
    return TCL_ERROR;
  }

  switch( iCmd ){
    case 0: {
      int rc;
      int bBlock = 0;

      if( objc>3 ){
        Tcl_WrongNumArgs(interp, 2, objv, "?BLOCKING?");
        return TCL_ERROR;
      }
      if( objc==3 && Tcl_GetBooleanFromObj(interp, objv[2], &bBlock) ){
        return TCL_ERROR;
      }

      rc = sqlite3_bgckpt_checkpoint(pCkpt, bBlock);
      if( rc!=SQLITE_OK ){
        Tcl_SetObjResult(interp, Tcl_NewStringObj(sqlite3ErrName(rc), -1));
        return TCL_ERROR;
      }
      break;
    }

    case 1: {
      Tcl_DeleteCommand(interp, Tcl_GetString(objv[0]));
      break;
    }
  }

  return TCL_OK;
}

/*
** Tclcmd: bgckpt CMDNAME FILENAME
*/
static int SQLITE_TCLAPI bgckpt_cmd(
  void * clientData,
  Tcl_Interp *interp,
  int objc,
  Tcl_Obj *CONST objv[]
){
  const char *zCmd;
  const char *zFilename;
  int rc;
  Checkpointer *pCkpt;

  if( objc!=3 ){
    Tcl_WrongNumArgs(interp, 1, objv, "CMDNAME FILENAME");
    return TCL_ERROR;
  }
  zCmd = Tcl_GetString(objv[1]);
  zFilename = Tcl_GetString(objv[2]);

  rc = sqlite3_bgckpt_create(zFilename, &pCkpt);
  if( rc!=SQLITE_OK ){
    Tcl_SetObjResult(interp, Tcl_NewStringObj(sqlite3ErrName(rc), -1));
    return TCL_ERROR;
  }

  Tcl_CreateObjCommand(interp, zCmd, bgckpt_obj_cmd, (void*)pCkpt, bgckpt_del);
  Tcl_SetObjResult(interp, objv[1]);
  return TCL_OK;
}

int Bgckpt_Init(Tcl_Interp *interp){
  Tcl_CreateObjCommand(interp, "bgckpt", bgckpt_cmd, 0, 0);
  return TCL_OK;
}
#endif   /* SQLITE_TEST */

#else
#if defined(INCLUDE_SQLITE_TCL_H)
#  include "sqlite_tcl.h"
#else
#  include "tcl.h"
#  ifndef SQLITE_TCLAPI
#    define SQLITE_TCLAPI
#  endif
#endif
int Bgckpt_Init(Tcl_Interp *interp){ return TCL_OK; }
#endif

Changes to main.mk.

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  $(TOP)/src/test_window.c \
  $(TOP)/src/test_wsd.c

# Extensions to be statically loaded.
#
TESTSRC += \
  $(TOP)/ext/misc/amatch.c \

  $(TOP)/ext/misc/carray.c \
  $(TOP)/ext/misc/closure.c \
  $(TOP)/ext/misc/csv.c \
  $(TOP)/ext/misc/eval.c \
  $(TOP)/ext/misc/explain.c \
  $(TOP)/ext/misc/fileio.c \
  $(TOP)/ext/misc/fuzzer.c \







>







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  $(TOP)/src/test_window.c \
  $(TOP)/src/test_wsd.c

# Extensions to be statically loaded.
#
TESTSRC += \
  $(TOP)/ext/misc/amatch.c \
  $(TOP)/ext/misc/bgckpt.c \
  $(TOP)/ext/misc/carray.c \
  $(TOP)/ext/misc/closure.c \
  $(TOP)/ext/misc/csv.c \
  $(TOP)/ext/misc/eval.c \
  $(TOP)/ext/misc/explain.c \
  $(TOP)/ext/misc/fileio.c \
  $(TOP)/ext/misc/fuzzer.c \

Changes to src/btree.c.

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    if( page1[18]>1 ){
      pBt->btsFlags |= BTS_READ_ONLY;
    }
    if( page1[19]>1 ){
      goto page1_init_failed;
    }
#else
    if( page1[18]>2 ){
      pBt->btsFlags |= BTS_READ_ONLY;
    }
    if( page1[19]>2 ){
      goto page1_init_failed;
    }

    /* If the write version is set to 2, this database should be accessed
    ** in WAL mode. If the log is not already open, open it now. Then 
    ** return SQLITE_OK and return without populating BtShared.pPage1.
    ** The caller detects this and calls this function again. This is
    ** required as the version of page 1 currently in the page1 buffer
    ** may not be the latest version - there may be a newer one in the log
    ** file.
    */
    if( page1[19]==2 && (pBt->btsFlags & BTS_NO_WAL)==0 ){
      int isOpen = 0;
      rc = sqlite3PagerOpenWal(pBt->pPager, &isOpen);
      if( rc!=SQLITE_OK ){
        goto page1_init_failed;
      }else{
        setDefaultSyncFlag(pBt, SQLITE_DEFAULT_WAL_SYNCHRONOUS+1);
        if( isOpen==0 ){
          releasePageOne(pPage1);
          return SQLITE_OK;
................................................................................
** "write version" (single byte at byte offset 19) fields in the database
** header to iVersion.
*/
int sqlite3BtreeSetVersion(Btree *pBtree, int iVersion){
  BtShared *pBt = pBtree->pBt;
  int rc;                         /* Return code */
 
  assert( iVersion==1 || iVersion==2 );

  /* If setting the version fields to 1, do not automatically open the
  ** WAL connection, even if the version fields are currently set to 2.
  */
  pBt->btsFlags &= ~BTS_NO_WAL;
  if( iVersion==1 ) pBt->btsFlags |= BTS_NO_WAL;








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    if( page1[18]>1 ){
      pBt->btsFlags |= BTS_READ_ONLY;
    }
    if( page1[19]>1 ){
      goto page1_init_failed;
    }
#else
    if( page1[18]>3 ){
      pBt->btsFlags |= BTS_READ_ONLY;
    }
    if( page1[19]>3 ){
      goto page1_init_failed;
    }

    /* If the write version is set to 2, this database should be accessed
    ** in WAL mode. If the log is not already open, open it now. Then 
    ** return SQLITE_OK and return without populating BtShared.pPage1.
    ** The caller detects this and calls this function again. This is
    ** required as the version of page 1 currently in the page1 buffer
    ** may not be the latest version - there may be a newer one in the log
    ** file.
    */
    if( page1[19]>=2 && (pBt->btsFlags & BTS_NO_WAL)==0 ){
      int isOpen = 0;
      rc = sqlite3PagerOpenWal(pBt->pPager, (page1[19]==3), &isOpen);
      if( rc!=SQLITE_OK ){
        goto page1_init_failed;
      }else{
        setDefaultSyncFlag(pBt, SQLITE_DEFAULT_WAL_SYNCHRONOUS+1);
        if( isOpen==0 ){
          releasePageOne(pPage1);
          return SQLITE_OK;
................................................................................
** "write version" (single byte at byte offset 19) fields in the database
** header to iVersion.
*/
int sqlite3BtreeSetVersion(Btree *pBtree, int iVersion){
  BtShared *pBt = pBtree->pBt;
  int rc;                         /* Return code */
 
  assert( iVersion==1 || iVersion==2 || iVersion==3 );

  /* If setting the version fields to 1, do not automatically open the
  ** WAL connection, even if the version fields are currently set to 2.
  */
  pBt->btsFlags &= ~BTS_NO_WAL;
  if( iVersion==1 ) pBt->btsFlags |= BTS_NO_WAL;

Changes to src/os_unix.c.

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  /* Access to the unixShmNode object is serialized by the caller */
  pShmNode = pFile->pInode->pShmNode;
  assert( pShmNode->nRef==0 || sqlite3_mutex_held(pShmNode->pShmMutex) );
  assert( pShmNode->nRef>0 || unixMutexHeld() );

  /* Shared locks never span more than one byte */
  assert( n==1 || lockType!=F_RDLCK );

  /* Locks are within range */
  assert( n>=1 && n<=SQLITE_SHM_NLOCK );

  if( pShmNode->hShm>=0 ){
    /* Initialize the locking parameters */
    f.l_type = lockType;
................................................................................
  assert( pShmNode->pInode==pDbFd->pInode );
  assert( ofst>=0 && ofst+n<=SQLITE_SHM_NLOCK );
  assert( n>=1 );
  assert( flags==(SQLITE_SHM_LOCK | SQLITE_SHM_SHARED)
       || flags==(SQLITE_SHM_LOCK | SQLITE_SHM_EXCLUSIVE)
       || flags==(SQLITE_SHM_UNLOCK | SQLITE_SHM_SHARED)
       || flags==(SQLITE_SHM_UNLOCK | SQLITE_SHM_EXCLUSIVE) );
  assert( n==1 || (flags & SQLITE_SHM_EXCLUSIVE)!=0 );
  assert( pShmNode->hShm>=0 || pDbFd->pInode->bProcessLock==1 );
  assert( pShmNode->hShm<0 || pDbFd->pInode->bProcessLock==0 );

  mask = (1<<(ofst+n)) - (1<<ofst);
  assert( n>1 || mask==(1<<ofst) );
  sqlite3_mutex_enter(pShmNode->pShmMutex);
  if( flags & SQLITE_SHM_UNLOCK ){







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  /* Access to the unixShmNode object is serialized by the caller */
  pShmNode = pFile->pInode->pShmNode;
  assert( pShmNode->nRef==0 || sqlite3_mutex_held(pShmNode->pShmMutex) );
  assert( pShmNode->nRef>0 || unixMutexHeld() );

  /* Shared locks never span more than one byte */
  /* assert( n==1 || lockType!=F_RDLCK ); */

  /* Locks are within range */
  assert( n>=1 && n<=SQLITE_SHM_NLOCK );

  if( pShmNode->hShm>=0 ){
    /* Initialize the locking parameters */
    f.l_type = lockType;
................................................................................
  assert( pShmNode->pInode==pDbFd->pInode );
  assert( ofst>=0 && ofst+n<=SQLITE_SHM_NLOCK );
  assert( n>=1 );
  assert( flags==(SQLITE_SHM_LOCK | SQLITE_SHM_SHARED)
       || flags==(SQLITE_SHM_LOCK | SQLITE_SHM_EXCLUSIVE)
       || flags==(SQLITE_SHM_UNLOCK | SQLITE_SHM_SHARED)
       || flags==(SQLITE_SHM_UNLOCK | SQLITE_SHM_EXCLUSIVE) );
  /* assert( n==1 || (flags & SQLITE_SHM_EXCLUSIVE)!=0 ); */
  assert( pShmNode->hShm>=0 || pDbFd->pInode->bProcessLock==1 );
  assert( pShmNode->hShm<0 || pDbFd->pInode->bProcessLock==0 );

  mask = (1<<(ofst+n)) - (1<<ofst);
  assert( n>1 || mask==(1<<ofst) );
  sqlite3_mutex_enter(pShmNode->pShmMutex);
  if( flags & SQLITE_SHM_UNLOCK ){

Changes to src/pager.c.

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7497


7498
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7500
7501
7502
7503
7504
....
7653
7654
7655
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7657
7658
7659
7660
7661
7662
7663
7664
7665
7666
7667
....
7674
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7681
7682
7683
7684
7685
7686
7687
7688
....
7700
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7702
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7704
7705
7706

7707
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7709
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7713
....
7716
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7723
7724
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7728
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....
7740
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7748
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7751
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7754
....
7755
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7758
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7760
7761
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7763
7764
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7766
7767
7768
7769
#endif

/*
** The maximum legal page number is (2^31 - 1).
*/
#define PAGER_MAX_PGNO 2147483647

/*
** The argument to this macro is a file descriptor (type sqlite3_file*).
** Return 0 if it is not open, or non-zero (but not 1) if it is.
**
** This is so that expressions can be written as:
**
**   if( isOpen(pPager->jfd) ){ ...
**
** instead of
**
**   if( pPager->jfd->pMethods ){ ...
*/
#define isOpen(pFd) ((pFd)->pMethods!=0)

#ifdef SQLITE_DIRECT_OVERFLOW_READ
/*
** Return true if page pgno can be read directly from the database file
** by the b-tree layer. This is the case if:
**
**   * the database file is open,
**   * there are no dirty pages in the cache, and
................................................................................
        ** a rollback transaction that switches from journal_mode=off
        ** to journal_mode=wal.
        */
        assert( p->eLock>=RESERVED_LOCK );
        assert( isOpen(p->jfd) 
             || p->journalMode==PAGER_JOURNALMODE_OFF 
             || p->journalMode==PAGER_JOURNALMODE_WAL 

        );
      }
      assert( pPager->dbOrigSize==pPager->dbFileSize );
      assert( pPager->dbOrigSize==pPager->dbHintSize );
      break;

    case PAGER_WRITER_DBMOD:
................................................................................
      assert( p->eLock==EXCLUSIVE_LOCK );
      assert( pPager->errCode==SQLITE_OK );
      assert( !pagerUseWal(pPager) );
      assert( p->eLock>=EXCLUSIVE_LOCK );
      assert( isOpen(p->jfd) 
           || p->journalMode==PAGER_JOURNALMODE_OFF 
           || p->journalMode==PAGER_JOURNALMODE_WAL 

           || (sqlite3OsDeviceCharacteristics(p->fd)&SQLITE_IOCAP_BATCH_ATOMIC)
      );
      assert( pPager->dbOrigSize<=pPager->dbHintSize );
      break;

    case PAGER_WRITER_FINISHED:
      assert( p->eLock==EXCLUSIVE_LOCK );
      assert( pPager->errCode==SQLITE_OK );
      assert( !pagerUseWal(pPager) );
      assert( isOpen(p->jfd) 
           || p->journalMode==PAGER_JOURNALMODE_OFF 
           || p->journalMode==PAGER_JOURNALMODE_WAL 

           || (sqlite3OsDeviceCharacteristics(p->fd)&SQLITE_IOCAP_BATCH_ATOMIC)
      );
      break;

    case PAGER_ERROR:
      /* There must be at least one outstanding reference to the pager if
      ** in ERROR state. Otherwise the pager should have already dropped
................................................................................
          ** https://bugzilla.mozilla.org/show_bug.cgi?id=1072773
          */
          rc = sqlite3OsSync(pPager->jfd, pPager->syncFlags);
        }
      }
      pPager->journalOff = 0;
    }else if( pPager->journalMode==PAGER_JOURNALMODE_PERSIST
      || (pPager->exclusiveMode && pPager->journalMode!=PAGER_JOURNALMODE_WAL)
    ){
      rc = zeroJournalHdr(pPager, hasMaster||pPager->tempFile);
      pPager->journalOff = 0;
    }else{
      /* This branch may be executed with Pager.journalMode==MEMORY if
      ** a hot-journal was just rolled back. In this case the journal
      ** file should be closed and deleted. If this connection writes to
      ** the database file, it will do so using an in-memory journal.
      */
      int bDelete = !pPager->tempFile;
      assert( sqlite3JournalIsInMemory(pPager->jfd)==0 );
      assert( pPager->journalMode==PAGER_JOURNALMODE_DELETE 
           || pPager->journalMode==PAGER_JOURNALMODE_MEMORY 
           || pPager->journalMode==PAGER_JOURNALMODE_WAL 

      );
      sqlite3OsClose(pPager->jfd);
      if( bDelete ){
        rc = sqlite3OsDelete(pPager->pVfs, pPager->zJournal, pPager->extraSync);
      }
    }
  }
................................................................................

        rc = pagerPagecount(pPager, &nPage);
        if( rc ) return rc;
        if( nPage==0 ){
          rc = sqlite3OsDelete(pPager->pVfs, pPager->zWal, 0);
        }else{
          testcase( sqlite3PcachePagecount(pPager->pPCache)==0 );
          rc = sqlite3PagerOpenWal(pPager, 0);
        }
      }else if( pPager->journalMode==PAGER_JOURNALMODE_WAL ){
        pPager->journalMode = PAGER_JOURNALMODE_DELETE;
      }
    }
  }
  return rc;
}
#endif
................................................................................

  /* The eMode parameter is always valid */
  assert(      eMode==PAGER_JOURNALMODE_DELETE
            || eMode==PAGER_JOURNALMODE_TRUNCATE
            || eMode==PAGER_JOURNALMODE_PERSIST
            || eMode==PAGER_JOURNALMODE_OFF 
            || eMode==PAGER_JOURNALMODE_WAL 

            || eMode==PAGER_JOURNALMODE_MEMORY );

  /* This routine is only called from the OP_JournalMode opcode, and
  ** the logic there will never allow a temporary file to be changed
  ** to WAL mode.
  */
  assert( pPager->tempFile==0 || eMode!=PAGER_JOURNALMODE_WAL );
................................................................................
    */
    assert( (PAGER_JOURNALMODE_TRUNCATE & 5)==1 );
    assert( (PAGER_JOURNALMODE_PERSIST & 5)==1 );
    assert( (PAGER_JOURNALMODE_DELETE & 5)==0 );
    assert( (PAGER_JOURNALMODE_MEMORY & 5)==4 );
    assert( (PAGER_JOURNALMODE_OFF & 5)==0 );
    assert( (PAGER_JOURNALMODE_WAL & 5)==5 );


    assert( isOpen(pPager->fd) || pPager->exclusiveMode );
    if( !pPager->exclusiveMode && (eOld & 5)==1 && (eMode & 1)==0 ){



      /* In this case we would like to delete the journal file. If it is
      ** not possible, then that is not a problem. Deleting the journal file
      ** here is an optimization only.
      **
      ** Before deleting the journal file, obtain a RESERVED lock on the
      ** database file. This ensures that the journal file is not deleted
................................................................................

/*
** Call sqlite3WalOpen() to open the WAL handle. If the pager is in 
** exclusive-locking mode when this function is called, take an EXCLUSIVE
** lock on the database file and use heap-memory to store the wal-index
** in. Otherwise, use the normal shared-memory.
*/
static int pagerOpenWal(Pager *pPager){
  int rc = SQLITE_OK;

  assert( pPager->pWal==0 && pPager->tempFile==0 );
  assert( pPager->eLock==SHARED_LOCK || pPager->eLock==EXCLUSIVE_LOCK );

  /* If the pager is already in exclusive-mode, the WAL module will use 
  ** heap-memory for the wal-index instead of the VFS shared-memory 
................................................................................

  /* Open the connection to the log file. If this operation fails, 
  ** (e.g. due to malloc() failure), return an error code.
  */
  if( rc==SQLITE_OK ){
    rc = sqlite3WalOpen(pPager->pVfs,
        pPager->fd, pPager->zWal, pPager->exclusiveMode,
        pPager->journalSizeLimit, &pPager->pWal
    );
  }
  pagerFixMaplimit(pPager);

  return rc;
}

................................................................................
**
** If the pager is open on a temp-file (or in-memory database), or if
** the WAL file is already open, set *pbOpen to 1 and return SQLITE_OK
** without doing anything.
*/
int sqlite3PagerOpenWal(
  Pager *pPager,                  /* Pager object */

  int *pbOpen                     /* OUT: Set to true if call is a no-op */
){
  int rc = SQLITE_OK;             /* Return code */

  assert( assert_pager_state(pPager) );
  assert( pPager->eState==PAGER_OPEN   || pbOpen );
  assert( pPager->eState==PAGER_READER || !pbOpen );
................................................................................

  if( !pPager->tempFile && !pPager->pWal ){
    if( !sqlite3PagerWalSupported(pPager) ) return SQLITE_CANTOPEN;

    /* Close any rollback journal previously open */
    sqlite3OsClose(pPager->jfd);

    rc = pagerOpenWal(pPager);
    if( rc==SQLITE_OK ){
      pPager->journalMode = PAGER_JOURNALMODE_WAL;
      pPager->eState = PAGER_OPEN;
    }
  }else{
    *pbOpen = 1;
  }

  return rc;
................................................................................
** EXCLUSIVE lock on the database file. If this cannot be obtained, an
** error (SQLITE_BUSY) is returned and the log connection is not closed.
** If successful, the EXCLUSIVE lock is not released before returning.
*/
int sqlite3PagerCloseWal(Pager *pPager, sqlite3 *db){
  int rc = SQLITE_OK;

  assert( pPager->journalMode==PAGER_JOURNALMODE_WAL );



  /* If the log file is not already open, but does exist in the file-system,
  ** it may need to be checkpointed before the connection can switch to
  ** rollback mode. Open it now so this can happen.
  */
  if( !pPager->pWal ){
    int logexists = 0;
................................................................................
    rc = pagerLockDb(pPager, SHARED_LOCK);
    if( rc==SQLITE_OK ){
      rc = sqlite3OsAccess(
          pPager->pVfs, pPager->zWal, SQLITE_ACCESS_EXISTS, &logexists
      );
    }
    if( rc==SQLITE_OK && logexists ){
      rc = pagerOpenWal(pPager);
    }
  }
    
  /* Checkpoint and close the log. Because an EXCLUSIVE lock is held on
  ** the database file, the log and log-summary files will be deleted.
  */
  if( rc==SQLITE_OK && pPager->pWal ){







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976
977
978
979
980
981
982
983
984
985
....
2116
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2118
2119
2120
2121
2122
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2124
2125
2126
2127
2128
2129
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2140
2141
2142
2143
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2145
....
3411
3412
3413
3414
3415
3416
3417
3418
3419
3420
3421
3422
3423
3424
3425
3426
3427
....
7444
7445
7446
7447
7448
7449
7450
7451
7452
7453
7454
7455
7456
7457
7458
....
7479
7480
7481
7482
7483
7484
7485
7486
7487
7488
7489
7490
7491
7492
7493
7494
7495
7496
7497
7498
....
7647
7648
7649
7650
7651
7652
7653
7654
7655
7656
7657
7658
7659
7660
7661
....
7668
7669
7670
7671
7672
7673
7674
7675
7676
7677
7678
7679
7680
7681
7682
....
7694
7695
7696
7697
7698
7699
7700
7701
7702
7703
7704
7705
7706
7707
7708
....
7711
7712
7713
7714
7715
7716
7717
7718
7719
7720
7721
7722
7723
7724
7725
7726
7727
....
7735
7736
7737
7738
7739
7740
7741
7742
7743
7744
7745
7746
7747
7748
7749
7750
7751
....
7752
7753
7754
7755
7756
7757
7758
7759
7760
7761
7762
7763
7764
7765
7766
#endif

/*
** The maximum legal page number is (2^31 - 1).
*/
#define PAGER_MAX_PGNO 2147483647















#ifdef SQLITE_DIRECT_OVERFLOW_READ
/*
** Return true if page pgno can be read directly from the database file
** by the b-tree layer. This is the case if:
**
**   * the database file is open,
**   * there are no dirty pages in the cache, and
................................................................................
        ** a rollback transaction that switches from journal_mode=off
        ** to journal_mode=wal.
        */
        assert( p->eLock>=RESERVED_LOCK );
        assert( isOpen(p->jfd) 
             || p->journalMode==PAGER_JOURNALMODE_OFF 
             || p->journalMode==PAGER_JOURNALMODE_WAL 
             || p->journalMode==PAGER_JOURNALMODE_WAL2
        );
      }
      assert( pPager->dbOrigSize==pPager->dbFileSize );
      assert( pPager->dbOrigSize==pPager->dbHintSize );
      break;

    case PAGER_WRITER_DBMOD:
................................................................................
      assert( p->eLock==EXCLUSIVE_LOCK );
      assert( pPager->errCode==SQLITE_OK );
      assert( !pagerUseWal(pPager) );
      assert( p->eLock>=EXCLUSIVE_LOCK );
      assert( isOpen(p->jfd) 
           || p->journalMode==PAGER_JOURNALMODE_OFF 
           || p->journalMode==PAGER_JOURNALMODE_WAL 
           || p->journalMode==PAGER_JOURNALMODE_WAL2
           || (sqlite3OsDeviceCharacteristics(p->fd)&SQLITE_IOCAP_BATCH_ATOMIC)
      );
      assert( pPager->dbOrigSize<=pPager->dbHintSize );
      break;

    case PAGER_WRITER_FINISHED:
      assert( p->eLock==EXCLUSIVE_LOCK );
      assert( pPager->errCode==SQLITE_OK );
      assert( !pagerUseWal(pPager) );
      assert( isOpen(p->jfd) 
           || p->journalMode==PAGER_JOURNALMODE_OFF 
           || p->journalMode==PAGER_JOURNALMODE_WAL 
           || p->journalMode==PAGER_JOURNALMODE_WAL2
           || (sqlite3OsDeviceCharacteristics(p->fd)&SQLITE_IOCAP_BATCH_ATOMIC)
      );
      break;

    case PAGER_ERROR:
      /* There must be at least one outstanding reference to the pager if
      ** in ERROR state. Otherwise the pager should have already dropped
................................................................................
          ** https://bugzilla.mozilla.org/show_bug.cgi?id=1072773
          */
          rc = sqlite3OsSync(pPager->jfd, pPager->syncFlags);
        }
      }
      pPager->journalOff = 0;
    }else if( pPager->journalMode==PAGER_JOURNALMODE_PERSIST
      || (pPager->exclusiveMode && pPager->journalMode<PAGER_JOURNALMODE_WAL)
    ){
      rc = zeroJournalHdr(pPager, hasMaster||pPager->tempFile);
      pPager->journalOff = 0;
    }else{
      /* This branch may be executed with Pager.journalMode==MEMORY if
      ** a hot-journal was just rolled back. In this case the journal
      ** file should be closed and deleted. If this connection writes to
      ** the database file, it will do so using an in-memory journal.
      */
      int bDelete = !pPager->tempFile;
      assert( sqlite3JournalIsInMemory(pPager->jfd)==0 );
      assert( pPager->journalMode==PAGER_JOURNALMODE_DELETE 
           || pPager->journalMode==PAGER_JOURNALMODE_MEMORY 
           || pPager->journalMode==PAGER_JOURNALMODE_WAL
           || pPager->journalMode==PAGER_JOURNALMODE_WAL2
      );
      sqlite3OsClose(pPager->jfd);
      if( bDelete ){
        rc = sqlite3OsDelete(pPager->pVfs, pPager->zJournal, pPager->extraSync);
      }
    }
  }
................................................................................

        rc = pagerPagecount(pPager, &nPage);
        if( rc ) return rc;
        if( nPage==0 ){
          rc = sqlite3OsDelete(pPager->pVfs, pPager->zWal, 0);
        }else{
          testcase( sqlite3PcachePagecount(pPager->pPCache)==0 );
          rc = sqlite3PagerOpenWal(pPager, 0, 0);
        }
      }else if( pPager->journalMode>=PAGER_JOURNALMODE_WAL ){
        pPager->journalMode = PAGER_JOURNALMODE_DELETE;
      }
    }
  }
  return rc;
}
#endif
................................................................................

  /* The eMode parameter is always valid */
  assert(      eMode==PAGER_JOURNALMODE_DELETE
            || eMode==PAGER_JOURNALMODE_TRUNCATE
            || eMode==PAGER_JOURNALMODE_PERSIST
            || eMode==PAGER_JOURNALMODE_OFF 
            || eMode==PAGER_JOURNALMODE_WAL 
            || eMode==PAGER_JOURNALMODE_WAL2
            || eMode==PAGER_JOURNALMODE_MEMORY );

  /* This routine is only called from the OP_JournalMode opcode, and
  ** the logic there will never allow a temporary file to be changed
  ** to WAL mode.
  */
  assert( pPager->tempFile==0 || eMode!=PAGER_JOURNALMODE_WAL );
................................................................................
    */
    assert( (PAGER_JOURNALMODE_TRUNCATE & 5)==1 );
    assert( (PAGER_JOURNALMODE_PERSIST & 5)==1 );
    assert( (PAGER_JOURNALMODE_DELETE & 5)==0 );
    assert( (PAGER_JOURNALMODE_MEMORY & 5)==4 );
    assert( (PAGER_JOURNALMODE_OFF & 5)==0 );
    assert( (PAGER_JOURNALMODE_WAL & 5)==5 );
    assert( (PAGER_JOURNALMODE_WAL2 & 5)==4 );

    assert( isOpen(pPager->fd) || pPager->exclusiveMode );
    if( !pPager->exclusiveMode && (eOld & 5)==1 && (eMode & 1)==0 
     && eMode!=PAGER_JOURNALMODE_WAL2       /* TODO: fix this if possible */
    ){

      /* In this case we would like to delete the journal file. If it is
      ** not possible, then that is not a problem. Deleting the journal file
      ** here is an optimization only.
      **
      ** Before deleting the journal file, obtain a RESERVED lock on the
      ** database file. This ensures that the journal file is not deleted
................................................................................

/*
** Call sqlite3WalOpen() to open the WAL handle. If the pager is in 
** exclusive-locking mode when this function is called, take an EXCLUSIVE
** lock on the database file and use heap-memory to store the wal-index
** in. Otherwise, use the normal shared-memory.
*/
static int pagerOpenWal(Pager *pPager, int bWal2){
  int rc = SQLITE_OK;

  assert( pPager->pWal==0 && pPager->tempFile==0 );
  assert( pPager->eLock==SHARED_LOCK || pPager->eLock==EXCLUSIVE_LOCK );

  /* If the pager is already in exclusive-mode, the WAL module will use 
  ** heap-memory for the wal-index instead of the VFS shared-memory 
................................................................................

  /* Open the connection to the log file. If this operation fails, 
  ** (e.g. due to malloc() failure), return an error code.
  */
  if( rc==SQLITE_OK ){
    rc = sqlite3WalOpen(pPager->pVfs,
        pPager->fd, pPager->zWal, pPager->exclusiveMode,
        pPager->journalSizeLimit, bWal2, &pPager->pWal
    );
  }
  pagerFixMaplimit(pPager);

  return rc;
}

................................................................................
**
** If the pager is open on a temp-file (or in-memory database), or if
** the WAL file is already open, set *pbOpen to 1 and return SQLITE_OK
** without doing anything.
*/
int sqlite3PagerOpenWal(
  Pager *pPager,                  /* Pager object */
  int bWal2,                      /* Open in wal2 mode if not already open */
  int *pbOpen                     /* OUT: Set to true if call is a no-op */
){
  int rc = SQLITE_OK;             /* Return code */

  assert( assert_pager_state(pPager) );
  assert( pPager->eState==PAGER_OPEN   || pbOpen );
  assert( pPager->eState==PAGER_READER || !pbOpen );
................................................................................

  if( !pPager->tempFile && !pPager->pWal ){
    if( !sqlite3PagerWalSupported(pPager) ) return SQLITE_CANTOPEN;

    /* Close any rollback journal previously open */
    sqlite3OsClose(pPager->jfd);

    rc = pagerOpenWal(pPager, bWal2);
    if( rc==SQLITE_OK ){
      pPager->journalMode = bWal2?PAGER_JOURNALMODE_WAL2:PAGER_JOURNALMODE_WAL;
      pPager->eState = PAGER_OPEN;
    }
  }else{
    *pbOpen = 1;
  }

  return rc;
................................................................................
** EXCLUSIVE lock on the database file. If this cannot be obtained, an
** error (SQLITE_BUSY) is returned and the log connection is not closed.
** If successful, the EXCLUSIVE lock is not released before returning.
*/
int sqlite3PagerCloseWal(Pager *pPager, sqlite3 *db){
  int rc = SQLITE_OK;

  assert( pPager->journalMode==PAGER_JOURNALMODE_WAL 
       || pPager->journalMode==PAGER_JOURNALMODE_WAL2
  );

  /* If the log file is not already open, but does exist in the file-system,
  ** it may need to be checkpointed before the connection can switch to
  ** rollback mode. Open it now so this can happen.
  */
  if( !pPager->pWal ){
    int logexists = 0;
................................................................................
    rc = pagerLockDb(pPager, SHARED_LOCK);
    if( rc==SQLITE_OK ){
      rc = sqlite3OsAccess(
          pPager->pVfs, pPager->zWal, SQLITE_ACCESS_EXISTS, &logexists
      );
    }
    if( rc==SQLITE_OK && logexists ){
      rc = pagerOpenWal(pPager, 0);
    }
  }
    
  /* Checkpoint and close the log. Because an EXCLUSIVE lock is held on
  ** the database file, the log and log-summary files will be deleted.
  */
  if( rc==SQLITE_OK && pPager->pWal ){

Changes to src/pager.h.

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#define PAGER_JOURNALMODE_QUERY     (-1)  /* Query the value of journalmode */
#define PAGER_JOURNALMODE_DELETE      0   /* Commit by deleting journal file */
#define PAGER_JOURNALMODE_PERSIST     1   /* Commit by zeroing journal header */
#define PAGER_JOURNALMODE_OFF         2   /* Journal omitted.  */
#define PAGER_JOURNALMODE_TRUNCATE    3   /* Commit by truncating journal */
#define PAGER_JOURNALMODE_MEMORY      4   /* In-memory journal file */
#define PAGER_JOURNALMODE_WAL         5   /* Use write-ahead logging */


















/*
** Flags that make up the mask passed to sqlite3PagerGet().
*/
#define PAGER_GET_NOCONTENT     0x01  /* Do not load data from disk */
#define PAGER_GET_READONLY      0x02  /* Read-only page is acceptable */

................................................................................
int sqlite3PagerSharedLock(Pager *pPager);


#ifndef SQLITE_OMIT_WAL
  int sqlite3PagerCheckpoint(Pager *pPager, sqlite3*, int, int*, int*);
  int sqlite3PagerWalSupported(Pager *pPager);
  int sqlite3PagerWalCallback(Pager *pPager);
  int sqlite3PagerOpenWal(Pager *pPager, int *pisOpen);
  int sqlite3PagerCloseWal(Pager *pPager, sqlite3*);
# ifdef SQLITE_ENABLE_SNAPSHOT
  int sqlite3PagerSnapshotGet(Pager *pPager, sqlite3_snapshot **ppSnapshot);
  int sqlite3PagerSnapshotOpen(Pager *pPager, sqlite3_snapshot *pSnapshot);
  int sqlite3PagerSnapshotRecover(Pager *pPager);
  int sqlite3PagerSnapshotCheck(Pager *pPager, sqlite3_snapshot *pSnapshot);
  void sqlite3PagerSnapshotUnlock(Pager *pPager);







>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>







 







|







77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
...
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
#define PAGER_JOURNALMODE_QUERY     (-1)  /* Query the value of journalmode */
#define PAGER_JOURNALMODE_DELETE      0   /* Commit by deleting journal file */
#define PAGER_JOURNALMODE_PERSIST     1   /* Commit by zeroing journal header */
#define PAGER_JOURNALMODE_OFF         2   /* Journal omitted.  */
#define PAGER_JOURNALMODE_TRUNCATE    3   /* Commit by truncating journal */
#define PAGER_JOURNALMODE_MEMORY      4   /* In-memory journal file */
#define PAGER_JOURNALMODE_WAL         5   /* Use write-ahead logging */
#define PAGER_JOURNALMODE_WAL2        6   /* Use write-ahead logging mode 2 */

#define isWalMode(x) ((x)==PAGER_JOURNALMODE_WAL || (x)==PAGER_JOURNALMODE_WAL2)

/*
** The argument to this macro is a file descriptor (type sqlite3_file*).
** Return 0 if it is not open, or non-zero (but not 1) if it is.
**
** This is so that expressions can be written as:
**
**   if( isOpen(pPager->jfd) ){ ...
**
** instead of
**
**   if( pPager->jfd->pMethods ){ ...
*/
#define isOpen(pFd) ((pFd)->pMethods!=0)

/*
** Flags that make up the mask passed to sqlite3PagerGet().
*/
#define PAGER_GET_NOCONTENT     0x01  /* Do not load data from disk */
#define PAGER_GET_READONLY      0x02  /* Read-only page is acceptable */

................................................................................
int sqlite3PagerSharedLock(Pager *pPager);


#ifndef SQLITE_OMIT_WAL
  int sqlite3PagerCheckpoint(Pager *pPager, sqlite3*, int, int*, int*);
  int sqlite3PagerWalSupported(Pager *pPager);
  int sqlite3PagerWalCallback(Pager *pPager);
  int sqlite3PagerOpenWal(Pager *pPager, int, int *pisOpen);
  int sqlite3PagerCloseWal(Pager *pPager, sqlite3*);
# ifdef SQLITE_ENABLE_SNAPSHOT
  int sqlite3PagerSnapshotGet(Pager *pPager, sqlite3_snapshot **ppSnapshot);
  int sqlite3PagerSnapshotOpen(Pager *pPager, sqlite3_snapshot *pSnapshot);
  int sqlite3PagerSnapshotRecover(Pager *pPager);
  int sqlite3PagerSnapshotCheck(Pager *pPager, sqlite3_snapshot *pSnapshot);
  void sqlite3PagerSnapshotUnlock(Pager *pPager);

Changes to src/pragma.c.

256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271

272
273
274
275
276
277
278
** defined in pager.h. This function returns the associated lowercase
** journal-mode name.
*/
const char *sqlite3JournalModename(int eMode){
  static char * const azModeName[] = {
    "delete", "persist", "off", "truncate", "memory"
#ifndef SQLITE_OMIT_WAL
     , "wal"
#endif
  };
  assert( PAGER_JOURNALMODE_DELETE==0 );
  assert( PAGER_JOURNALMODE_PERSIST==1 );
  assert( PAGER_JOURNALMODE_OFF==2 );
  assert( PAGER_JOURNALMODE_TRUNCATE==3 );
  assert( PAGER_JOURNALMODE_MEMORY==4 );
  assert( PAGER_JOURNALMODE_WAL==5 );

  assert( eMode>=0 && eMode<=ArraySize(azModeName) );

  if( eMode==ArraySize(azModeName) ) return 0;
  return azModeName[eMode];
}

/*







|








>







256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
** defined in pager.h. This function returns the associated lowercase
** journal-mode name.
*/
const char *sqlite3JournalModename(int eMode){
  static char * const azModeName[] = {
    "delete", "persist", "off", "truncate", "memory"
#ifndef SQLITE_OMIT_WAL
     , "wal", "wal2"
#endif
  };
  assert( PAGER_JOURNALMODE_DELETE==0 );
  assert( PAGER_JOURNALMODE_PERSIST==1 );
  assert( PAGER_JOURNALMODE_OFF==2 );
  assert( PAGER_JOURNALMODE_TRUNCATE==3 );
  assert( PAGER_JOURNALMODE_MEMORY==4 );
  assert( PAGER_JOURNALMODE_WAL==5 );
  assert( PAGER_JOURNALMODE_WAL2==6 );
  assert( eMode>=0 && eMode<=ArraySize(azModeName) );

  if( eMode==ArraySize(azModeName) ) return 0;
  return azModeName[eMode];
}

/*

Changes to src/test_tclsh.c.

94
95
96
97
98
99
100

101
102
103
104
105
106
107
...
161
162
163
164
165
166
167


168
169
170
171
172
173
174
#if defined(SQLITE_ENABLE_SESSION) && defined(SQLITE_ENABLE_PREUPDATE_HOOK)
  extern int TestSession_Init(Tcl_Interp*);
#endif
  extern int Md5_Init(Tcl_Interp*);
  extern int Fts5tcl_Init(Tcl_Interp *);
  extern int SqliteRbu_Init(Tcl_Interp*);
  extern int Sqlitetesttcl_Init(Tcl_Interp*);

#if defined(SQLITE_ENABLE_FTS3) || defined(SQLITE_ENABLE_FTS4)
  extern int Sqlitetestfts3_Init(Tcl_Interp *interp);
#endif
#ifdef SQLITE_ENABLE_ZIPVFS
  extern int Zipvfs_Init(Tcl_Interp*);
#endif
  extern int TestExpert_Init(Tcl_Interp*);
................................................................................
  SqlitetestSyscall_Init(interp);
#if defined(SQLITE_ENABLE_SESSION) && defined(SQLITE_ENABLE_PREUPDATE_HOOK)
  TestSession_Init(interp);
#endif
  Fts5tcl_Init(interp);
  SqliteRbu_Init(interp);
  Sqlitetesttcl_Init(interp);



#if defined(SQLITE_ENABLE_FTS3) || defined(SQLITE_ENABLE_FTS4)
  Sqlitetestfts3_Init(interp);
#endif
  TestExpert_Init(interp);
  Sqlitetest_window_Init(interp);








>







 







>
>







94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
...
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
#if defined(SQLITE_ENABLE_SESSION) && defined(SQLITE_ENABLE_PREUPDATE_HOOK)
  extern int TestSession_Init(Tcl_Interp*);
#endif
  extern int Md5_Init(Tcl_Interp*);
  extern int Fts5tcl_Init(Tcl_Interp *);
  extern int SqliteRbu_Init(Tcl_Interp*);
  extern int Sqlitetesttcl_Init(Tcl_Interp*);
  extern int Bgckpt_Init(Tcl_Interp*);
#if defined(SQLITE_ENABLE_FTS3) || defined(SQLITE_ENABLE_FTS4)
  extern int Sqlitetestfts3_Init(Tcl_Interp *interp);
#endif
#ifdef SQLITE_ENABLE_ZIPVFS
  extern int Zipvfs_Init(Tcl_Interp*);
#endif
  extern int TestExpert_Init(Tcl_Interp*);
................................................................................
  SqlitetestSyscall_Init(interp);
#if defined(SQLITE_ENABLE_SESSION) && defined(SQLITE_ENABLE_PREUPDATE_HOOK)
  TestSession_Init(interp);
#endif
  Fts5tcl_Init(interp);
  SqliteRbu_Init(interp);
  Sqlitetesttcl_Init(interp);
  Bgckpt_Init(interp);


#if defined(SQLITE_ENABLE_FTS3) || defined(SQLITE_ENABLE_FTS4)
  Sqlitetestfts3_Init(interp);
#endif
  TestExpert_Init(interp);
  Sqlitetest_window_Init(interp);

Changes to src/vdbe.c.

6634
6635
6636
6637
6638
6639
6640

6641
6642
6643
6644
6645
6646
6647
....
6651
6652
6653
6654
6655
6656
6657
6658
6659
6660
6661
6662
6663
6664
6665
6666






6667




6668
6669
6670
6671
6672
6673
6674
6675
6676

6677
6678
6679
6680
6681
6682
6683
6684
6685
6686
6687
6688
6689
6690
6691
6692
6693
6694
6695
6696
6697

6698
6699


6700
6701
6702
6703
6704
6705
6706
  eNew = pOp->p3;
  assert( eNew==PAGER_JOURNALMODE_DELETE 
       || eNew==PAGER_JOURNALMODE_TRUNCATE 
       || eNew==PAGER_JOURNALMODE_PERSIST 
       || eNew==PAGER_JOURNALMODE_OFF
       || eNew==PAGER_JOURNALMODE_MEMORY
       || eNew==PAGER_JOURNALMODE_WAL

       || eNew==PAGER_JOURNALMODE_QUERY
  );
  assert( pOp->p1>=0 && pOp->p1<db->nDb );
  assert( p->readOnly==0 );

  pBt = db->aDb[pOp->p1].pBt;
  pPager = sqlite3BtreePager(pBt);
................................................................................

#ifndef SQLITE_OMIT_WAL
  zFilename = sqlite3PagerFilename(pPager, 1);

  /* Do not allow a transition to journal_mode=WAL for a database
  ** in temporary storage or if the VFS does not support shared memory 
  */
  if( eNew==PAGER_JOURNALMODE_WAL
   && (sqlite3Strlen30(zFilename)==0           /* Temp file */
       || !sqlite3PagerWalSupported(pPager))   /* No shared-memory support */
  ){
    eNew = eOld;
  }

  if( (eNew!=eOld)
   && (eOld==PAGER_JOURNALMODE_WAL || eNew==PAGER_JOURNALMODE_WAL)






  ){




    if( !db->autoCommit || db->nVdbeRead>1 ){
      rc = SQLITE_ERROR;
      sqlite3VdbeError(p,
          "cannot change %s wal mode from within a transaction",
          (eNew==PAGER_JOURNALMODE_WAL ? "into" : "out of")
      );
      goto abort_due_to_error;
    }else{
 

      if( eOld==PAGER_JOURNALMODE_WAL ){
        /* If leaving WAL mode, close the log file. If successful, the call
        ** to PagerCloseWal() checkpoints and deletes the write-ahead-log 
        ** file. An EXCLUSIVE lock may still be held on the database file 
        ** after a successful return. 
        */
        rc = sqlite3PagerCloseWal(pPager, db);
        if( rc==SQLITE_OK ){
          sqlite3PagerSetJournalMode(pPager, eNew);
        }
      }else if( eOld==PAGER_JOURNALMODE_MEMORY ){
        /* Cannot transition directly from MEMORY to WAL.  Use mode OFF
        ** as an intermediate */
        sqlite3PagerSetJournalMode(pPager, PAGER_JOURNALMODE_OFF);
      }
  
      /* Open a transaction on the database file. Regardless of the journal
      ** mode, this transaction always uses a rollback journal.
      */
      assert( sqlite3BtreeIsInTrans(pBt)==0 );
      if( rc==SQLITE_OK ){

        rc = sqlite3BtreeSetVersion(pBt, (eNew==PAGER_JOURNALMODE_WAL ? 2 : 1));
      }


    }
  }
#endif /* ifndef SQLITE_OMIT_WAL */

  if( rc ) eNew = eOld;
  eNew = sqlite3PagerSetJournalMode(pPager, eNew);








>







 







|






|
<
>
>
>
>
>
>
|
>
>
>
>







<
|
>
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
>
|
<
>
>







6634
6635
6636
6637
6638
6639
6640
6641
6642
6643
6644
6645
6646
6647
6648
....
6652
6653
6654
6655
6656
6657
6658
6659
6660
6661
6662
6663
6664
6665
6666

6667
6668
6669
6670
6671
6672
6673
6674
6675
6676
6677
6678
6679
6680
6681
6682
6683
6684

6685
6686
6687
6688
6689
6690
6691
6692
6693
6694
6695
6696
6697
6698
6699
6700
6701
6702
6703
6704
6705
6706
6707
6708
6709

6710
6711
6712
6713
6714
6715
6716
6717
6718
  eNew = pOp->p3;
  assert( eNew==PAGER_JOURNALMODE_DELETE 
       || eNew==PAGER_JOURNALMODE_TRUNCATE 
       || eNew==PAGER_JOURNALMODE_PERSIST 
       || eNew==PAGER_JOURNALMODE_OFF
       || eNew==PAGER_JOURNALMODE_MEMORY
       || eNew==PAGER_JOURNALMODE_WAL
       || eNew==PAGER_JOURNALMODE_WAL2
       || eNew==PAGER_JOURNALMODE_QUERY
  );
  assert( pOp->p1>=0 && pOp->p1<db->nDb );
  assert( p->readOnly==0 );

  pBt = db->aDb[pOp->p1].pBt;
  pPager = sqlite3BtreePager(pBt);
................................................................................

#ifndef SQLITE_OMIT_WAL
  zFilename = sqlite3PagerFilename(pPager, 1);

  /* Do not allow a transition to journal_mode=WAL for a database
  ** in temporary storage or if the VFS does not support shared memory 
  */
  if( isWalMode(eNew)
   && (sqlite3Strlen30(zFilename)==0           /* Temp file */
       || !sqlite3PagerWalSupported(pPager))   /* No shared-memory support */
  ){
    eNew = eOld;
  }

  if( eNew!=eOld && (isWalMode(eNew) || isWalMode(eOld)) ){


    /* Prevent changing directly to wal2 from wal mode. And vice versa. */
    if( isWalMode(eNew) && isWalMode(eOld) ){
      rc = SQLITE_ERROR;
      sqlite3VdbeError(p, "cannot change from %s to %s mode",
          sqlite3JournalModename(eOld), sqlite3JournalModename(eNew)
      );
      goto abort_due_to_error;
    }

    /* Prevent switching into or out of wal/wal2 mode mid-transaction */
    if( !db->autoCommit || db->nVdbeRead>1 ){
      rc = SQLITE_ERROR;
      sqlite3VdbeError(p,
          "cannot change %s wal mode from within a transaction",
          (eNew==PAGER_JOURNALMODE_WAL ? "into" : "out of")
      );
      goto abort_due_to_error;

    }
 
    if( isWalMode(eOld) ){
      /* If leaving WAL mode, close the log file. If successful, the call
      ** to PagerCloseWal() checkpoints and deletes the write-ahead-log 
      ** file. An EXCLUSIVE lock may still be held on the database file 
      ** after a successful return. 
      */
      rc = sqlite3PagerCloseWal(pPager, db);
      if( rc==SQLITE_OK ){
        sqlite3PagerSetJournalMode(pPager, eNew);
      }
    }else if( eOld==PAGER_JOURNALMODE_MEMORY ){
      /* Cannot transition directly from MEMORY to WAL.  Use mode OFF
      ** as an intermediate */
      sqlite3PagerSetJournalMode(pPager, PAGER_JOURNALMODE_OFF);
    }

    /* Open a transaction on the database file. Regardless of the journal
    ** mode, this transaction always uses a rollback journal.
    */
    assert( sqlite3BtreeIsInTrans(pBt)==0 );
    if( rc==SQLITE_OK ){
      /* 1==rollback, 2==wal, 3==wal2 */
      rc = sqlite3BtreeSetVersion(pBt, 

          1 + isWalMode(eNew) + (eNew==PAGER_JOURNALMODE_WAL2)
      );
    }
  }
#endif /* ifndef SQLITE_OMIT_WAL */

  if( rc ) eNew = eOld;
  eNew = sqlite3PagerSetJournalMode(pPager, eNew);

Changes to src/wal.c.

97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
...
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
































































































































































247
248
249
250
251
252
253
...
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285

286
287
288
289
290
291
292
293
294
...
303
304
305
306
307
308
309





























310
311
312
313
314
315
316
...
322
323
324
325
326
327
328






329
330
331
332
333
334
335
336
337
338
339
340
341
342
343





































344
345
346
347
348
349
350
...
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
...
469
470
471
472
473
474
475

476
477
478
479
480
481
482

483
484
485
486
487
488
489
...
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
...
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
...
874
875
876
877
878
879
880






























881
882
883
884
885
886
887
...
937
938
939
940
941
942
943





































944
945
946
947
948
949
950
...
954
955
956
957
958
959
960
















961
962
963
964
965
966
967
968
969
970
971




972
973
974
975
976
977
978
...
983
984
985
986
987
988
989


990








991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
....
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047

1048







1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
....
1117
1118
1119
1120
1121
1122
1123








1124






































































































































1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139




1140
1141
1142
1143
1144
1145
1146
....
1158
1159
1160
1161
1162
1163
1164
1165























1166
1167








1168
1169


1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188


1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200





1201
1202





1203
1204
1205
1206
1207

1208
1209
1210
1211
1212


1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259

1260



1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279



1280
1281
1282
1283

1284
1285
1286
1287
1288
1289
1290

1291






1292
1293
1294

1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318


1319
1320
1321
1322
1323
1324
1325
....
1335
1336
1337
1338
1339
1340
1341

1342
1343
1344
1345
1346


1347
1348
1349
1350
1351
1352
1353
....
1359
1360
1361
1362
1363
1364
1365


1366
1367
1368
1369
1370
1371
1372
1373
1374
1375

1376
1377
1378
1379
1380
1381
1382

1383





1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
....
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614





1615
1616
1617
1618
1619

1620
1621




1622
1623
1624
1625

1626
1627
1628







1629
1630
1631
1632
1633
1634
1635
1636
1637
....
1644
1645
1646
1647
1648
1649
1650
1651

1652
1653
1654
1655
1656
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** being considered valid at the same time and being checkpointing together
** following a crash.
**
** READER ALGORITHM
**
** To read a page from the database (call it page number P), a reader
** first checks the WAL to see if it contains page P.  If so, then the
** last valid instance of page P that is a followed by a commit frame
** or is a commit frame itself becomes the value read.  If the WAL
** contains no copies of page P that are valid and which are a commit
** frame or are followed by a commit frame, then page P is read from
** the database file.
**
** To start a read transaction, the reader records the index of the last
** valid frame in the WAL.  The reader uses this recorded "mxFrame" value
................................................................................
**
** Note that entries are added in order of increasing K.  Hence, one
** reader might be using some value K0 and a second reader that started
** at a later time (after additional transactions were added to the WAL
** and to the wal-index) might be using a different value K1, where K1>K0.
** Both readers can use the same hash table and mapping section to get
** the correct result.  There may be entries in the hash table with
** K>K0 but to the first reader, those entries will appear to be unused
** slots in the hash table and so the first reader will get an answer as
** if no values greater than K0 had ever been inserted into the hash table
** in the first place - which is what reader one wants.  Meanwhile, the
** second reader using K1 will see additional values that were inserted
** later, which is exactly what reader two wants.  
**
** When a rollback occurs, the value of K is decreased. Hash table entries
** that correspond to frames greater than the new K value are removed
** from the hash table at this point.
*/
































































































































































#ifndef SQLITE_OMIT_WAL

#include "wal.h"

/*
** Trace output macros
*/
................................................................................
# define AtomicStore(PTR,VAL)  __atomic_store_n((PTR),(VAL),__ATOMIC_RELAXED)
#else
# define AtomicLoad(PTR)       (*(PTR))
# define AtomicStore(PTR,VAL)  (*(PTR) = (VAL))
#endif

/*
** The maximum (and only) versions of the wal and wal-index formats
** that may be interpreted by this version of SQLite.
**
** If a client begins recovering a WAL file and finds that (a) the checksum
** values in the wal-header are correct and (b) the version field is not
** WAL_MAX_VERSION, recovery fails and SQLite returns SQLITE_CANTOPEN.
**
** Similarly, if a client successfully reads a wal-index header (i.e. the 
** checksum test is successful) and finds that the version field is not
** WALINDEX_MAX_VERSION, then no read-transaction is opened and SQLite
** returns SQLITE_CANTOPEN.
*/

#define WAL_MAX_VERSION      3007000
#define WALINDEX_MAX_VERSION 3007000

/*
** Index numbers for various locking bytes.   WAL_NREADER is the number
** of available reader locks and should be at least 3.  The default
** is SQLITE_SHM_NLOCK==8 and  WAL_NREADER==5.
**
** Technically, the various VFSes are free to implement these locks however
................................................................................
#define WAL_WRITE_LOCK         0
#define WAL_ALL_BUT_WRITE      1
#define WAL_CKPT_LOCK          1
#define WAL_RECOVER_LOCK       2
#define WAL_READ_LOCK(I)       (3+(I))
#define WAL_NREADER            (SQLITE_SHM_NLOCK-3)































/* Object declarations */
typedef struct WalIndexHdr WalIndexHdr;
typedef struct WalIterator WalIterator;
typedef struct WalCkptInfo WalCkptInfo;


................................................................................
** For all versions of SQLite through 3.10.0 and probably beyond,
** the locking bytes (WalCkptInfo.aLock) start at offset 120 and
** the total header size is 136 bytes.
**
** The szPage value can be any power of 2 between 512 and 32768, inclusive.
** Or it can be 1 to represent a 65536-byte page.  The latter case was
** added in 3.7.1 when support for 64K pages was added.  






*/
struct WalIndexHdr {
  u32 iVersion;                   /* Wal-index version */
  u32 unused;                     /* Unused (padding) field */
  u32 iChange;                    /* Counter incremented each transaction */
  u8 isInit;                      /* 1 when initialized */
  u8 bigEndCksum;                 /* True if checksums in WAL are big-endian */
  u16 szPage;                     /* Database page size in bytes. 1==64K */
  u32 mxFrame;                    /* Index of last valid frame in the WAL */
  u32 nPage;                      /* Size of database in pages */
  u32 aFrameCksum[2];             /* Checksum of last frame in log */
  u32 aSalt[2];                   /* Two salt values copied from WAL header */
  u32 aCksum[2];                  /* Checksum over all prior fields */
};






































/*
** A copy of the following object occurs in the wal-index immediately
** following the second copy of the WalIndexHdr.  This object stores
** information used by checkpoint.
**
** nBackfill is the number of frames in the WAL that have been written
** back into the database. (We call the act of moving content from WAL to
................................................................................
/*
** An open write-ahead log file is represented by an instance of the
** following object.
*/
struct Wal {
  sqlite3_vfs *pVfs;         /* The VFS used to create pDbFd */
  sqlite3_file *pDbFd;       /* File handle for the database file */
  sqlite3_file *pWalFd;      /* File handle for WAL file */
  u32 iCallback;             /* Value to pass to log callback (or 0) */
  i64 mxWalSize;             /* Truncate WAL to this size upon reset */
  int nWiData;               /* Size of array apWiData */
  int szFirstBlock;          /* Size of first block written to WAL file */
  volatile u32 **apWiData;   /* Pointer to wal-index content in memory */
  u32 szPage;                /* Database page size */
  i16 readLock;              /* Which read lock is being held.  -1 for none */
................................................................................
  u8 padToSectorBoundary;    /* Pad transactions out to the next sector */
  u8 bShmUnreliable;         /* SHM content is read-only and unreliable */
  WalIndexHdr hdr;           /* Wal-index header for current transaction */
  u32 minFrame;              /* Ignore wal frames before this one */
  u32 iReCksum;              /* On commit, recalculate checksums from here */
  u32 nPriorFrame;           /* For sqlite3WalInfo() */
  const char *zWalName;      /* Name of WAL file */

  u32 nCkpt;                 /* Checkpoint sequence counter in the wal-header */
#ifdef SQLITE_DEBUG
  u8 lockError;              /* True if a locking error has occurred */
#endif
#ifdef SQLITE_ENABLE_SNAPSHOT
  WalIndexHdr *pSnapshot;    /* Start transaction here if not NULL */
#endif

};

/*
** Candidate values for Wal.exclusiveMode.
*/
#define WAL_NORMAL_MODE     0
#define WAL_EXCLUSIVE_MODE  1     
................................................................................
*/
static void walIndexWriteHdr(Wal *pWal){
  volatile WalIndexHdr *aHdr = walIndexHdr(pWal);
  const int nCksum = offsetof(WalIndexHdr, aCksum);

  assert( pWal->writeLock );
  pWal->hdr.isInit = 1;
  pWal->hdr.iVersion = WALINDEX_MAX_VERSION;
  walChecksumBytes(1, (u8*)&pWal->hdr, nCksum, 0, pWal->hdr.aCksum);
  memcpy((void*)&aHdr[1], (const void*)&pWal->hdr, sizeof(WalIndexHdr));
  walShmBarrier(pWal);
  memcpy((void*)&aHdr[0], (const void*)&pWal->hdr, sizeof(WalIndexHdr));
}

/*
................................................................................
  */
  pgno = sqlite3Get4byte(&aFrame[0]);
  if( pgno==0 ){
    return 0;
  }

  /* A frame is only valid if a checksum of the WAL header,
  ** all prior frams, the first 16 bytes of this frame-header, 
  ** and the frame-data matches the checksum in the last 8 
  ** bytes of this frame-header.
  */
  nativeCksum = (pWal->hdr.bigEndCksum==SQLITE_BIGENDIAN);
  walChecksumBytes(nativeCksum, aFrame, 8, aCksum, aCksum);
  walChecksumBytes(nativeCksum, aData, pWal->szPage, aCksum, aCksum);
  if( aCksum[0]!=sqlite3Get4byte(&aFrame[16]) 
................................................................................
static void walUnlockExclusive(Wal *pWal, int lockIdx, int n){
  if( pWal->exclusiveMode ) return;
  (void)sqlite3OsShmLock(pWal->pDbFd, lockIdx, n,
                         SQLITE_SHM_UNLOCK | SQLITE_SHM_EXCLUSIVE);
  WALTRACE(("WAL%p: release EXCLUSIVE-%s cnt=%d\n", pWal,
             walLockName(lockIdx), n));
}































/*
** Compute a hash on a page number.  The resulting hash value must land
** between 0 and (HASHTABLE_NSLOT-1).  The walHashNext() function advances
** the hash to the next value in the event of a collision.
*/
static int walHash(u32 iPage){
................................................................................
    }else{
      pLoc->iZero = HASHTABLE_NPAGE_ONE + (iHash-1)*HASHTABLE_NPAGE;
    }
    pLoc->aPgno = &pLoc->aPgno[-1];
  }
  return rc;
}






































/*
** Return the number of the wal-index page that contains the hash-table
** and page-number array that contain entries corresponding to WAL frame
** iFrame. The wal-index is broken up into 32KB pages. Wal-index pages 
** are numbered starting from 0.
*/
................................................................................
       && (iHash>=1 || iFrame<=HASHTABLE_NPAGE_ONE)
       && (iHash<=1 || iFrame>(HASHTABLE_NPAGE_ONE+HASHTABLE_NPAGE))
       && (iHash>=2 || iFrame<=HASHTABLE_NPAGE_ONE+HASHTABLE_NPAGE)
       && (iHash<=2 || iFrame>(HASHTABLE_NPAGE_ONE+2*HASHTABLE_NPAGE))
  );
  return iHash;
}

















/*
** Return the page number associated with frame iFrame in this WAL.
*/
static u32 walFramePgno(Wal *pWal, u32 iFrame){
  int iHash = walFramePage(iFrame);
  if( iHash==0 ){
    return pWal->apWiData[0][WALINDEX_HDR_SIZE/sizeof(u32) + iFrame - 1];
  }
  return pWal->apWiData[iHash][(iFrame-1-HASHTABLE_NPAGE_ONE)%HASHTABLE_NPAGE];
}





/*
** Remove entries from the hash table that point to WAL slots greater
** than pWal->hdr.mxFrame.
**
** This function is called whenever pWal->hdr.mxFrame is decreased due
** to a rollback or savepoint.
................................................................................
** actually needed.
*/
static void walCleanupHash(Wal *pWal){
  WalHashLoc sLoc;                /* Hash table location */
  int iLimit = 0;                 /* Zero values greater than this */
  int nByte;                      /* Number of bytes to zero in aPgno[] */
  int i;                          /* Used to iterate through aHash[] */











  assert( pWal->writeLock );
  testcase( pWal->hdr.mxFrame==HASHTABLE_NPAGE_ONE-1 );
  testcase( pWal->hdr.mxFrame==HASHTABLE_NPAGE_ONE );
  testcase( pWal->hdr.mxFrame==HASHTABLE_NPAGE_ONE+1 );

  if( pWal->hdr.mxFrame==0 ) return;

  /* Obtain pointers to the hash-table and page-number array containing 
  ** the entry that corresponds to frame pWal->hdr.mxFrame. It is guaranteed
  ** that the page said hash-table and array reside on is already mapped.
  */
  assert( pWal->nWiData>walFramePage(pWal->hdr.mxFrame) );
  assert( pWal->apWiData[walFramePage(pWal->hdr.mxFrame)] );
  walHashGet(pWal, walFramePage(pWal->hdr.mxFrame), &sLoc);

  /* Zero all hash-table entries that correspond to frame numbers greater
  ** than pWal->hdr.mxFrame.
  */
  iLimit = pWal->hdr.mxFrame - sLoc.iZero;
  assert( iLimit>0 );
  for(i=0; i<HASHTABLE_NSLOT; i++){
    if( sLoc.aHash[i]>iLimit ){
      sLoc.aHash[i] = 0;
    }
  }
  
  /* Zero the entries in the aPgno array that correspond to frames with
  ** frame numbers greater than pWal->hdr.mxFrame. 
  */
  nByte = (int)((char *)sLoc.aHash - (char *)&sLoc.aPgno[iLimit+1]);
  memset((void *)&sLoc.aPgno[iLimit+1], 0, nByte);

#ifdef SQLITE_ENABLE_EXPENSIVE_ASSERT
  /* Verify that the every entry in the mapping region is still reachable
  ** via the hash table even after the cleanup.
  */
................................................................................
      }
      assert( sLoc.aHash[iKey]==j );
    }
  }
#endif /* SQLITE_ENABLE_EXPENSIVE_ASSERT */
}


/*
** Set an entry in the wal-index that will map database page number
** pPage into WAL frame iFrame.
*/
static int walIndexAppend(Wal *pWal, u32 iFrame, u32 iPage){
  int rc;                         /* Return code */
  WalHashLoc sLoc;                /* Wal-index hash table location */









  rc = walHashGet(pWal, walFramePage(iFrame), &sLoc);

  /* Assuming the wal-index file was successfully mapped, populate the
  ** page number array and hash table entry.
  */
  if( rc==SQLITE_OK ){
    int iKey;                     /* Hash table key */
    int idx;                      /* Value to write to hash-table slot */
    int nCollide;                 /* Number of hash collisions */

    idx = iFrame - sLoc.iZero;
    assert( idx <= HASHTABLE_NSLOT/2 + 1 );
    
    /* If this is the first entry to be added to this hash-table, zero the
    ** entire hash table and aPgno[] array before proceeding. 
    */
    if( idx==1 ){
      int nByte = (int)((u8 *)&sLoc.aHash[HASHTABLE_NSLOT]
................................................................................
#endif /* SQLITE_ENABLE_EXPENSIVE_ASSERT */
  }


  return rc;
}
















































































































































/*
** Recover the wal-index by reading the write-ahead log file. 
**
** This routine first tries to establish an exclusive lock on the
** wal-index to prevent other threads/processes from doing anything
** with the WAL or wal-index while recovery is running.  The
** WAL_RECOVER_LOCK is also held so that other threads will know
** that this thread is running recovery.  If unable to establish
** the necessary locks, this routine returns SQLITE_BUSY.
*/
static int walIndexRecover(Wal *pWal){
  int rc;                         /* Return Code */
  i64 nSize;                      /* Size of log file */
  u32 aFrameCksum[2] = {0, 0};
  int iLock;                      /* Lock offset to lock for checkpoint */





  /* Obtain an exclusive lock on all byte in the locking range not already
  ** locked by the caller. The caller is guaranteed to have locked the
  ** WAL_WRITE_LOCK byte, and may have also locked the WAL_CKPT_LOCK byte.
  ** If successful, the same bytes that are locked here are concurrent before
  ** this function returns.
  */
................................................................................
  }
  if( rc ){
    return rc;
  }

  WALTRACE(("WAL%p: recovery begin...\n", pWal));

  memset(&pWal->hdr, 0, sizeof(WalIndexHdr));
























  rc = sqlite3OsFileSize(pWal->pWalFd, &nSize);








  if( rc!=SQLITE_OK ){
    goto recovery_error;


  }

  if( nSize>WAL_HDRSIZE ){
    u8 aBuf[WAL_HDRSIZE];         /* Buffer to load WAL header into */
    u8 *aFrame = 0;               /* Malloc'd buffer to load entire frame */
    int szFrame;                  /* Number of bytes in buffer aFrame[] */
    u8 *aData;                    /* Pointer to data part of aFrame buffer */
    int iFrame;                   /* Index of last frame read */
    i64 iOffset;                  /* Next offset to read from log file */
    int szPage;                   /* Page size according to the log */
    u32 magic;                    /* Magic value read from WAL header */
    u32 version;                  /* Magic value read from WAL header */
    int isValid;                  /* True if this frame is valid */

    /* Read in the WAL header. */
    rc = sqlite3OsRead(pWal->pWalFd, aBuf, WAL_HDRSIZE, 0);
    if( rc!=SQLITE_OK ){
      goto recovery_error;
    }



    /* If the database page size is not a power of two, or is greater than
    ** SQLITE_MAX_PAGE_SIZE, conclude that the WAL file contains no valid 
    ** data. Similarly, if the 'magic' value is invalid, ignore the whole
    ** WAL file.
    */
    magic = sqlite3Get4byte(&aBuf[0]);
    szPage = sqlite3Get4byte(&aBuf[8]);
    if( (magic&0xFFFFFFFE)!=WAL_MAGIC 
     || szPage&(szPage-1) 
     || szPage>SQLITE_MAX_PAGE_SIZE 
     || szPage<512 





    ){
      goto finished;





    }
    pWal->hdr.bigEndCksum = (u8)(magic&0x00000001);
    pWal->szPage = szPage;
    pWal->nCkpt = sqlite3Get4byte(&aBuf[12]);
    memcpy(&pWal->hdr.aSalt, &aBuf[16], 8);


    /* Verify that the WAL header checksum is correct */
    walChecksumBytes(pWal->hdr.bigEndCksum==SQLITE_BIGENDIAN, 
        aBuf, WAL_HDRSIZE-2*4, 0, pWal->hdr.aFrameCksum
    );


    if( pWal->hdr.aFrameCksum[0]!=sqlite3Get4byte(&aBuf[24])
     || pWal->hdr.aFrameCksum[1]!=sqlite3Get4byte(&aBuf[28])
    ){
      goto finished;
    }

    /* Verify that the version number on the WAL format is one that
    ** are able to understand */
    version = sqlite3Get4byte(&aBuf[4]);
    if( version!=WAL_MAX_VERSION ){
      rc = SQLITE_CANTOPEN_BKPT;
      goto finished;
    }

    /* Malloc a buffer to read frames into. */
    szFrame = szPage + WAL_FRAME_HDRSIZE;
    aFrame = (u8 *)sqlite3_malloc64(szFrame);
    if( !aFrame ){
      rc = SQLITE_NOMEM_BKPT;
      goto recovery_error;
    }
    aData = &aFrame[WAL_FRAME_HDRSIZE];

    /* Read all frames from the log file. */
    iFrame = 0;
    for(iOffset=WAL_HDRSIZE; (iOffset+szFrame)<=nSize; iOffset+=szFrame){
      u32 pgno;                   /* Database page number for frame */
      u32 nTruncate;              /* dbsize field from frame header */

      /* Read and decode the next log frame. */
      iFrame++;
      rc = sqlite3OsRead(pWal->pWalFd, aFrame, szFrame, iOffset);
      if( rc!=SQLITE_OK ) break;
      isValid = walDecodeFrame(pWal, &pgno, &nTruncate, aData, aFrame);
      if( !isValid ) break;
      rc = walIndexAppend(pWal, iFrame, pgno);
      if( rc!=SQLITE_OK ) break;

      /* If nTruncate is non-zero, this is a commit record. */
      if( nTruncate ){
        pWal->hdr.mxFrame = iFrame;
        pWal->hdr.nPage = nTruncate;
        pWal->hdr.szPage = (u16)((szPage&0xff00) | (szPage>>16));
        testcase( szPage<=32768 );
        testcase( szPage>=65536 );
        aFrameCksum[0] = pWal->hdr.aFrameCksum[0];
        aFrameCksum[1] = pWal->hdr.aFrameCksum[1];

      }



    }

    sqlite3_free(aFrame);
  }

finished:
  if( rc==SQLITE_OK ){
    volatile WalCkptInfo *pInfo;
    int i;
    pWal->hdr.aFrameCksum[0] = aFrameCksum[0];
    pWal->hdr.aFrameCksum[1] = aFrameCksum[1];
    walIndexWriteHdr(pWal);

    /* Reset the checkpoint-header. This is safe because this thread is 
    ** currently holding locks that exclude all other readers, writers and
    ** checkpointers.
    */
    pInfo = walCkptInfo(pWal);
    pInfo->nBackfill = 0;



    pInfo->nBackfillAttempted = pWal->hdr.mxFrame;
    pInfo->aReadMark[0] = 0;
    for(i=1; i<WAL_NREADER; i++) pInfo->aReadMark[i] = READMARK_NOT_USED;
    if( pWal->hdr.mxFrame ) pInfo->aReadMark[1] = pWal->hdr.mxFrame;


    /* If more than one frame was recovered from the log file, report an
    ** event via sqlite3_log(). This is to help with identifying performance
    ** problems caused by applications routinely shutting down without
    ** checkpointing the log file.
    */
    if( pWal->hdr.nPage ){

      sqlite3_log(SQLITE_NOTICE_RECOVER_WAL,






          "recovered %d frames from WAL file %s",
          pWal->hdr.mxFrame, pWal->zWalName
      );

    }
  }

recovery_error:
  WALTRACE(("WAL%p: recovery %s\n", pWal, rc ? "failed" : "ok"));
  walUnlockExclusive(pWal, iLock, WAL_READ_LOCK(0)-iLock);
  walUnlockExclusive(pWal, WAL_READ_LOCK(1), WAL_NREADER-1);
  return rc;
}

/*
** Close an open wal-index.
*/
static void walIndexClose(Wal *pWal, int isDelete){
  if( pWal->exclusiveMode==WAL_HEAPMEMORY_MODE || pWal->bShmUnreliable ){
    int i;
    for(i=0; i<pWal->nWiData; i++){
      sqlite3_free((void *)pWal->apWiData[i]);
      pWal->apWiData[i] = 0;
    }
  }
  if( pWal->exclusiveMode!=WAL_HEAPMEMORY_MODE ){
    sqlite3OsShmUnmap(pWal->pDbFd, isDelete);
  }


}

/* 
** Open a connection to the WAL file zWalName. The database file must 
** already be opened on connection pDbFd. The buffer that zWalName points
** to must remain valid for the lifetime of the returned Wal* handle.
**
................................................................................
*/
int sqlite3WalOpen(
  sqlite3_vfs *pVfs,              /* vfs module to open wal and wal-index */
  sqlite3_file *pDbFd,            /* The open database file */
  const char *zWalName,           /* Name of the WAL file */
  int bNoShm,                     /* True to run in heap-memory mode */
  i64 mxWalSize,                  /* Truncate WAL to this size on reset */

  Wal **ppWal                     /* OUT: Allocated Wal handle */
){
  int rc;                         /* Return Code */
  Wal *pRet;                      /* Object to allocate and return */
  int flags;                      /* Flags passed to OsOpen() */



  assert( zWalName && zWalName[0] );
  assert( pDbFd );

  /* In the amalgamation, the os_unix.c and os_win.c source files come before
  ** this source file.  Verify that the #defines of the locking byte offsets
  ** in os_unix.c and os_win.c agree with the WALINDEX_LOCK_OFFSET value.
................................................................................
#ifdef WIN_SHM_BASE
  assert( WIN_SHM_BASE==WALINDEX_LOCK_OFFSET );
#endif
#ifdef UNIX_SHM_BASE
  assert( UNIX_SHM_BASE==WALINDEX_LOCK_OFFSET );
#endif




  /* Allocate an instance of struct Wal to return. */
  *ppWal = 0;
  pRet = (Wal*)sqlite3MallocZero(sizeof(Wal) + pVfs->szOsFile);
  if( !pRet ){
    return SQLITE_NOMEM_BKPT;
  }

  pRet->pVfs = pVfs;
  pRet->pWalFd = (sqlite3_file *)&pRet[1];

  pRet->pDbFd = pDbFd;
  pRet->readLock = -1;
  pRet->mxWalSize = mxWalSize;
  pRet->zWalName = zWalName;
  pRet->syncHeader = 1;
  pRet->padToSectorBoundary = 1;
  pRet->exclusiveMode = (bNoShm ? WAL_HEAPMEMORY_MODE: WAL_NORMAL_MODE);







  /* Open file handle on the write-ahead log file. */
  flags = (SQLITE_OPEN_READWRITE|SQLITE_OPEN_CREATE|SQLITE_OPEN_WAL);
  rc = sqlite3OsOpen(pVfs, zWalName, pRet->pWalFd, flags, &flags);
  if( rc==SQLITE_OK && flags&SQLITE_OPEN_READONLY ){
    pRet->readOnly = WAL_RDONLY;
  }

  if( rc!=SQLITE_OK ){
    walIndexClose(pRet, 0);
    sqlite3OsClose(pRet->pWalFd);
    sqlite3_free(pRet);
  }else{
    int iDC = sqlite3OsDeviceCharacteristics(pDbFd);
    if( iDC & SQLITE_IOCAP_SEQUENTIAL ){ pRet->syncHeader = 0; }
    if( iDC & SQLITE_IOCAP_POWERSAFE_OVERWRITE ){
      pRet->padToSectorBoundary = 0;
    }
................................................................................
*/
static void walIteratorFree(WalIterator *p){
  sqlite3_free(p);
}

/*
** Construct a WalInterator object that can be used to loop over all 
** pages in the WAL following frame nBackfill in ascending order. Frames
** nBackfill or earlier may be included - excluding them is an optimization
** only. The caller must hold the checkpoint lock.
**
** On success, make *pp point to the newly allocated WalInterator object
** return SQLITE_OK. Otherwise, return an error code. If this routine
** returns an error, the value of *pp is undefined.
**
** The calling routine should invoke walIteratorFree() to destroy the
** WalIterator object when it has finished with it.
*/
static int walIteratorInit(Wal *pWal, u32 nBackfill, WalIterator **pp){





  WalIterator *p;                 /* Return value */
  int nSegment;                   /* Number of segments to merge */
  u32 iLast;                      /* Last frame in log */
  int nByte;                      /* Number of bytes to allocate */
  int i;                          /* Iterator variable */

  ht_slot *aTmp;                  /* Temp space used by merge-sort */
  int rc = SQLITE_OK;             /* Return Code */





  /* This routine only runs while holding the checkpoint lock. And
  ** it only runs if there is actually content in the log (mxFrame>0).
  */

  assert( pWal->ckptLock && pWal->hdr.mxFrame>0 );
  iLast = pWal->hdr.mxFrame;








  /* Allocate space for the WalIterator object. */
  nSegment = walFramePage(iLast) + 1;
  nByte = sizeof(WalIterator) 
        + (nSegment-1)*sizeof(struct WalSegment)
        + iLast*sizeof(ht_slot);
  p = (WalIterator *)sqlite3_malloc64(nByte);
  if( !p ){
    return SQLITE_NOMEM_BKPT;
  }
................................................................................
  aTmp = (ht_slot *)sqlite3_malloc64(
      sizeof(ht_slot) * (iLast>HASHTABLE_NPAGE?HASHTABLE_NPAGE:iLast)
  );
  if( !aTmp ){
    rc = SQLITE_NOMEM_BKPT;
  }

  for(i=walFramePage(nBackfill+1); rc==SQLITE_OK && i<nSegment; i++){

    WalHashLoc sLoc;

    rc = walHashGet(pWal, i, &sLoc);
    if( rc==SQLITE_OK ){
      int j;                      /* Counter variable */
      int nEntry;                 /* Number of entries in this segment */
      ht_slot *aIndex;            /* Sorted index for this segment */










      sLoc.aPgno++;
      if( (i+1)==nSegment ){

        nEntry = (int)(iLast - sLoc.iZero);
      }else{
        nEntry = (int)((u32*)sLoc.aHash - (u32*)sLoc.aPgno);
      }
      aIndex = &((ht_slot *)&p->aSegment[p->nSegment])[sLoc.iZero];
      sLoc.iZero++;
  
      for(j=0; j<nEntry; j++){
        aIndex[j] = (ht_slot)j;
      }
      walMergesort((u32 *)sLoc.aPgno, aTmp, aIndex, &nEntry);
      p->aSegment[i].iZero = sLoc.iZero;
      p->aSegment[i].nEntry = nEntry;
      p->aSegment[i].aIndex = aIndex;
      p->aSegment[i].aPgno = (u32 *)sLoc.aPgno;
    }
  }
  sqlite3_free(aTmp);

  if( rc!=SQLITE_OK ){
    walIteratorFree(p);
    p = 0;
................................................................................
** new wal-index header. It should be passed a pseudo-random value (i.e. 
** one obtained from sqlite3_randomness()).
*/
static void walRestartHdr(Wal *pWal, u32 salt1){
  volatile WalCkptInfo *pInfo = walCkptInfo(pWal);
  int i;                          /* Loop counter */
  u32 *aSalt = pWal->hdr.aSalt;   /* Big-endian salt values */

  pWal->nCkpt++;
  pWal->hdr.mxFrame = 0;
  sqlite3Put4byte((u8*)&aSalt[0], 1 + sqlite3Get4byte((u8*)&aSalt[0]));
  memcpy(&pWal->hdr.aSalt[1], &salt1, 4);
  walIndexWriteHdr(pWal);
  pInfo->nBackfill = 0;
  pInfo->nBackfillAttempted = 0;
................................................................................
  WalIterator *pIter = 0;         /* Wal iterator context */
  u32 iDbpage = 0;                /* Next database page to write */
  u32 iFrame = 0;                 /* Wal frame containing data for iDbpage */
  u32 mxSafeFrame;                /* Max frame that can be backfilled */
  u32 mxPage;                     /* Max database page to write */
  int i;                          /* Loop counter */
  volatile WalCkptInfo *pInfo;    /* The checkpoint status information */




  szPage = walPagesize(pWal);
  testcase( szPage<=32768 );
  testcase( szPage>=65536 );
  pInfo = walCkptInfo(pWal);

  if( pInfo->nBackfill<pWal->hdr.mxFrame ){












    /* EVIDENCE-OF: R-62920-47450 The busy-handler callback is never invoked
    ** in the SQLITE_CHECKPOINT_PASSIVE mode. */
    assert( eMode!=SQLITE_CHECKPOINT_PASSIVE || xBusy==0 );

    /* Compute in mxSafeFrame the index of the last frame of the WAL that is
    ** safe to write into the database.  Frames beyond mxSafeFrame might
    ** overwrite database pages that are in use by active readers and thus
    ** cannot be backfilled from the WAL.
    */

    mxSafeFrame = pWal->hdr.mxFrame;
    mxPage = pWal->hdr.nPage;
    for(i=1; i<WAL_NREADER; i++){
      /* Thread-sanitizer reports that the following is an unsafe read,
      ** as some other thread may be in the process of updating the value
      ** of the aReadMark[] slot. The assumption here is that if that is
      ** happening, the other client may only be increasing the value,
      ** not decreasing it. So assuming either that either the "old" or
      ** "new" version of the value is read, and not some arbitrary value
      ** that would never be written by a real client, things are still 
      ** safe.  */
      u32 y = pInfo->aReadMark[i];
      if( mxSafeFrame>y ){
        assert( y<=pWal->hdr.mxFrame );
        rc = walBusyLock(pWal, xBusy, pBusyArg, WAL_READ_LOCK(i), 1);
        if( rc==SQLITE_OK ){
          pInfo->aReadMark[i] = (i==1 ? mxSafeFrame : READMARK_NOT_USED);
          walUnlockExclusive(pWal, WAL_READ_LOCK(i), 1);
        }else if( rc==SQLITE_BUSY ){
          mxSafeFrame = y;
          xBusy = 0;
        }else{
          goto walcheckpoint_out;
        }
      }
    }


    /* Allocate the iterator */
    if( pInfo->nBackfill<mxSafeFrame ){

      rc = walIteratorInit(pWal, pInfo->nBackfill, &pIter);
      assert( rc==SQLITE_OK || pIter==0 );
    }

    if( pIter
     && (rc = walBusyLock(pWal, xBusy, pBusyArg, WAL_READ_LOCK(0),1))==SQLITE_OK
    ){
      u32 nBackfill = pInfo->nBackfill;


      pInfo->nBackfillAttempted = mxSafeFrame;

      /* Sync the WAL to disk */

      rc = sqlite3OsSync(pWal->pWalFd, CKPT_SYNC_FLAGS(sync_flags));

      /* If the database may grow as a result of this checkpoint, hint
      ** about the eventual size of the db file to the VFS layer.
      */
      if( rc==SQLITE_OK ){
        i64 nReq = ((i64)mxPage * szPage);
        i64 nSize;                    /* Current size of database file */
        rc = sqlite3OsFileSize(pWal->pDbFd, &nSize);
        if( rc==SQLITE_OK && nSize<nReq ){
          sqlite3OsFileControlHint(pWal->pDbFd, SQLITE_FCNTL_SIZE_HINT, &nReq);
        }
      }


      /* Iterate through the contents of the WAL, copying data to the db file */
      while( rc==SQLITE_OK && 0==walIteratorNext(pIter, &iDbpage, &iFrame) ){
        i64 iOffset;

        assert( walFramePgno(pWal, iFrame)==iDbpage );


        if( db->u1.isInterrupted ){
          rc = db->mallocFailed ? SQLITE_NOMEM_BKPT : SQLITE_INTERRUPT;
          break;
        }
        if( iFrame<=nBackfill || iFrame>mxSafeFrame || iDbpage>mxPage ){

          continue;
        }
        iOffset = walFrameOffset(iFrame, szPage) + WAL_FRAME_HDRSIZE;



        /* testcase( IS_BIG_INT(iOffset) ); // requires a 4GiB WAL file */
        rc = sqlite3OsRead(pWal->pWalFd, zBuf, szPage, iOffset);
        if( rc!=SQLITE_OK ) break;
        iOffset = (iDbpage-1)*(i64)szPage;
        testcase( IS_BIG_INT(iOffset) );
        rc = sqlite3OsWrite(pWal->pDbFd, zBuf, szPage, iOffset);
        if( rc!=SQLITE_OK ) break;
      }

      /* If work was actually accomplished... */
      if( rc==SQLITE_OK ){
        if( mxSafeFrame==walIndexHdr(pWal)->mxFrame ){

          i64 szDb = pWal->hdr.nPage*(i64)szPage;
          testcase( IS_BIG_INT(szDb) );
          rc = sqlite3OsTruncate(pWal->pDbFd, szDb);

          if( rc==SQLITE_OK ){
            rc = sqlite3OsSync(pWal->pDbFd, CKPT_SYNC_FLAGS(sync_flags));
          }
        }
        if( rc==SQLITE_OK ){
          pInfo->nBackfill = mxSafeFrame;
        }
      }

      /* Release the reader lock held while backfilling */
      walUnlockExclusive(pWal, WAL_READ_LOCK(0), 1);
    }

    if( rc==SQLITE_BUSY ){
      /* Reset the return code so as not to report a checkpoint failure
      ** just because there are active readers.  */
      rc = SQLITE_OK;
    }
................................................................................
  }

  /* If this is an SQLITE_CHECKPOINT_RESTART or TRUNCATE operation, and the
  ** entire wal file has been copied into the database file, then block 
  ** until all readers have finished using the wal file. This ensures that 
  ** the next process to write to the database restarts the wal file.
  */
  if( rc==SQLITE_OK && eMode!=SQLITE_CHECKPOINT_PASSIVE ){
    assert( pWal->writeLock );
    if( pInfo->nBackfill<pWal->hdr.mxFrame ){
      rc = SQLITE_BUSY;
    }else if( eMode>=SQLITE_CHECKPOINT_RESTART ){
      u32 salt1;
      sqlite3_randomness(4, &salt1);
      assert( pInfo->nBackfill==pWal->hdr.mxFrame );
................................................................................
          ** writer clients should see that the entire log file has been
          ** checkpointed and behave accordingly. This seems unsafe though,
          ** as it would leave the system in a state where the contents of
          ** the wal-index header do not match the contents of the 
          ** file-system. To avoid this, update the wal-index header to
          ** indicate that the log file contains zero valid frames.  */
          walRestartHdr(pWal, salt1);
          rc = sqlite3OsTruncate(pWal->pWalFd, 0);
        }
        walUnlockExclusive(pWal, WAL_READ_LOCK(1), WAL_NREADER-1);
      }
    }
  }

 walcheckpoint_out:
................................................................................
}

/*
** If the WAL file is currently larger than nMax bytes in size, truncate
** it to exactly nMax bytes. If an error occurs while doing so, ignore it.
*/
static void walLimitSize(Wal *pWal, i64 nMax){

  i64 sz;
  int rx;
  sqlite3BeginBenignMalloc();
  rx = sqlite3OsFileSize(pWal->pWalFd, &sz);
  if( rx==SQLITE_OK && (sz > nMax ) ){
    rx = sqlite3OsTruncate(pWal->pWalFd, nMax);
  }
  sqlite3EndBenignMalloc();
  if( rx ){
    sqlite3_log(rx, "cannot limit WAL size: %s", pWal->zWalName);

  }
}

/*
** Close a connection to a log file.
*/
int sqlite3WalClose(
................................................................................
    ** the wal and wal-index files.
    **
    ** The EXCLUSIVE lock is not released before returning.
    */
    if( zBuf!=0
     && SQLITE_OK==(rc = sqlite3OsLock(pWal->pDbFd, SQLITE_LOCK_EXCLUSIVE))
    ){

      if( pWal->exclusiveMode==WAL_NORMAL_MODE ){
        pWal->exclusiveMode = WAL_EXCLUSIVE_MODE;
      }

      rc = sqlite3WalCheckpoint(pWal, db, 
          SQLITE_CHECKPOINT_PASSIVE, 0, 0, sync_flags, nBuf, zBuf, 0, 0
      );
      if( rc==SQLITE_OK ){
        int bPersist = -1;
        sqlite3OsFileControlHint(
            pWal->pDbFd, SQLITE_FCNTL_PERSIST_WAL, &bPersist
        );
        if( bPersist!=1 ){
          /* Try to delete the WAL file if the checkpoint completed and
          ** fsyned (rc==SQLITE_OK) and if we are not in persistent-wal
          ** mode (!bPersist) */
          isDelete = 1;
        }else if( pWal->mxWalSize>=0 ){
          /* Try to truncate the WAL file to zero bytes if the checkpoint
          ** completed and fsynced (rc==SQLITE_OK) and we are in persistent
          ** WAL mode (bPersist) and if the PRAGMA journal_size_limit is a
          ** non-negative value (pWal->mxWalSize>=0).  Note that we truncate
          ** to zero bytes as truncating to the journal_size_limit might
          ** leave a corrupt WAL file on disk. */
          walLimitSize(pWal, 0);
        }









      }
    }

    walIndexClose(pWal, isDelete);
    sqlite3OsClose(pWal->pWalFd);
    if( isDelete ){
      sqlite3BeginBenignMalloc();
      sqlite3OsDelete(pWal->pVfs, pWal->zWalName, 0);

      sqlite3EndBenignMalloc();
    }
    WALTRACE(("WAL%p: closed\n", pWal));
    sqlite3_free((void *)pWal->apWiData);
    sqlite3_free(pWal);
  }
  return rc;
................................................................................
    }
  }

  /* If the header is read successfully, check the version number to make
  ** sure the wal-index was not constructed with some future format that
  ** this version of SQLite cannot understand.
  */

  if( badHdr==0 && pWal->hdr.iVersion!=WALINDEX_MAX_VERSION ){

    rc = SQLITE_CANTOPEN_BKPT;
  }
  if( pWal->bShmUnreliable ){
    if( rc!=SQLITE_OK ){
      walIndexClose(pWal, 0);
      pWal->bShmUnreliable = 0;
      assert( pWal->nWiData>0 && pWal->apWiData[0]==0 );
................................................................................
  ** into pWal->hdr.
  */
  memcpy(&pWal->hdr, (void*)walIndexHdr(pWal), sizeof(WalIndexHdr));

  /* Make sure some writer hasn't come in and changed the WAL file out
  ** from under us, then disconnected, while we were not looking.
  */
  rc = sqlite3OsFileSize(pWal->pWalFd, &szWal);
  if( rc!=SQLITE_OK ){
    goto begin_unreliable_shm_out;
  }
  if( szWal<WAL_HDRSIZE ){
    /* If the wal file is too small to contain a wal-header and the
    ** wal-index header has mxFrame==0, then it must be safe to proceed
    ** reading the database file only. However, the page cache cannot
................................................................................
    ** since this client's last read transaction.  */
    *pChanged = 1;
    rc = (pWal->hdr.mxFrame==0 ? SQLITE_OK : WAL_RETRY);
    goto begin_unreliable_shm_out;
  }

  /* Check the salt keys at the start of the wal file still match. */
  rc = sqlite3OsRead(pWal->pWalFd, aBuf, WAL_HDRSIZE, 0);
  if( rc!=SQLITE_OK ){
    goto begin_unreliable_shm_out;
  }
  if( memcmp(&pWal->hdr.aSalt, &aBuf[16], 8) ){
    /* Some writer has wrapped the WAL file while we were not looking.
    ** Return WAL_RETRY which will cause the in-memory WAL-index to be
    ** rebuilt. */
................................................................................
      iOffset+szFrame<=szWal; 
      iOffset+=szFrame
  ){
    u32 pgno;                   /* Database page number for frame */
    u32 nTruncate;              /* dbsize field from frame header */

    /* Read and decode the next log frame. */
    rc = sqlite3OsRead(pWal->pWalFd, aFrame, szFrame, iOffset);
    if( rc!=SQLITE_OK ) break;
    if( !walDecodeFrame(pWal, &pgno, &nTruncate, aData, aFrame) ) break;

    /* If nTruncate is non-zero, then a complete transaction has been
    ** appended to this wal file. Set rc to WAL_RETRY and break out of
    ** the loop.  */
    if( nTruncate ){
................................................................................
** checkpoint process do as much work as possible.  This routine might
** update values of the aReadMark[] array in the header, but if it does
** so it takes care to hold an exclusive lock on the corresponding
** WAL_READ_LOCK() while changing values.
*/
static int walTryBeginRead(Wal *pWal, int *pChanged, int useWal, int cnt){
  volatile WalCkptInfo *pInfo;    /* Checkpoint information in wal-index */
  u32 mxReadMark;                 /* Largest aReadMark[] value */
  int mxI;                        /* Index of largest aReadMark[] value */
  int i;                          /* Loop counter */
  int rc = SQLITE_OK;             /* Return code  */
  u32 mxFrame;                    /* Wal frame to lock to */

  assert( pWal->readLock<0 );     /* Not currently locked */

  /* useWal may only be set for read/write connections */
  assert( (pWal->readOnly & WAL_SHM_RDONLY)==0 || useWal==0 );

  /* Take steps to avoid spinning forever if there is a protocol error.
  **
  ** Circumstances that cause a RETRY should only last for the briefest
................................................................................
      return walBeginShmUnreliable(pWal, pChanged);
    }
  }

  assert( pWal->nWiData>0 );
  assert( pWal->apWiData[0]!=0 );
  pInfo = walCkptInfo(pWal);























  if( !useWal && pInfo->nBackfill==pWal->hdr.mxFrame
#ifdef SQLITE_ENABLE_SNAPSHOT
   && (pWal->pSnapshot==0 || pWal->hdr.mxFrame==0)
#endif
  ){
    /* The WAL has been completely backfilled (or it is empty).
    ** and can be safely ignored.
    */
    rc = walLockShared(pWal, WAL_READ_LOCK(0));
    walShmBarrier(pWal);
    if( rc==SQLITE_OK ){
      if( memcmp((void *)walIndexHdr(pWal), &pWal->hdr, sizeof(WalIndexHdr)) ){
        /* It is not safe to allow the reader to continue here if frames
        ** may have been appended to the log before READ_LOCK(0) was obtained.
        ** When holding READ_LOCK(0), the reader ignores the entire log file,
        ** which implies that the database file contains a trustworthy
        ** snapshot. Since holding READ_LOCK(0) prevents a checkpoint from
        ** happening, this is usually correct.
        **
        ** However, if frames have been appended to the log (or if the log 
        ** is wrapped and written for that matter) before the READ_LOCK(0)
        ** is obtained, that is not necessarily true. A checkpointer may
        ** have started to backfill the appended frames but crashed before
        ** it finished. Leaving a corrupt image in the database file.
        */
        walUnlockShared(pWal, WAL_READ_LOCK(0));
        return WAL_RETRY;
      }
      pWal->readLock = 0;
      return SQLITE_OK;
    }else if( rc!=SQLITE_BUSY ){
      return rc;
    }
  }

  /* If we get this far, it means that the reader will want to use
  ** the WAL to get at content from recent commits.  The job now is
  ** to select one of the aReadMark[] entries that is closest to
  ** but not exceeding pWal->hdr.mxFrame and lock that entry.
  */
  mxReadMark = 0;
  mxI = 0;
  mxFrame = pWal->hdr.mxFrame;
#ifdef SQLITE_ENABLE_SNAPSHOT
  if( pWal->pSnapshot && pWal->pSnapshot->mxFrame<mxFrame ){
    mxFrame = pWal->pSnapshot->mxFrame;
  }
#endif
  for(i=1; i<WAL_NREADER; i++){
    u32 thisMark = AtomicLoad(pInfo->aReadMark+i);
    if( mxReadMark<=thisMark && thisMark<=mxFrame ){
      assert( thisMark!=READMARK_NOT_USED );
      mxReadMark = thisMark;
      mxI = i;
    }
  }
  if( (pWal->readOnly & WAL_SHM_RDONLY)==0
   && (mxReadMark<mxFrame || mxI==0)
  ){
    for(i=1; i<WAL_NREADER; i++){
      rc = walLockExclusive(pWal, WAL_READ_LOCK(i), 1);
      if( rc==SQLITE_OK ){
        mxReadMark = AtomicStore(pInfo->aReadMark+i,mxFrame);
        mxI = i;
        walUnlockExclusive(pWal, WAL_READ_LOCK(i), 1);
        break;
      }else if( rc!=SQLITE_BUSY ){
        return rc;
      }
    }
  }
  if( mxI==0 ){
    assert( rc==SQLITE_BUSY || (pWal->readOnly & WAL_SHM_RDONLY)!=0 );
    return rc==SQLITE_BUSY ? WAL_RETRY : SQLITE_READONLY_CANTINIT;
  }

  rc = walLockShared(pWal, WAL_READ_LOCK(mxI));
  if( rc ){
    return rc==SQLITE_BUSY ? WAL_RETRY : rc;
  }
  /* Now that the read-lock has been obtained, check that neither the
  ** value in the aReadMark[] array or the contents of the wal-index
  ** header have changed.
  **
  ** It is necessary to check that the wal-index header did not change
  ** between the time it was read and when the shared-lock was obtained
  ** on WAL_READ_LOCK(mxI) was obtained to account for the possibility
  ** that the log file may have been wrapped by a writer, or that frames
  ** that occur later in the log than pWal->hdr.mxFrame may have been
  ** copied into the database by a checkpointer. If either of these things
  ** happened, then reading the database with the current value of
  ** pWal->hdr.mxFrame risks reading a corrupted snapshot. So, retry
  ** instead.
  **
  ** Before checking that the live wal-index header has not changed
  ** since it was read, set Wal.minFrame to the first frame in the wal
  ** file that has not yet been checkpointed. This client will not need
  ** to read any frames earlier than minFrame from the wal file - they
  ** can be safely read directly from the database file.
  **
  ** Because a ShmBarrier() call is made between taking the copy of 
  ** nBackfill and checking that the wal-header in shared-memory still
  ** matches the one cached in pWal->hdr, it is guaranteed that the 
  ** checkpointer that set nBackfill was not working with a wal-index
  ** header newer than that cached in pWal->hdr. If it were, that could
  ** cause a problem. The checkpointer could omit to checkpoint
  ** a version of page X that lies before pWal->minFrame (call that version
  ** A) on the basis that there is a newer version (version B) of the same
  ** page later in the wal file. But if version B happens to like past
  ** frame pWal->hdr.mxFrame - then the client would incorrectly assume
  ** that it can read version A from the database file. However, since
  ** we can guarantee that the checkpointer that set nBackfill could not
  ** see any pages past pWal->hdr.mxFrame, this problem does not come up.
  */
  pWal->minFrame = AtomicLoad(&pInfo->nBackfill)+1;
  walShmBarrier(pWal);
  if( AtomicLoad(pInfo->aReadMark+mxI)!=mxReadMark
   || memcmp((void *)walIndexHdr(pWal), &pWal->hdr, sizeof(WalIndexHdr))
  ){
    walUnlockShared(pWal, WAL_READ_LOCK(mxI));
    return WAL_RETRY;
  }else{
    assert( mxReadMark<=pWal->hdr.mxFrame );
    pWal->readLock = (i16)mxI;

  }
  return rc;
}

#ifdef SQLITE_ENABLE_SNAPSHOT
/*
** Attempt to reduce the value of the WalCkptInfo.nBackfillAttempted 
................................................................................
          rc = walHashGet(pWal, walFramePage(i), &sLoc);
          if( rc!=SQLITE_OK ) break;
          pgno = sLoc.aPgno[i-sLoc.iZero];
          iDbOff = (i64)(pgno-1) * szPage;

          if( iDbOff+szPage<=szDb ){
            iWalOff = walFrameOffset(i, szPage) + WAL_FRAME_HDRSIZE;
            rc = sqlite3OsRead(pWal->pWalFd, pBuf1, szPage, iWalOff);

            if( rc==SQLITE_OK ){
              rc = sqlite3OsRead(pWal->pDbFd, pBuf2, szPage, iDbOff);
            }

            if( rc!=SQLITE_OK || 0==memcmp(pBuf1, pBuf2, szPage) ){
              break;
................................................................................
  do{
    rc = walTryBeginRead(pWal, pChanged, 0, ++cnt);
  }while( rc==WAL_RETRY );
  testcase( (rc&0xff)==SQLITE_BUSY );
  testcase( (rc&0xff)==SQLITE_IOERR );
  testcase( rc==SQLITE_PROTOCOL );
  testcase( rc==SQLITE_OK );





  pWal->nPriorFrame = pWal->hdr.mxFrame;
#ifdef SQLITE_ENABLE_SNAPSHOT
  if( rc==SQLITE_OK ){
    if( pSnapshot && memcmp(pSnapshot, &pWal->hdr, sizeof(WalIndexHdr))!=0 ){
      /* At this point the client has a lock on an aReadMark[] slot holding
      ** a value equal to or smaller than pSnapshot->mxFrame, but pWal->hdr
................................................................................

/*
** Finish with a read transaction.  All this does is release the
** read-lock.
*/
void sqlite3WalEndReadTransaction(Wal *pWal){
  sqlite3WalEndWriteTransaction(pWal);
  if( pWal->readLock>=0 ){



    walUnlockShared(pWal, WAL_READ_LOCK(pWal->readLock));

    pWal->readLock = -1;
  }
}





















































































/*
** Search the wal file for page pgno. If found, set *piRead to the frame that
** contains the page. Otherwise, if pgno is not in the wal file, set *piRead
** to zero.
**
** Return SQLITE_OK if successful, or an error code if an error occurs. If an
................................................................................
** error does occur, the final value of *piRead is undefined.
*/
int sqlite3WalFindFrame(
  Wal *pWal,                      /* WAL handle */
  Pgno pgno,                      /* Database page number to read data for */
  u32 *piRead                     /* OUT: Frame number (or zero) */
){



  u32 iRead = 0;                  /* If !=0, WAL frame to return data from */
  u32 iLast = pWal->hdr.mxFrame;  /* Last page in WAL for this reader */
  int iHash;                      /* Used to loop through N hash tables */
  int iMinHash;

  /* This routine is only be called from within a read transaction. */
  assert( pWal->readLock>=0 || pWal->lockError );

  /* If the "last page" field of the wal-index header snapshot is 0, then
  ** no data will be read from the wal under any circumstances. Return early
  ** in this case as an optimization.  Likewise, if pWal->readLock==0, 
  ** then the WAL is ignored by the reader so return early, as if the 
  ** WAL were empty.
  */







  if( iLast==0 || (pWal->readLock==0 && pWal->bShmUnreliable==0) ){
    *piRead = 0;
    return SQLITE_OK;
  }

  /* Each iteration of the following for() loop searches one
  ** hash table (each hash table indexes up to HASHTABLE_NPAGE frames).
  **
  ** This code might run concurrently to the code in walIndexAppend()
  ** that adds entries to the wal-index (and possibly to this hash 
  ** table). This means the value just read from the hash 
  ** slot (aHash[iKey]) may have been added before or after the 
  ** current read transaction was opened. Values added after the
  ** read transaction was opened may have been written incorrectly -
  ** i.e. these slots may contain garbage data. However, we assume
  ** that any slots written before the current read transaction was
  ** opened remain unmodified.
  **
  ** For the reasons above, the if(...) condition featured in the inner
  ** loop of the following block is more stringent that would be required 
  ** if we had exclusive access to the hash-table:
  **
  **   (aPgno[iFrame]==pgno): 
  **     This condition filters out normal hash-table collisions.
  **
  **   (iFrame<=iLast): 
  **     This condition filters out entries that were added to the hash
  **     table after the current read-transaction had started.
  */
  iMinHash = walFramePage(pWal->minFrame);
  for(iHash=walFramePage(iLast); iHash>=iMinHash; iHash--){
    WalHashLoc sLoc;              /* Hash table location */
    int iKey;                     /* Hash slot index */
    int nCollide;                 /* Number of hash collisions remaining */
    int rc;                       /* Error code */

    rc = walHashGet(pWal, iHash, &sLoc);
    if( rc!=SQLITE_OK ){
      return rc;








    }
    nCollide = HASHTABLE_NSLOT;
    for(iKey=walHash(pgno); sLoc.aHash[iKey]; iKey=walNextHash(iKey)){
      u32 iFrame = sLoc.aHash[iKey] + sLoc.iZero;
      if( iFrame<=iLast && iFrame>=pWal->minFrame
       && sLoc.aPgno[sLoc.aHash[iKey]]==pgno ){
        assert( iFrame>iRead || CORRUPT_DB );
        iRead = iFrame;
      }
      if( (nCollide--)==0 ){
        return SQLITE_CORRUPT_BKPT;
      }
    }

    if( iRead ) break;





  }


#ifdef SQLITE_ENABLE_EXPENSIVE_ASSERT
  /* If expensive assert() statements are available, do a linear search
  ** of the wal-index file content. Make sure the results agree with the
  ** result obtained using the hash indexes above.  */



  if( rc==SQLITE_OK ){
    u32 iRead2 = 0;
    u32 iTest;
    assert( pWal->bShmUnreliable || pWal->minFrame>0 );
    for(iTest=iLast; iTest>=pWal->minFrame && iTest>0; iTest--){
      if( walFramePgno(pWal, iTest)==pgno ){
        iRead2 = iTest;
................................................................................
/*
** Read the contents of frame iRead from the wal file into buffer pOut
** (which is nOut bytes in size). Return SQLITE_OK if successful, or an
** error code otherwise.
*/
int sqlite3WalReadFrame(
  Wal *pWal,                      /* WAL handle */
  u32 iRead,                      /* Frame to read */
  int nOut,                       /* Size of buffer pOut in bytes */
  u8 *pOut                        /* Buffer to write page data to */
){
  int sz;


  i64 iOffset;


  sz = pWal->hdr.szPage;
  sz = (sz&0xfe00) + ((sz&0x0001)<<16);
  testcase( sz<=32768 );
  testcase( sz>=65536 );










  iOffset = walFrameOffset(iRead, sz) + WAL_FRAME_HDRSIZE;
  /* testcase( IS_BIG_INT(iOffset) ); // requires a 4GiB WAL */
  return sqlite3OsRead(pWal->pWalFd, pOut, (nOut>sz ? sz : nOut), iOffset);
}

/* 
** Return the size of the database in pages (or zero, if unknown).
*/
Pgno sqlite3WalDbsize(Wal *pWal){
  if( pWal && ALWAYS(pWal->readLock>=0) ){
    return pWal->hdr.nPage;
  }
  return 0;
}

/*
** Take the WRITER lock on the WAL file. Return SQLITE_OK if successful,
................................................................................
              u8 aNew[4];
              u8 *aOld = &((u8*)pPage1->pData)[40];
              int sz;
              i64 iOffset;
              sz = pWal->hdr.szPage;
              sz = (sz&0xfe00) + ((sz&0x0001)<<16);
              iOffset = walFrameOffset(i+sLoc.iZero, sz) + WAL_FRAME_HDRSIZE+40;
              rc = sqlite3OsRead(pWal->pWalFd, aNew, sizeof(aNew), iOffset);
              if( rc==SQLITE_OK && memcmp(aOld, aNew, sizeof(aNew)) ){
                rc = SQLITE_BUSY_SNAPSHOT;
              }
            }else if( sqlite3BitvecTestNotNull(pAllRead, sLoc.aPgno[i]) ){
              *piConflict = sLoc.aPgno[i];
              rc = SQLITE_BUSY_SNAPSHOT;
            }else if( (pPg = sqlite3PagerLookup(pPager, sLoc.aPgno[i])) ){
................................................................................
**
** Otherwise, if the callback function does not return an error, this
** function returns SQLITE_OK.
*/
int sqlite3WalUndo(Wal *pWal, int (*xUndo)(void *, Pgno), void *pUndoCtx){
  int rc = SQLITE_OK;
  if( pWal->writeLock ){
    Pgno iMax = pWal->hdr.mxFrame;


    Pgno iFrame;
  


    /* Restore the clients cache of the wal-index header to the state it
    ** was in before the client began writing to the database. 
    */
    memcpy(&pWal->hdr, (void *)walIndexHdr(pWal), sizeof(WalIndexHdr));



    for(iFrame=pWal->hdr.mxFrame+1; 
        ALWAYS(rc==SQLITE_OK) && iFrame<=iMax; 
        iFrame++
    ){
      /* This call cannot fail. Unless the page for which the page number
      ** is passed as the second argument is (a) in the cache and 
      ** (b) has an outstanding reference, then xUndo is either a no-op
      ** (if (a) is false) or simply expels the page from the cache (if (b)
      ** is false).
      **
      ** If the upper layer is doing a rollback, it is guaranteed that there
      ** are no outstanding references to any page other than page 1. And
      ** page 1 is never written to the log until the transaction is
      ** committed. As a result, the call to xUndo may not fail.
      */




      assert( walFramePgno(pWal, iFrame)!=1 );


      rc = xUndo(pUndoCtx, walFramePgno(pWal, iFrame));
    }
    if( iMax!=pWal->hdr.mxFrame ) walCleanupHash(pWal);
  }
  return rc;
}

/* 
** Argument aWalData must point to an array of WAL_SAVEPOINT_NDATA u32 
** values. This function populates the array with values required to 
** "rollback" the write position of the WAL handle back to the current 
** point in the event of a savepoint rollback (via WalSavepointUndo()).
*/
void sqlite3WalSavepoint(Wal *pWal, u32 *aWalData){
  aWalData[0] = pWal->hdr.mxFrame;



  aWalData[1] = pWal->hdr.aFrameCksum[0];
  aWalData[2] = pWal->hdr.aFrameCksum[1];
  aWalData[3] = pWal->nCkpt;

}

/* 
** Move the write position of the WAL back to the point identified by
** the values in the aWalData[] array. aWalData must point to an array
** of WAL_SAVEPOINT_NDATA u32 values that has been previously populated
** by a call to WalSavepoint().
*/
int sqlite3WalSavepointUndo(Wal *pWal, u32 *aWalData){
  int rc = SQLITE_OK;



  assert( pWal->writeLock || aWalData[0]==pWal->hdr.mxFrame );
  assert( aWalData[3]!=pWal->nCkpt || aWalData[0]<=pWal->hdr.mxFrame );



  if( aWalData[3]!=pWal->nCkpt ){
    /* This savepoint was opened immediately after the write-transaction
    ** was started. Right after that, the writer decided to wrap around
    ** to the start of the log. Update the savepoint values to match.
    */
    aWalData[0] = 0;
    aWalData[3] = pWal->nCkpt;
  }

  if( aWalData[0]<pWal->hdr.mxFrame ){
    pWal->hdr.mxFrame = aWalData[0];
    pWal->hdr.aFrameCksum[0] = aWalData[1];
    pWal->hdr.aFrameCksum[1] = aWalData[2];
    walCleanupHash(pWal);
  }

  return rc;
}

/*



























** This function is called just before writing a set of frames to the log
** file (see sqlite3WalFrames()). It checks to see if, instead of appending
** to the current log file, it is possible to overwrite the start of the
** existing log file with the new frames (i.e. "reset" the log). If so,
** it sets pWal->hdr.mxFrame to 0. Otherwise, pWal->hdr.mxFrame is left
** unchanged.



**
** SQLITE_OK is returned if no error is encountered (regardless of whether
** or not pWal->hdr.mxFrame is modified). An SQLite error code is returned
** if an error occurs.
*/
static int walRestartLog(Wal *pWal){
  int rc = SQLITE_OK;































  if( pWal->readLock==0 ){
    volatile WalCkptInfo *pInfo = walCkptInfo(pWal);
    assert( pInfo->nBackfill==pWal->hdr.mxFrame );
    if( pInfo->nBackfill>0 ){
      u32 salt1;
      sqlite3_randomness(4, &salt1);
      rc = walLockExclusive(pWal, WAL_READ_LOCK(1), WAL_NREADER-1);
      if( rc==SQLITE_OK ){
................................................................................
    ** only - never from the wal file. This means that if a writer holding
    ** a lock on aReadmark[0] were to commit a transaction but not close the
    ** read-transaction, subsequent read operations would read directly from
    ** the database file - ignoring the new pages just appended
    ** to the wal file. */
    rc = walUpgradeReadlock(pWal);
  }

  return rc;
}

/*
** Information about the current state of the WAL file and where
** the next fsync should occur - passed from sqlite3WalFrames() into
** walWriteToLog().
................................................................................
  PgHdr *pPage,               /* The page of the frame to be written */
  int nTruncate,              /* The commit flag.  Usually 0.  >0 for commit */
  sqlite3_int64 iOffset       /* Byte offset at which to write */
){
  int rc;                         /* Result code from subfunctions */
  void *pData;                    /* Data actually written */
  u8 aFrame[WAL_FRAME_HDRSIZE];   /* Buffer to assemble frame-header in */












#if defined(SQLITE_HAS_CODEC)
  if( (pData = sqlite3PagerCodec(pPage))==0 ) return SQLITE_NOMEM_BKPT;
#else
  pData = pPage->pData;
#endif
  walEncodeFrame(p->pWal, pPage->pgno, nTruncate, pData, aFrame);
  rc = walWriteToLog(p, aFrame, sizeof(aFrame), iOffset);
................................................................................
** one or more frames have been overwritten. It updates the checksums for
** all frames written to the wal file by the current transaction starting
** with the earliest to have been overwritten.
**
** SQLITE_OK is returned if successful, or an SQLite error code otherwise.
*/
static int walRewriteChecksums(Wal *pWal, u32 iLast){
  const int szPage = pWal->szPage;/* Database page size */
  int rc = SQLITE_OK;             /* Return code */

  u8 *aBuf;                       /* Buffer to load data from wal file into */
  u8 aFrame[WAL_FRAME_HDRSIZE];   /* Buffer to assemble frame-headers in */
  u32 iRead;                      /* Next frame to read from wal file */
  i64 iCksumOff;


  aBuf = sqlite3_malloc(szPage + WAL_FRAME_HDRSIZE);
  if( aBuf==0 ) return SQLITE_NOMEM_BKPT;

  /* Find the checksum values to use as input for the recalculating the
  ** first checksum. If the first frame is frame 1 (implying that the current
  ** transaction restarted the wal file), these values must be read from the
................................................................................
  ** previous frame.  */
  assert( pWal->iReCksum>0 );
  if( pWal->iReCksum==1 ){
    iCksumOff = 24;
  }else{
    iCksumOff = walFrameOffset(pWal->iReCksum-1, szPage) + 16;
  }
  rc = sqlite3OsRead(pWal->pWalFd, aBuf, sizeof(u32)*2, iCksumOff);
  pWal->hdr.aFrameCksum[0] = sqlite3Get4byte(aBuf);
  pWal->hdr.aFrameCksum[1] = sqlite3Get4byte(&aBuf[sizeof(u32)]);

  iRead = pWal->iReCksum;
  pWal->iReCksum = 0;
  for(; rc==SQLITE_OK && iRead<=iLast; iRead++){
    i64 iOff = walFrameOffset(iRead, szPage);
    rc = sqlite3OsRead(pWal->pWalFd, aBuf, szPage+WAL_FRAME_HDRSIZE, iOff);
    if( rc==SQLITE_OK ){
      u32 iPgno, nDbSize;
      iPgno = sqlite3Get4byte(aBuf);
      nDbSize = sqlite3Get4byte(&aBuf[4]);

      walEncodeFrame(pWal, iPgno, nDbSize, &aBuf[WAL_FRAME_HDRSIZE], aFrame);
      rc = sqlite3OsWrite(pWal->pWalFd, aFrame, sizeof(aFrame), iOff);
    }
  }

  sqlite3_free(aBuf);
  return rc;
}

................................................................................
  PgHdr *pLast = 0;               /* Last frame in list */
  int nExtra = 0;                 /* Number of extra copies of last page */
  int szFrame;                    /* The size of a single frame */
  i64 iOffset;                    /* Next byte to write in WAL file */
  WalWriter w;                    /* The writer */
  u32 iFirst = 0;                 /* First frame that may be overwritten */
  WalIndexHdr *pLive;             /* Pointer to shared header */



  assert( pList );
  assert( pWal->writeLock );

  /* If this frame set completes a transaction, then nTruncate>0.  If
  ** nTruncate==0 then this frame set does not complete the transaction. */
  assert( (isCommit!=0)==(nTruncate!=0) );

#if defined(SQLITE_TEST) && defined(SQLITE_DEBUG)
  { int cnt; for(cnt=0, p=pList; p; p=p->pDirty, cnt++){}
    WALTRACE(("WAL%p: frame write begin. %d frames. mxFrame=%d. %s\n",
              pWal, cnt, pWal->hdr.mxFrame, isCommit ? "Commit" : "Spill"));
  }
#endif

  pLive = (WalIndexHdr*)walIndexHdr(pWal);
  if( memcmp(&pWal->hdr, (void *)pLive, sizeof(WalIndexHdr))!=0 ){
    iFirst = pLive->mxFrame+1;


  }

  /* See if it is possible to write these frames into the start of the
  ** log file, instead of appending to it at pWal->hdr.mxFrame.
  */
  if( SQLITE_OK!=(rc = walRestartLog(pWal)) ){
    return rc;
  }

  /* If this is the first frame written into the log, write the WAL
  ** header to the start of the WAL file. See comments at the top of
  ** this source file for a description of the WAL header format.
  */
  iFrame = pWal->hdr.mxFrame;










  if( iFrame==0 ){

    u8 aWalHdr[WAL_HDRSIZE];      /* Buffer to assemble wal-header in */
    u32 aCksum[2];                /* Checksum for wal-header */

    sqlite3Put4byte(&aWalHdr[0], (WAL_MAGIC | SQLITE_BIGENDIAN));
    sqlite3Put4byte(&aWalHdr[4], WAL_MAX_VERSION);
    sqlite3Put4byte(&aWalHdr[8], szPage);












    sqlite3Put4byte(&aWalHdr[12], pWal->nCkpt);
    if( pWal->nCkpt==0 ) sqlite3_randomness(8, pWal->hdr.aSalt);
    memcpy(&aWalHdr[16], pWal->hdr.aSalt, 8);
    walChecksumBytes(1, aWalHdr, WAL_HDRSIZE-2*4, 0, aCksum);
    sqlite3Put4byte(&aWalHdr[24], aCksum[0]);
    sqlite3Put4byte(&aWalHdr[28], aCksum[1]);
    
    pWal->szPage = szPage;
    pWal->hdr.bigEndCksum = SQLITE_BIGENDIAN;
    pWal->hdr.aFrameCksum[0] = aCksum[0];
    pWal->hdr.aFrameCksum[1] = aCksum[1];
    pWal->truncateOnCommit = 1;

    rc = sqlite3OsWrite(pWal->pWalFd, aWalHdr, sizeof(aWalHdr), 0);
    WALTRACE(("WAL%p: wal-header write %s\n", pWal, rc ? "failed" : "ok"));
    if( rc!=SQLITE_OK ){
      return rc;
    }

    /* Sync the header (unless SQLITE_IOCAP_SEQUENTIAL is true or unless
    ** all syncing is turned off by PRAGMA synchronous=OFF).  Otherwise
    ** an out-of-order write following a WAL restart could result in
    ** database corruption.  See the ticket:
    **
    **     https://sqlite.org/src/info/ff5be73dee
    */
    if( pWal->syncHeader ){
      rc = sqlite3OsSync(pWal->pWalFd, CKPT_SYNC_FLAGS(sync_flags));
      if( rc ) return rc;
    }
  }
  assert( (int)pWal->szPage==szPage );

  /* Setup information needed to write frames into the WAL */
  w.pWal = pWal;
  w.pFd = pWal->pWalFd;
  w.iSyncPoint = 0;
  w.syncFlags = sync_flags;
  w.szPage = szPage;
  iOffset = walFrameOffset(iFrame+1, szPage);
  szFrame = szPage + WAL_FRAME_HDRSIZE;

  /* Write all frames into the log file exactly once */
................................................................................

    /* Check if this page has already been written into the wal file by
    ** the current transaction. If so, overwrite the existing frame and
    ** set Wal.writeLock to WAL_WRITELOCK_RECKSUM - indicating that 
    ** checksums must be recomputed when the transaction is committed.  */
    if( iFirst && (p->pDirty || isCommit==0) ){
      u32 iWrite = 0;
      VVA_ONLY(rc =) sqlite3WalFindFrame(pWal, p->pgno, &iWrite);
      assert( rc==SQLITE_OK || iWrite==0 );



      if( iWrite>=iFirst ){
        i64 iOff = walFrameOffset(iWrite, szPage) + WAL_FRAME_HDRSIZE;
        void *pData;
        if( pWal->iReCksum==0 || iWrite<pWal->iReCksum ){
          pWal->iReCksum = iWrite;
        }
#if defined(SQLITE_HAS_CODEC)
        if( (pData = sqlite3PagerCodec(p))==0 ) return SQLITE_NOMEM;
#else
        pData = p->pData;
#endif
        rc = sqlite3OsWrite(pWal->pWalFd, pData, szPage, iOff);
        if( rc ) return rc;
        p->flags &= ~PGHDR_WAL_APPEND;
        continue;
      }
    }

    iFrame++;
................................................................................
  ** boundary is crossed.  Only the part of the WAL prior to the last
  ** sector boundary is synced; the part of the last frame that extends
  ** past the sector boundary is written after the sync.
  */
  if( isCommit && WAL_SYNC_FLAGS(sync_flags)!=0 ){
    int bSync = 1;
    if( pWal->padToSectorBoundary ){
      int sectorSize = sqlite3SectorSize(pWal->pWalFd);
      w.iSyncPoint = ((iOffset+sectorSize-1)/sectorSize)*sectorSize;
      bSync = (w.iSyncPoint==iOffset);
      testcase( bSync );
      while( iOffset<w.iSyncPoint ){
        rc = walWriteOneFrame(&w, pLast, nTruncate, iOffset);
        if( rc ) return rc;
        iOffset += szFrame;
................................................................................
  }

  /* Append data to the wal-index. It is not necessary to lock the 
  ** wal-index to do this as the SQLITE_SHM_WRITE lock held on the wal-index
  ** guarantees that there are no other writers, and no data that may
  ** be in use by existing readers is being overwritten.
  */
  iFrame = pWal->hdr.mxFrame;
  for(p=pList; p && rc==SQLITE_OK; p=p->pDirty){
    if( (p->flags & PGHDR_WAL_APPEND)==0 ) continue;
    iFrame++;
    rc = walIndexAppend(pWal, iFrame, p->pgno);
  }
  while( rc==SQLITE_OK && nExtra>0 ){
    iFrame++;
    nExtra--;
    rc = walIndexAppend(pWal, iFrame, pLast->pgno);
  }

  if( rc==SQLITE_OK ){
    /* Update the private copy of the header. */
    pWal->hdr.szPage = (u16)((szPage&0xff00) | (szPage>>16));
    testcase( szPage<=32768 );
    testcase( szPage>=65536 );
    pWal->hdr.mxFrame = iFrame;
    if( isCommit ){
      pWal->hdr.iChange++;
      pWal->hdr.nPage = nTruncate;
    }
    /* If this is a commit, update the wal-index header too. */
    if( isCommit ){
      walIndexWriteHdr(pWal);









      pWal->iCallback = iFrame;

    }
  }

  WALTRACE(("WAL%p: frame write %s\n", pWal, rc ? "failed" : "ok"));
  return rc;
}

................................................................................
    if( isChanged && pWal->pDbFd->pMethods->iVersion>=3 ){
      sqlite3OsUnfetch(pWal->pDbFd, 0, 0);
    }
  }

  /* Copy data from the log to the database file. */
  if( rc==SQLITE_OK ){
    if( pWal->hdr.mxFrame && walPagesize(pWal)!=nBuf ){


      rc = SQLITE_CORRUPT_BKPT;
    }else{
      rc = walCheckpoint(pWal, db, eMode2, xBusy2, pBusyArg, sync_flags, zBuf);
    }

    /* If no error occurred, set the output variables. */
    if( rc==SQLITE_OK || rc==SQLITE_BUSY ){
      if( pnLog ) *pnLog = (int)pWal->hdr.mxFrame;




      if( pnCkpt ) *pnCkpt = (int)(walCkptInfo(pWal)->nBackfill);








    }
  }

  if( isChanged ){
    /* If a new wal-index header was loaded before the checkpoint was 
    ** performed, then the pager-cache associated with pWal is now
    ** out of date. So zero the cached wal-index header to ensure that
................................................................................
** If op is negative, then do a dry-run of the op==1 case but do
** not actually change anything. The pager uses this to see if it
** should acquire the database exclusive lock prior to invoking
** the op==1 case.
*/
int sqlite3WalExclusiveMode(Wal *pWal, int op){
  int rc;

  assert( pWal->writeLock==0 );
  assert( pWal->exclusiveMode!=WAL_HEAPMEMORY_MODE || op==-1 );

  /* pWal->readLock is usually set, but might be -1 if there was a 
  ** prior error while attempting to acquire are read-lock. This cannot 
  ** happen if the connection is actually in exclusive mode (as no xShmLock
  ** locks are taken in this case). Nor should the pager attempt to
  ** upgrade to exclusive-mode following such an error.
  */
  assert( pWal->readLock>=0 || pWal->lockError );
  assert( pWal->readLock>=0 || (op<=0 && pWal->exclusiveMode==0) );

  if( op==0 ){
    if( pWal->exclusiveMode!=WAL_NORMAL_MODE ){
      pWal->exclusiveMode = WAL_NORMAL_MODE;



      if( walLockShared(pWal, WAL_READ_LOCK(pWal->readLock))!=SQLITE_OK ){


        pWal->exclusiveMode = WAL_EXCLUSIVE_MODE;
      }
      rc = pWal->exclusiveMode==WAL_NORMAL_MODE;
    }else{
      /* Already in locking_mode=NORMAL */
      rc = 0;
    }
  }else if( op>0 ){
    assert( pWal->exclusiveMode==WAL_NORMAL_MODE );
    assert( pWal->readLock>=0 );



    walUnlockShared(pWal, WAL_READ_LOCK(pWal->readLock));

    pWal->exclusiveMode = WAL_EXCLUSIVE_MODE;
    rc = 1;
  }else{
    rc = pWal->exclusiveMode==WAL_NORMAL_MODE;
  }
  return rc;
}
................................................................................
  return (pWal ? pWal->szPage : 0);
}
#endif

/* Return the sqlite3_file object for the WAL file
*/
sqlite3_file *sqlite3WalFile(Wal *pWal){
  return pWal->pWalFd;
}

/* 
** Return the values required by sqlite3_wal_info().
*/
int sqlite3WalInfo(Wal *pWal, u32 *pnPrior, u32 *pnFrame){
  int rc = SQLITE_OK;







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249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
...
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443

444
445
446

447
448
449
450
451
452
453
...
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
...
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
...
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
...
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
...
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
....
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
....
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
....
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
....
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
....
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348

1349
1350
1351
1352
1353
1354
1355
....
1362
1363
1364
1365
1366
1367
1368

1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
....
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614


1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
....
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669

1670
1671
1672
1673
1674
1675
1676
1677
1678

1679
1680
1681
1682











1683




1684
1685
1686
1687











1688
1689
1690
1691
1692
1693

1694
1695
1696
1697
1698
1699




1700
1701




1702
1703
1704
1705
1706











1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720












1721






1722
1723
1724
1725
1726
1727
1728









1729
1730
1731
1732
1733

1734

1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747

1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763

1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
....
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
....
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871

1872
1873
1874
1875
1876
1877
1878
....
2074
2075
2076
2077
2078
2079
2080
2081
2082
2083
2084
2085
2086
2087
2088
2089
2090
2091
2092
2093
2094
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113
2114
2115

2116
2117
2118
2119
2120
2121
2122
2123
2124

2125
2126
2127
2128
2129
2130
2131
....
2138
2139
2140
2141
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
2159
2160
2161
2162
2163
2164

2165
2166
2167
2168
2169
2170
2171
2172
2173
2174
2175
2176
2177
2178
2179
2180
2181
2182
2183
2184
2185
2186
2187
....
2235
2236
2237
2238
2239
2240
2241
2242
2243
2244
2245
2246
2247
2248
2249
....
2297
2298
2299
2300
2301
2302
2303
2304
2305
2306
2307
2308
2309
2310
2311
2312
2313
2314
2315
2316
2317
2318
2319
2320
2321
2322
2323
2324
2325
2326
2327
2328
2329
2330
2331
2332
2333
2334
2335
2336
2337
2338
2339
2340
2341
2342
2343
2344
2345
2346
2347
2348
2349
2350
2351
2352
2353
2354
2355
2356
2357
2358
2359
2360
2361
2362
2363
2364
2365
2366
2367
2368
2369
2370
2371
2372
2373
2374
2375
2376
2377
2378

2379
2380
2381
2382
2383

2384
2385
2386
2387
2388
2389
2390
2391
2392

2393
2394
2395
2396
2397
2398
2399
2400
2401
2402
2403
2404
2405
2406
2407
2408
2409
2410
2411
2412
2413
2414
2415
2416
2417
2418
2419
2420
2421
2422
2423
2424
2425
2426
2427
2428
2429
2430
2431
2432
2433
2434
2435
2436

2437
2438
2439
2440
2441
2442
2443
2444
2445
....
2446
2447
2448
2449
2450
2451
2452
2453
2454
2455
2456
2457
2458
2459
2460
....
2471
2472
2473
2474
2475
2476
2477
2478
2479
2480
2481
2482
2483
2484
2485
....
2488
2489
2490
2491
2492
2493
2494
2495
2496
2497
2498
2499
2500
2501
2502
2503
2504
2505
2506
2507
2508
2509
2510
2511
2512
2513
....
2528
2529
2530
2531
2532
2533
2534
2535
2536
2537
2538
2539
2540
2541
2542
2543
2544
2545
2546
2547
2548
2549
2550
2551
2552
2553
2554
2555
2556
2557
2558
2559
2560
2561
2562
2563
2564
2565
2566
2567
2568
2569
2570
2571
2572
2573
2574

2575
2576
2577
2578
2579
2580
2581
2582
2583
2584
2585
....
2752
2753
2754
2755
2756
2757
2758
2759
2760
2761
2762
2763
2764
2765
2766
2767
2768
....
2863
2864
2865
2866
2867
2868
2869
2870
2871
2872
2873
2874
2875
2876
2877
....
2880
2881
2882
2883
2884
2885
2886
2887
2888
2889
2890
2891
2892
2893
2894
....
2915
2916
2917
2918
2919
2920
2921
2922
2923
2924
2925
2926
2927
2928
2929
....
2997
2998
2999
3000
3001
3002
3003



3004

3005
3006
3007
3008
3009
3010
3011
3012
3013
....
3072
3073
3074
3075
3076
3077
3078
3079
3080
3081
3082
3083
3084
3085
3086
3087
3088
3089
3090
3091
3092
3093
3094
3095
3096
3097
3098
3099
3100
3101
3102
3103
3104
3105
3106
3107
3108
3109
3110
3111
3112
3113
3114
3115
3116
3117
3118
3119
3120
3121
3122
3123
3124
3125
3126
3127
3128
3129
3130
3131
3132
3133
3134
3135
3136
3137
3138
3139
3140
3141
3142
3143
3144
3145
3146
3147
3148
3149
3150
3151
3152
3153
3154
3155
3156
3157
3158
3159
3160
3161
3162
3163
3164
3165
3166
3167
3168
3169
3170
3171
3172
3173
3174
3175
3176
3177
3178
3179
3180
3181
3182
3183
3184
3185
3186
3187
3188
3189
3190
3191
3192
3193
3194
3195
3196
3197
3198
3199
3200
3201
3202
3203
3204
3205
3206
3207
3208
3209
3210
3211
3212
3213
3214
3215
3216
3217
3218
3219
3220
3221
3222
3223
3224
3225
3226
3227
3228
3229
3230
3231
3232
3233
....
3275
3276
3277
3278
3279
3280
3281
3282
3283
3284
3285
3286
3287
3288
3289
....
3333
3334
3335
3336
3337
3338
3339
3340
3341
3342
3343
3344
3345
3346
3347
3348
3349
3350
....
3412
3413
3414
3415
3416
3417
3418
3419
3420
3421
3422
3423
3424
3425
3426
3427
3428
3429
3430
3431
3432
3433
3434
3435
3436
3437
3438
3439
3440
3441
3442
3443
3444
3445
3446
3447
3448
3449
3450
3451
3452
3453
3454
3455
3456
3457
3458
3459
3460
3461
3462
3463
3464
3465
3466
3467
3468
3469
3470
3471
3472
3473
3474
3475
3476
3477
3478
3479
3480
3481
3482
3483
3484
3485
3486
3487
3488
3489
3490
3491
3492
3493
3494
3495
3496
3497
3498
3499
3500
3501
3502
3503
3504
3505
3506
3507
3508
3509
3510
3511
3512
3513
3514
3515
3516
3517
3518
....
3519
3520
3521
3522
3523
3524
3525
3526
3527
3528
3529



3530
3531
3532
3533
3534
3535
3536
3537


3538
3539
3540
3541
3542
3543
3544
3545
3546
3547
3548
3549





3550
3551























3552



3553
3554
3555
3556
3557
3558
3559
3560
3561







3562




3563
3564
3565
3566
3567
3568
3569
3570
3571
3572
3573
3574
3575
3576
3577
3578
3579
3580
3581
3582
3583
3584
3585
3586
....
3598
3599
3600
3601
3602
3603
3604
3605
3606
3607
3608
3609
3610
3611
3612
3613
3614
3615
3616
3617
3618
3619
3620
3621
3622
3623
3624
3625
3626
3627
3628
3629
3630
3631
3632
3633
3634
3635
3636
3637
3638
3639
3640
3641
3642
3643
3644
3645
....
3809
3810
3811
3812
3813
3814
3815
3816
3817
3818
3819
3820
3821
3822
3823
....
3907
3908
3909
3910
3911
3912
3913
3914
3915
3916
3917
3918
3919
3920
3921
3922
3923
3924
3925
3926
3927

3928


3929
3930
3931
3932
3933
3934
3935
3936
3937
3938
3939
3940
3941
3942
3943
3944
3945
3946
3947
3948
3949
3950
3951
3952
3953
3954
3955
3956
3957
3958
3959
3960
3961
3962
3963
3964
3965
3966

3967
3968
3969
3970
3971
3972
3973
3974
3975
3976
3977
3978
3979
3980
3981

3982
3983
3984
3985
3986
3987
3988
3989
3990
3991
3992
3993
3994
3995
3996
3997
3998
3999
4000
4001
4002
4003
4004
4005
4006
4007
4008
4009
4010
4011
4012
4013
4014
4015
4016
4017
4018
4019
4020
4021
4022
4023
4024
4025
4026
4027
4028
4029
4030
4031
4032
4033
4034



4035
4036
4037
4038
4039
4040
4041
4042
4043
4044
4045
4046
4047
4048
4049
4050
4051
4052
4053
4054
4055
4056
4057
4058
4059
4060
4061
4062
4063
4064
4065
4066
4067
4068
4069
4070
4071
4072
4073
4074
4075
4076
4077
4078
4079
4080
4081
4082
4083
....
4104
4105
4106
4107
4108
4109
4110
4111
4112
4113
4114
4115
4116
4117
4118
....
4163
4164
4165
4166
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....
4197
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4200
4201
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4203

4204
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4210
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....
4219
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....
4264
4265
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4277
4278
4279
4280







4281
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4283
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4286
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4288
4289
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4291
4292
4293
4294
4295
4296
4297
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4299
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4310
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4321
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4323
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4325
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4329

4330
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4332
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4360
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4364
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....
4373
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4380
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....
4430
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....
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....
4591
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....
4676
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....
4833
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4846
4847
** being considered valid at the same time and being checkpointing together
** following a crash.
**
** READER ALGORITHM
**
** To read a page from the database (call it page number P), a reader
** first checks the WAL to see if it contains page P.  If so, then the
** last valid instance of page P that is followed by a commit frame
** or is a commit frame itself becomes the value read.  If the WAL
** contains no copies of page P that are valid and which are a commit
** frame or are followed by a commit frame, then page P is read from
** the database file.
**
** To start a read transaction, the reader records the index of the last
** valid frame in the WAL.  The reader uses this recorded "mxFrame" value
................................................................................
**
** Note that entries are added in order of increasing K.  Hence, one
** reader might be using some value K0 and a second reader that started
** at a later time (after additional transactions were added to the WAL
** and to the wal-index) might be using a different value K1, where K1>K0.
** Both readers can use the same hash table and mapping section to get
** the correct result.  There may be entries in the hash table with
** K>K0, but to the first reader those entries will appear to be unused
** slots in the hash table and so the first reader will get an answer as
** if no values greater than K0 had ever been inserted into the hash table
** in the first place - which is what reader one wants.  Meanwhile, the
** second reader using K1 will see additional values that were inserted
** later, which is exactly what reader two wants.  
**
** When a rollback occurs, the value of K is decreased. Hash table entries
** that correspond to frames greater than the new K value are removed
** from the hash table at this point.
*/

/*
** WAL2 NOTES
**
** This file also contains the implementation of "wal2" mode - activated
** using "PRAGMA journal_mode = wal2". Wal2 mode is very similar to wal
** mode, except that it uses two wal files instead of one. Under some
** circumstances, wal2 mode provides more concurrency than legacy wal 
** mode.
**
** THE PROBLEM WAL2 SOLVES:
**
** In legacy wal mode, if a writer wishes to write to the database while
** a checkpoint is ongoing, it may append frames to the existing wal file.
** This means that after the checkpoint has finished, the wal file consists
** of a large block of checkpointed frames, followed by a block of
** uncheckpointed frames. In a deployment that features a high volume of
** write traffic, this may mean that the wal file is never completely
** checkpointed. And so grows indefinitely.
**
** An alternative is to use "PRAGMA wal_checkpoint=RESTART" or similar to
** force a complete checkpoint of the wal file. But this must:
**
**   1) Wait on all existing readers to finish,
**   2) Wait on any existing writer, and then block all new writers,
**   3) Do the checkpoint,
**   4) Wait on any new readers that started during steps 2 and 3. Writers
**      are still blocked during this step.
**
** This means that in order to avoid the wal file growing indefinitely 
** in a busy system, writers must periodically pause to allow a checkpoint
** to complete. In a system with long running readers, such pauses may be
** for a non-trivial amount of time.
**
** OVERVIEW OF SOLUTION
**
** Wal2 mode uses two wal files. After writers have grown the first wal 
** file to a pre-configured size, they begin appending transactions to 
** the second wal file. Once all existing readers are reading snapshots
** new enough to include the entire first wal file, a checkpointer can
** checkpoint it.
**
** Meanwhile, writers are writing transactions to the second wal file.
** Once that wal file has grown larger than the pre-configured size, each
** new writer checks if:
**
**    * the first wal file has been checkpointed, and if so, if
**    * there are no readers still reading from the first wal file (once
**      it has been checkpointed, new readers read only from the second
**      wal file).
**
** If both these conditions are true, the writer may switch back to the
** first wal file. Eventually, a checkpointer can checkpoint the second
** wal file, and so on.
**
** The wal file that writers are currently appending to (the one they
** don't have to check the above two criteria before writing to) is called
** the "current" wal file.
**
** The first wal file takes the same name as the wal file in legacy wal
** mode systems - "<db>-wal". The second is named "<db>-wal2".
**
** WAL FILE FORMAT
**
** The file format used for each wal file in wal2 mode is the same as for
** legacy wal mode.  Except, the file format field is set to 3021000 
** instead of 3007000.
**
** WAL-INDEX FORMAT
**
** The wal-index format is also very similar. Even though there are two
** wal files, there is still a single wal-index shared-memory area (*-shm
** file with the default unix or win32 VFS). The wal-index header is the
** same size, with the following exceptions it has the same format:
**
**   * The version field is set to 3021000 instead of 3007000.
**
**   * An unused 32-bit field in the legacy wal-index header is
**     now used to store (a) a single bit indicating which of the
**     two wal files writers should append to and (b) the number
**     of frames in the second wal file (31 bits).
**
** The first hash table in the wal-index contains entries corresponding
** to the first HASHTABLE_NPAGE_ONE frames stored in the first wal file.
** The second hash table in the wal-index contains entries indexing the
** first HASHTABLE_NPAGE frames in the second wal file. The third hash
** table contains the next HASHTABLE_NPAGE frames in the first wal file,
** and so on.
**
** LOCKS
**
** Read-locks are simpler than for legacy wal mode. There are no locking
** slots that contain frame numbers. Instead, there are four distinct
** combinations of read locks a reader may hold:
**
**   WAL_LOCK_PART1:       "part" lock on first wal, none of second.
**   WAL_LOCK_PART1_FULL2: "part" lock on first wal, "full" of second.
**   WAL_LOCK_PART2: no lock on first wal, "part" lock on second.
**   WAL_LOCK_PART2_FULL1: "full" lock on first wal, "part" lock on second.
**
** When a reader reads the wal-index header as part of opening a read
** transaction, it takes a "part" lock on the current wal file. "Part" 
** because the wal file may grow while the read transaction is active, in 
** which case the reader would be reading only part of the wal file. 
** A part lock prevents a checkpointer from checkpointing the wal file 
** on which it is held.
**
** If there is data in the non-current wal file that has not been 
** checkpointed, the reader takes a "full" lock on that wal file. A 
** "full" lock indicates that the reader is using the entire wal file.
** A full lock prevents a writer from overwriting the wal file on which
** it is held, but does not prevent a checkpointer from checkpointing 
** it.
**
** There is still a single WRITER and a single CHECKPOINTER lock. The
** recovery procedure still takes the same exclusive lock on the entire
** range of SQLITE_SHM_NLOCK shm-locks. This works because the read-locks
** above use four of the six read-locking slots used by legacy wal mode.
** See the header comment for function walLockReader() for details.
**
** STARTUP/RECOVERY
**
** The read and write version fields of the database header in a wal2
** database are set to 0x03, instead of 0x02 as in legacy wal mode.
**
** The wal file format used in wal2 mode is the same as the format used
** in legacy wal mode. However, in order to support recovery, there are two
** differences in the way wal file header fields are populated, as follows:
**
**   * When the first wal file is first created, the "nCkpt" field in
**     the wal file header is set to 0. Thereafter, each time the writer
**     switches wal file, it sets the nCkpt field in the new wal file
**     header to ((nCkpt0 + 1) & 0x0F), where nCkpt0 is the value in
**     the previous wal file header. This means that the first wal file
**     always has an even value in the nCkpt field, and the second wal
**     file always has an odd value.
**
**   * When a writer switches wal file, it sets the salt values in the
**     new wal file to a copy of the checksum for the final frame in
**     the previous wal file.
**
** Recovery proceeds as follows:
**
** 1. Each wal file is recovered separately. Except, if the first wal 
**    file does not exist or is zero bytes in size, the second wal file
**    is truncated to zero bytes before it is "recovered".
**
** 2. If both wal files contain valid headers, then the nCkpt fields
**    are compared to see which of the two wal files is older. If the
**    salt keys in the second wal file match the final frame checksum 
**    in the older wal file, then both wal files are used. Otherwise,
**    the newer wal file is ignored.
**
** 3. Or, if only one or neither of the wal files has a valid header, 
**    then only a single or no wal files are recovered into the 
**    reconstructed wal-index.
**
** Refer to header comments for walIndexRecover() for further details.
*/

#ifndef SQLITE_OMIT_WAL

#include "wal.h"

/*
** Trace output macros
*/
................................................................................
# define AtomicStore(PTR,VAL)  __atomic_store_n((PTR),(VAL),__ATOMIC_RELAXED)
#else
# define AtomicLoad(PTR)       (*(PTR))
# define AtomicStore(PTR,VAL)  (*(PTR) = (VAL))
#endif

/*
** Both the wal-file and the wal-index contain version fields 
** indicating the current version of the system. If a client
** reads the header of a wal file (as part of recovery), or the
** wal-index (as part of opening a read transaction) and (a) the
** header checksum is correct but (b) the version field is not
** recognized, the operation fails with SQLITE_CANTOPEN.
**
** Currently, clients support both version-1 ("journal_mode=wal") and
** version-2 ("journal_mode=wal2"). Legacy clients may support version-1
** only.

*/
#define WAL_VERSION1 3007000      /* For "journal_mode=wal" */
#define WAL_VERSION2 3021000      /* For "journal_mode=wal2" */


/*
** Index numbers for various locking bytes.   WAL_NREADER is the number
** of available reader locks and should be at least 3.  The default
** is SQLITE_SHM_NLOCK==8 and  WAL_NREADER==5.
**
** Technically, the various VFSes are free to implement these locks however
................................................................................
#define WAL_WRITE_LOCK         0
#define WAL_ALL_BUT_WRITE      1
#define WAL_CKPT_LOCK          1
#define WAL_RECOVER_LOCK       2
#define WAL_READ_LOCK(I)       (3+(I))
#define WAL_NREADER            (SQLITE_SHM_NLOCK-3)

/*
** Values that may be stored in Wal.readLock in wal2 mode.
**
** In wal mode, the Wal.readLock member is set to -1 when no read-lock
** is held, or else is the index of the read-mark on which a lock is
** held.
**
** In wal2 mode, Wal.readLock must be set to one of the following values.
** A value of -1 still indicates that no read-lock is held, but the other
** values are symbolic. See the implementation of walLockReader() for
** details of how the symbols map to OS level locks.
*/
#define WAL_LOCK_NONE        -1
#define WAL_LOCK_PART1        1
#define WAL_LOCK_PART1_FULL2  2
#define WAL_LOCK_PART2        3
#define WAL_LOCK_PART2_FULL1  4

/* 
** This constant is used in wal2 mode only.
**
** In wal2 mode, when committing a transaction, if the current wal file 
** is sufficiently large and there are no conflicting locks held, the
** writer writes the new transaction into the start of the other wal
** file. Usually, "sufficiently large" is defined by the value configured
** using "PRAGMA journal_size_limit". However, if no such value has been
** configured, sufficiently large defaults to WAL_DEFAULT_WALSIZE frames.
*/
#define WAL_DEFAULT_WALSIZE 1000

/* Object declarations */
typedef struct WalIndexHdr WalIndexHdr;
typedef struct WalIterator WalIterator;
typedef struct WalCkptInfo WalCkptInfo;


................................................................................
** For all versions of SQLite through 3.10.0 and probably beyond,
** the locking bytes (WalCkptInfo.aLock) start at offset 120 and
** the total header size is 136 bytes.
**
** The szPage value can be any power of 2 between 512 and 32768, inclusive.
** Or it can be 1 to represent a 65536-byte page.  The latter case was
** added in 3.7.1 when support for 64K pages was added.  
**
** WAL2 mode notes: Member variable mxFrame2 is only used in wal2 mode
** (when iVersion is set to WAL_VERSION2). The lower 31 bits store
** the maximum frame number in file *-wal2. The most significant bit
** is a flag - set if clients are currently appending to *-wal2, clear
** otherwise.
*/
struct WalIndexHdr {
  u32 iVersion;                   /* Wal-index version */
  u32 mxFrame2;                   /* See "WAL2 mode notes" above */
  u32 iChange;                    /* Counter incremented each transaction */
  u8 isInit;                      /* 1 when initialized */
  u8 bigEndCksum;                 /* True if checksums in WAL are big-endian */
  u16 szPage;                     /* Database page size in bytes. 1==64K */
  u32 mxFrame;                    /* Index of last valid frame in each WAL */
  u32 nPage;                      /* Size of database in pages */
  u32 aFrameCksum[2];             /* Checksum of last frame in log */
  u32 aSalt[2];                   /* Two salt values copied from WAL header */
  u32 aCksum[2];                  /* Checksum over all prior fields */
};

/*
** The following macros and functions are get/set methods for the maximum
** frame numbers and current wal file values stored in the WalIndexHdr
** structure. These are helpful because of the unorthodox way in which
** the values are stored in wal2 mode (see above). They are equivalent
** to functions with the following signatures.
**
**   u32  walidxGetMxFrame(WalIndexHdr*, int iWal);          // get mxFrame
**   void walidxSetMxFrame(WalIndexHdr*, int iWal, u32 val); // set mxFrame
**   int  walidxGetFile(WalIndexHdr*)                        // get file
**   void walidxSetFile(WalIndexHdr*, int val);              // set file
*/
#define walidxGetMxFrame(pHdr, iWal) \
  ((iWal) ? ((pHdr)->mxFrame2 & 0x7FFFFFF) : (pHdr)->mxFrame)

static void walidxSetMxFrame(WalIndexHdr *pHdr, int iWal, u32 mxFrame){
  if( iWal ){
    pHdr->mxFrame2 = (pHdr->mxFrame2 & 0x80000000) | mxFrame;
  }else{
    pHdr->mxFrame = mxFrame;
  }
  assert( walidxGetMxFrame(pHdr, iWal)==mxFrame );
}

#define walidxGetFile(pHdr) ((pHdr)->mxFrame2 >> 31)

#define walidxSetFile(pHdr, iWal) (                                   \
    (pHdr)->mxFrame2 = ((pHdr)->mxFrame2 & 0x7FFFFFFF) | ((iWal)<<31) \
)

/*
** Argument is a pointer to a Wal structure. Return true if the current
** cache of the wal-index header indicates "journal_mode=wal2" mode, or
** false otherwise.
*/
#define isWalMode2(pWal) ((pWal)->hdr.iVersion==WAL_VERSION2)

/*
** A copy of the following object occurs in the wal-index immediately
** following the second copy of the WalIndexHdr.  This object stores
** information used by checkpoint.
**
** nBackfill is the number of frames in the WAL that have been written
** back into the database. (We call the act of moving content from WAL to
................................................................................
/*
** An open write-ahead log file is represented by an instance of the
** following object.
*/
struct Wal {
  sqlite3_vfs *pVfs;         /* The VFS used to create pDbFd */
  sqlite3_file *pDbFd;       /* File handle for the database file */
  sqlite3_file *apWalFd[2];  /* File handle for "*-wal" and "*-wal2" */
  u32 iCallback;             /* Value to pass to log callback (or 0) */
  i64 mxWalSize;             /* Truncate WAL to this size upon reset */
  int nWiData;               /* Size of array apWiData */
  int szFirstBlock;          /* Size of first block written to WAL file */
  volatile u32 **apWiData;   /* Pointer to wal-index content in memory */
  u32 szPage;                /* Database page size */
  i16 readLock;              /* Which read lock is being held.  -1 for none */
................................................................................
  u8 padToSectorBoundary;    /* Pad transactions out to the next sector */
  u8 bShmUnreliable;         /* SHM content is read-only and unreliable */
  WalIndexHdr hdr;           /* Wal-index header for current transaction */
  u32 minFrame;              /* Ignore wal frames before this one */
  u32 iReCksum;              /* On commit, recalculate checksums from here */
  u32 nPriorFrame;           /* For sqlite3WalInfo() */
  const char *zWalName;      /* Name of WAL file */
  char *zWalName2;           /* Name of second WAL file */
  u32 nCkpt;                 /* Checkpoint sequence counter in the wal-header */
#ifdef SQLITE_DEBUG
  u8 lockError;              /* True if a locking error has occurred */
#endif
#ifdef SQLITE_ENABLE_SNAPSHOT
  WalIndexHdr *pSnapshot;    /* Start transaction here if not NULL */
#endif
  int bWal2;                 /* bWal2 flag passed to WalOpen() */
};

/*
** Candidate values for Wal.exclusiveMode.
*/
#define WAL_NORMAL_MODE     0
#define WAL_EXCLUSIVE_MODE  1     
................................................................................
*/
static void walIndexWriteHdr(Wal *pWal){
  volatile WalIndexHdr *aHdr = walIndexHdr(pWal);
  const int nCksum = offsetof(WalIndexHdr, aCksum);

  assert( pWal->writeLock );
  pWal->hdr.isInit = 1;
  assert( pWal->hdr.iVersion==WAL_VERSION1||pWal->hdr.iVersion==WAL_VERSION2 );
  walChecksumBytes(1, (u8*)&pWal->hdr, nCksum, 0, pWal->hdr.aCksum);
  memcpy((void*)&aHdr[1], (const void*)&pWal->hdr, sizeof(WalIndexHdr));
  walShmBarrier(pWal);
  memcpy((void*)&aHdr[0], (const void*)&pWal->hdr, sizeof(WalIndexHdr));
}

/*
................................................................................
  */
  pgno = sqlite3Get4byte(&aFrame[0]);
  if( pgno==0 ){
    return 0;
  }

  /* A frame is only valid if a checksum of the WAL header,
  ** all prior frames, the first 16 bytes of this frame-header, 
  ** and the frame-data matches the checksum in the last 8 
  ** bytes of this frame-header.
  */
  nativeCksum = (pWal->hdr.bigEndCksum==SQLITE_BIGENDIAN);
  walChecksumBytes(nativeCksum, aFrame, 8, aCksum, aCksum);
  walChecksumBytes(nativeCksum, aData, pWal->szPage, aCksum, aCksum);
  if( aCksum[0]!=sqlite3Get4byte(&aFrame[16]) 
................................................................................
static void walUnlockExclusive(Wal *pWal, int lockIdx, int n){
  if( pWal->exclusiveMode ) return;
  (void)sqlite3OsShmLock(pWal->pDbFd, lockIdx, n,
                         SQLITE_SHM_UNLOCK | SQLITE_SHM_EXCLUSIVE);
  WALTRACE(("WAL%p: release EXCLUSIVE-%s cnt=%d\n", pWal,
             walLockName(lockIdx), n));
}

/*
** This function is used to take and release read-locks in wal2 mode.
**
** Use of WAL_READ_LOCK(x) slots for (1<=x<=4).
**
** 1) Partial read of *-wal-1   (blocks checkpointer from checkpointing)
** 2) Full read of *-wal-2      (blocks writer from writing)
** 3) Partial read of *-wal-2   (blocks checkpointer from checkpointing)
** 4) Full read of *-wal-1      (blocks writer from writing)
*/
static int walLockReader(Wal *pWal, int eLock, int bLock){
  int i;                          /* Index of first readmark to lock */
  int n;                          /* Number of readmarks to lock */

  assert( pWal->hdr.iVersion==WAL_VERSION2 );
  if( pWal->exclusiveMode ) return SQLITE_OK;

  switch( eLock ){
    case WAL_LOCK_PART1      : i = 1; n = 1; break; 
    case WAL_LOCK_PART1_FULL2: i = 1; n = 2; break; 
    case WAL_LOCK_PART2      : i = 3; n = 1; break; 
    case WAL_LOCK_PART2_FULL1: i = 3; n = 2; break; 
    default: assert( !"cannot happen" );
  }

  return sqlite3OsShmLock(pWal->pDbFd, WAL_READ_LOCK(i), n,
      SQLITE_SHM_SHARED | (bLock ? SQLITE_SHM_LOCK : SQLITE_SHM_UNLOCK) 
  );
}

/*
** Compute a hash on a page number.  The resulting hash value must land
** between 0 and (HASHTABLE_NSLOT-1).  The walHashNext() function advances
** the hash to the next value in the event of a collision.
*/
static int walHash(u32 iPage){
................................................................................
    }else{
      pLoc->iZero = HASHTABLE_NPAGE_ONE + (iHash-1)*HASHTABLE_NPAGE;
    }
    pLoc->aPgno = &pLoc->aPgno[-1];
  }
  return rc;
}

static u32 walExternalEncode(int iWal, u32 iFrame){
  u32 iRet;
  if( iWal ){
    iRet = HASHTABLE_NPAGE_ONE + iFrame;
    iRet += ((iFrame-1) / HASHTABLE_NPAGE) * HASHTABLE_NPAGE;
  }else{
    iRet = iFrame;
    iFrame += HASHTABLE_NPAGE - HASHTABLE_NPAGE_ONE;
    iRet += ((iFrame-1) / HASHTABLE_NPAGE) * HASHTABLE_NPAGE;
  }
  return iRet;
}

/*
** Parameter iExternal is an external frame identifier. This function
** transforms it to a wal file number (0 or 1) and frame number within
** this wal file (reported via output parameter *piRead).
*/
static int walExternalDecode(u32 iExternal, u32 *piRead){
  int iHash = (iExternal+HASHTABLE_NPAGE-HASHTABLE_NPAGE_ONE-1)/HASHTABLE_NPAGE;

  if( 0==(iHash & 0x01) ){
    /* A frame in wal file 0 */
    *piRead = (iExternal <= HASHTABLE_NPAGE_ONE) ? iExternal :
      iExternal - (iHash/2) * HASHTABLE_NPAGE;
    return 0;
  }
  if( iHash==0 ){
    *piRead = iExternal;
    return 0;
  }else{
    *piRead = iExternal - HASHTABLE_NPAGE_ONE - ((iHash-1)/2) * HASHTABLE_NPAGE;
  }

  return (iHash % 2);
}

/*
** Return the number of the wal-index page that contains the hash-table
** and page-number array that contain entries corresponding to WAL frame
** iFrame. The wal-index is broken up into 32KB pages. Wal-index pages 
** are numbered starting from 0.
*/
................................................................................
       && (iHash>=1 || iFrame<=HASHTABLE_NPAGE_ONE)
       && (iHash<=1 || iFrame>(HASHTABLE_NPAGE_ONE+HASHTABLE_NPAGE))
       && (iHash>=2 || iFrame<=HASHTABLE_NPAGE_ONE+HASHTABLE_NPAGE)
       && (iHash<=2 || iFrame>(HASHTABLE_NPAGE_ONE+2*HASHTABLE_NPAGE))
  );
  return iHash;
}

/*
** Return the index of the hash-table corresponding to frame iFrame of wal
** file iWal.
*/
static int walFramePage2(int iWal, u32 iFrame){
  int iRet;
  assert( iWal==0 || iWal==1 );
  assert( iFrame>0 );
  if( iWal==0 ){
    iRet = 2*((iFrame+HASHTABLE_NPAGE-HASHTABLE_NPAGE_ONE-1)/HASHTABLE_NPAGE);
  }else{
    iRet = 1 + 2 * ((iFrame-1) / HASHTABLE_NPAGE);
  }
  return iRet;
}

/*
** Return the page number associated with frame iFrame in this WAL.
*/
static u32 walFramePgno(Wal *pWal, u32 iFrame){
  int iHash = walFramePage(iFrame);
  if( iHash==0 ){
    return pWal->apWiData[0][WALINDEX_HDR_SIZE/sizeof(u32) + iFrame - 1];
  }
  return pWal->apWiData[iHash][(iFrame-1-HASHTABLE_NPAGE_ONE)%HASHTABLE_NPAGE];
}

static u32 walFramePgno2(Wal *pWal, int iWal, u32 iFrame){
  return walFramePgno(pWal, walExternalEncode(iWal, iFrame));
}

/*
** Remove entries from the hash table that point to WAL slots greater
** than pWal->hdr.mxFrame.
**
** This function is called whenever pWal->hdr.mxFrame is decreased due
** to a rollback or savepoint.
................................................................................
** actually needed.
*/
static void walCleanupHash(Wal *pWal){
  WalHashLoc sLoc;                /* Hash table location */
  int iLimit = 0;                 /* Zero values greater than this */
  int nByte;                      /* Number of bytes to zero in aPgno[] */
  int i;                          /* Used to iterate through aHash[] */
  int iWal = walidxGetFile(&pWal->hdr);
  u32 mxFrame = walidxGetMxFrame(&pWal->hdr, iWal);

  u32 iExternal;
  if( isWalMode2(pWal) ){
    iExternal = walExternalEncode(iWal, mxFrame);
  }else{
    assert( iWal==0 );
    iExternal = mxFrame;
  }

  assert( pWal->writeLock );
  testcase( mxFrame==HASHTABLE_NPAGE_ONE-1 );
  testcase( mxFrame==HASHTABLE_NPAGE_ONE );
  testcase( mxFrame==HASHTABLE_NPAGE_ONE+1 );

  if( mxFrame==0 ) return;

  /* Obtain pointers to the hash-table and page-number array containing 
  ** the entry that corresponds to frame pWal->hdr.mxFrame. It is guaranteed
  ** that the page said hash-table and array reside on is already mapped.
  */
  assert( pWal->nWiData>walFramePage(iExternal) );
  assert( pWal->apWiData[walFramePage(iExternal)] );
  walHashGet(pWal, walFramePage(iExternal), &sLoc);

  /* Zero all hash-table entries that correspond to frame numbers greater
  ** than pWal->hdr.mxFrame.
  */
  iLimit = iExternal - sLoc.iZero;
  assert( iLimit>0 );
  for(i=0; i<HASHTABLE_NSLOT; i++){
    if( sLoc.aHash[i]>iLimit ){
      sLoc.aHash[i] = 0;
    }
  }
  
  /* Zero the entries in the aPgno array that correspond to frames with
  ** frame numbers greater than pWal->hdr.mxFrame.  */

  nByte = (int)((char *)sLoc.aHash - (char *)&sLoc.aPgno[iLimit+1]);
  memset((void *)&sLoc.aPgno[iLimit+1], 0, nByte);

#ifdef SQLITE_ENABLE_EXPENSIVE_ASSERT
  /* Verify that the every entry in the mapping region is still reachable
  ** via the hash table even after the cleanup.
  */
................................................................................
      }
      assert( sLoc.aHash[iKey]==j );
    }
  }
#endif /* SQLITE_ENABLE_EXPENSIVE_ASSERT */
}


/*
** Set an entry in the wal-index that will map database page number
** pPage into WAL frame iFrame.
*/
static int walIndexAppend(Wal *pWal, int iWal, u32 iFrame, u32 iPage){
  int rc;                         /* Return code */
  WalHashLoc sLoc;                /* Wal-index hash table location */
  u32 iExternal;
  
  if( isWalMode2(pWal) ){
    iExternal = walExternalEncode(iWal, iFrame);
  }else{
    assert( iWal==0 );
    iExternal = iFrame;
  }

  rc = walHashGet(pWal, walFramePage(iExternal), &sLoc);

  /* Assuming the wal-index file was successfully mapped, populate the
  ** page number array and hash table entry.
  */
  if( rc==SQLITE_OK ){
    int iKey;                     /* Hash table key */
    int idx;                      /* Value to write to hash-table slot */
    int nCollide;                 /* Number of hash collisions */

    idx = iExternal - sLoc.iZero;
    assert( idx <= HASHTABLE_NSLOT/2 + 1 );
    
    /* If this is the first entry to be added to this hash-table, zero the
    ** entire hash table and aPgno[] array before proceeding. 
    */
    if( idx==1 ){
      int nByte = (int)((u8 *)&sLoc.aHash[HASHTABLE_NSLOT]
................................................................................
#endif /* SQLITE_ENABLE_EXPENSIVE_ASSERT */
  }


  return rc;
}

/*
** Recover a single wal file - *-wal if iWal==0, or *-wal2 if iWal==1.
*/
static int walIndexRecoverOne(Wal *pWal, int iWal, u32 *pnCkpt, int *pbZero){
  i64 nSize;                      /* Size of log file */
  u32 aFrameCksum[2] = {0, 0};
  int rc;
  sqlite3_file *pWalFd = pWal->apWalFd[iWal];

  assert( iWal==0 || iWal==1 );

  memset(&pWal->hdr, 0, sizeof(WalIndexHdr));
  sqlite3_randomness(8, pWal->hdr.aSalt);

  rc = sqlite3OsFileSize(pWalFd, &nSize);
  if( rc==SQLITE_OK ){
    if( nSize>WAL_HDRSIZE ){
      u8 aBuf[WAL_HDRSIZE];         /* Buffer to load WAL header into */
      u8 *aFrame = 0;               /* Malloc'd buffer to load entire frame */
      int szFrame;                  /* Number of bytes in buffer aFrame[] */
      u8 *aData;                    /* Pointer to data part of aFrame buffer */
      int iFrame;                   /* Index of last frame read */
      i64 iOffset;                  /* Next offset to read from log file */
      int szPage;                   /* Page size according to the log */
      u32 magic;                    /* Magic value read from WAL header */
      u32 version;                  /* Magic value read from WAL header */
      int isValid;                  /* True if this frame is valid */
  
      /* Read in the WAL header. */
      rc = sqlite3OsRead(pWalFd, aBuf, WAL_HDRSIZE, 0);
      if( rc!=SQLITE_OK ){
        return rc;
      }
  
      /* If the database page size is not a power of two, or is greater than
      ** SQLITE_MAX_PAGE_SIZE, conclude that the WAL file contains no valid 
      ** data. Similarly, if the 'magic' value is invalid, ignore the whole
      ** WAL file.
      */
      magic = sqlite3Get4byte(&aBuf[0]);
      szPage = sqlite3Get4byte(&aBuf[8]);
      if( (magic&0xFFFFFFFE)!=WAL_MAGIC 
       || szPage&(szPage-1) 
       || szPage>SQLITE_MAX_PAGE_SIZE 
       || szPage<512 
      ){
        return SQLITE_OK;
      }
      pWal->hdr.bigEndCksum = (u8)(magic&0x00000001);
      pWal->szPage = szPage;
  
      /* Verify that the WAL header checksum is correct */
      walChecksumBytes(pWal->hdr.bigEndCksum==SQLITE_BIGENDIAN, 
          aBuf, WAL_HDRSIZE-2*4, 0, pWal->hdr.aFrameCksum
      );
      if( pWal->hdr.aFrameCksum[0]!=sqlite3Get4byte(&aBuf[24])
       || pWal->hdr.aFrameCksum[1]!=sqlite3Get4byte(&aBuf[28])
      ){
        return SQLITE_OK;
      }
  
      memcpy(&pWal->hdr.aSalt, &aBuf[16], 8);
      *pnCkpt = sqlite3Get4byte(&aBuf[12]);
  
      /* Verify that the version number on the WAL format is one that
      ** are able to understand */
      version = sqlite3Get4byte(&aBuf[4]);
      if( version!=WAL_VERSION1 && version!=WAL_VERSION2 ){
        return SQLITE_CANTOPEN_BKPT;
      }
      pWal->hdr.iVersion = version;
  
      /* Malloc a buffer to read frames into. */
      szFrame = szPage + WAL_FRAME_HDRSIZE;
      aFrame = (u8 *)sqlite3_malloc64(szFrame);
      if( !aFrame ){
        return SQLITE_NOMEM_BKPT;
      }
      aData = &aFrame[WAL_FRAME_HDRSIZE];
  
      /* Read all frames from the log file. */
      iFrame = 0;
      for(iOffset=WAL_HDRSIZE; (iOffset+szFrame)<=nSize; iOffset+=szFrame){
        u32 pgno;                   /* Database page number for frame */
        u32 nTruncate;              /* dbsize field from frame header */
  
        /* Read and decode the next log frame. */
        iFrame++;
        rc = sqlite3OsRead(pWalFd, aFrame, szFrame, iOffset);
        if( rc!=SQLITE_OK ) break;
        isValid = walDecodeFrame(pWal, &pgno, &nTruncate, aData, aFrame);
        if( !isValid ) break;
        rc = walIndexAppend(pWal, iWal, iFrame, pgno);
        if( rc!=SQLITE_OK ) break;
  
        /* If nTruncate is non-zero, this is a commit record. */
        if( nTruncate ){
          pWal->hdr.mxFrame = iFrame;
          pWal->hdr.nPage = nTruncate;
          pWal->hdr.szPage = (u16)((szPage&0xff00) | (szPage>>16));
          testcase( szPage<=32768 );
          testcase( szPage>=65536 );
          aFrameCksum[0] = pWal->hdr.aFrameCksum[0];
          aFrameCksum[1] = pWal->hdr.aFrameCksum[1];
        }
      }
  
      sqlite3_free(aFrame);
    }else if( pbZero && nSize==0 ){
      *pbZero = 1;
    }
  }

  pWal->hdr.aFrameCksum[0] = aFrameCksum[0];
  pWal->hdr.aFrameCksum[1] = aFrameCksum[1];

  return rc;
}

static int walOpenWal2(Wal *pWal){
  int rc = SQLITE_OK;
  if( !isOpen(pWal->apWalFd[1]) ){
    int f = (SQLITE_OPEN_READWRITE|SQLITE_OPEN_CREATE|SQLITE_OPEN_WAL);
    rc = sqlite3OsOpen(pWal->pVfs, pWal->zWalName2, pWal->apWalFd[1], f, &f);
  }
  return rc;
}

static int walTruncateWal2(Wal *pWal){
  int bIs;
  int rc;
  assert( !isOpen(pWal->apWalFd[1]) );
  rc = sqlite3OsAccess(pWal->pVfs, pWal->zWalName2, SQLITE_ACCESS_EXISTS, &bIs);
  if( rc==SQLITE_OK && bIs ){
    rc = walOpenWal2(pWal);
    if( rc==SQLITE_OK ){
      rc = sqlite3OsTruncate(pWal->apWalFd[1], 0);
      sqlite3OsClose(pWal->apWalFd[1]);
    }
  }
  return rc;
}

/*
** Recover the wal-index by reading the write-ahead log file. 
**
** This routine first tries to establish an exclusive lock on the
** wal-index to prevent other threads/processes from doing anything
** with the WAL or wal-index while recovery is running.  The
** WAL_RECOVER_LOCK is also held so that other threads will know
** that this thread is running recovery.  If unable to establish
** the necessary locks, this routine returns SQLITE_BUSY.
*/
static int walIndexRecover(Wal *pWal){
  int rc;                         /* Return Code */


  int iLock;                      /* Lock offset to lock for checkpoint */
  u32 nCkpt1 = 0xFFFFFFFF;
  u32 nCkpt2 = 0xFFFFFFFF;
  int bZero = 0;
  WalIndexHdr hdr;

  /* Obtain an exclusive lock on all byte in the locking range not already
  ** locked by the caller. The caller is guaranteed to have locked the
  ** WAL_WRITE_LOCK byte, and may have also locked the WAL_CKPT_LOCK byte.
  ** If successful, the same bytes that are locked here are concurrent before
  ** this function returns.
  */
................................................................................
  }
  if( rc ){
    return rc;
  }

  WALTRACE(("WAL%p: recovery begin...\n", pWal));

  /* Recover the *-wal file. If a valid version-1 header is recovered
  ** from it, do not open the *-wal2 file. Even if it exists.
  **
  ** Otherwise, if the *-wal2 file exists or if the "wal2" flag was 
  ** specified when sqlite3WalOpen() was called, open and recover
  ** the *-wal2 file. Except, if the *-wal file was zero bytes in size,
  ** truncate the *-wal2 to zero bytes in size.
  **
  ** After this block has run, if the *-wal2 file is open the system
  ** starts up in VERSION2 mode. In this case pWal->hdr contains the 
  ** wal-index header considering only *-wal2. Stack variable hdr
  ** contains the wal-index header considering only *-wal. The hash 
  ** tables are populated for both.  
  **
  ** Or, if the *-wal2 file is not open, start up in VERSION1 mode.
  ** pWal->hdr is already populated.
  */
  rc = walIndexRecoverOne(pWal, 0, &nCkpt1, &bZero);
  assert( pWal->hdr.iVersion==0 
      || pWal->hdr.iVersion==WAL_VERSION1 
      || pWal->hdr.iVersion==WAL_VERSION2 
  );
  if( rc==SQLITE_OK && bZero ){
    rc = walTruncateWal2(pWal);
  }

  if( rc==SQLITE_OK && pWal->hdr.iVersion!=WAL_VERSION1 ){
    int bOpen = 1;
    sqlite3_vfs *pVfs = pWal->pVfs;
    if( pWal->hdr.iVersion==0 && pWal->bWal2==0 ){
      rc = sqlite3OsAccess(pVfs, pWal->zWalName2, SQLITE_ACCESS_EXISTS, &bOpen);
    }
    if( rc==SQLITE_OK && bOpen ){
      rc = walOpenWal2(pWal);
      if( rc==SQLITE_OK ){

        hdr = pWal->hdr;
        rc = walIndexRecoverOne(pWal, 1, &nCkpt2, 0);
      }
    }











  }





  if( rc==SQLITE_OK ){
    volatile WalCkptInfo *pInfo;












    if( isOpen(pWal->apWalFd[1]) ){
      /* The case where *-wal2 may follow *-wal */
      if( nCkpt2<=0x0F && nCkpt2==nCkpt1+1 ){
        if( sqlite3Get4byte((u8*)(&pWal->hdr.aSalt[0]))==hdr.aFrameCksum[0]
            && sqlite3Get4byte((u8*)(&pWal->hdr.aSalt[1]))==hdr.aFrameCksum[1]
          ){

          walidxSetFile(&pWal->hdr, 1);
          walidxSetMxFrame(&pWal->hdr, 1, pWal->hdr.mxFrame);
          walidxSetMxFrame(&pWal->hdr, 0, hdr.mxFrame);
        }else{
          pWal->hdr = hdr;
        }




      }else





      /* When *-wal may follow *-wal2 */
      if( (nCkpt2==0x0F && nCkpt1==0) || (nCkpt2<0x0F && nCkpt2==nCkpt1-1) ){
        if( sqlite3Get4byte((u8*)(&hdr.aSalt[0]))==pWal->hdr.aFrameCksum[0]
         && sqlite3Get4byte((u8*)(&hdr.aSalt[1]))==pWal->hdr.aFrameCksum[1]
        ){











          SWAP(WalIndexHdr, pWal->hdr, hdr);
          walidxSetMxFrame(&pWal->hdr, 1, hdr.mxFrame);
        }else{
          walidxSetFile(&pWal->hdr, 1);
          walidxSetMxFrame(&pWal->hdr, 1, pWal->hdr.mxFrame);
          walidxSetMxFrame(&pWal->hdr, 0, 0);
        }
      }else

      /* Fallback */
      if( nCkpt1<=nCkpt2 ){
        pWal->hdr = hdr;
      }else{
        walidxSetFile(&pWal->hdr, 1);












        walidxSetMxFrame(&pWal->hdr, 1, pWal->hdr.mxFrame);






        walidxSetMxFrame(&pWal->hdr, 0, 0);
      }
      pWal->hdr.iVersion = WAL_VERSION2;
    }else{
      pWal->hdr.iVersion = WAL_VERSION1;
    }










    walIndexWriteHdr(pWal);

    /* Reset the checkpoint-header. This is safe because this thread is 
    ** currently holding locks that exclude all other readers, writers and
    ** checkpointers.  */

    pInfo = walCkptInfo(pWal);

    memset((void*)pInfo, 0, sizeof(WalCkptInfo));
    if( 0==isWalMode2(pWal) ){
      int i;
      pInfo->nBackfillAttempted = pWal->hdr.mxFrame;
      pInfo->aReadMark[0] = 0;
      for(i=1; i<WAL_NREADER; i++) pInfo->aReadMark[i] = READMARK_NOT_USED;
      if( pWal->hdr.mxFrame ) pInfo->aReadMark[1] = pWal->hdr.mxFrame;
    }

    /* If more than one frame was recovered from the log file, report an
    ** event via sqlite3_log(). This is to help with identifying performance
    ** problems caused by applications routinely shutting down without
    ** checkpointing the log file.  */

    if( pWal->hdr.nPage ){
      if( isWalMode2(pWal) ){
        sqlite3_log(SQLITE_NOTICE_RECOVER_WAL,
            "recovered (%d,%d) frames from WAL files %s[2] (%s mode)",
            walidxGetMxFrame(&pWal->hdr, 0), walidxGetMxFrame(&pWal->hdr, 1), 
            pWal->zWalName, isWalMode2(pWal) ? "wal2" : "wal"
        );
      }else{
        sqlite3_log(SQLITE_NOTICE_RECOVER_WAL,
            "recovered %d frames from WAL file %s",
            pWal->hdr.mxFrame, pWal->zWalName
        );
      }
    }
  }


  WALTRACE(("WAL%p: recovery %s\n", pWal, rc ? "failed" : "ok"));
  walUnlockExclusive(pWal, iLock, WAL_READ_LOCK(0)-iLock);
  walUnlockExclusive(pWal, WAL_READ_LOCK(1), WAL_NREADER-1);
  return rc;
}

/*
** Close an open wal-index and wal files.
*/
static void walIndexClose(Wal *pWal, int isDelete){
  if( pWal->exclusiveMode==WAL_HEAPMEMORY_MODE || pWal->bShmUnreliable ){
    int i;
    for(i=0; i<pWal->nWiData; i++){
      sqlite3_free((void *)pWal->apWiData[i]);
      pWal->apWiData[i] = 0;
    }
  }
  if( pWal->exclusiveMode!=WAL_HEAPMEMORY_MODE ){
    sqlite3OsShmUnmap(pWal->pDbFd, isDelete);
  }
  sqlite3OsClose(pWal->apWalFd[0]);
  sqlite3OsClose(pWal->apWalFd[1]);
}

/* 
** Open a connection to the WAL file zWalName. The database file must 
** already be opened on connection pDbFd. The buffer that zWalName points
** to must remain valid for the lifetime of the returned Wal* handle.
**
................................................................................
*/
int sqlite3WalOpen(
  sqlite3_vfs *pVfs,              /* vfs module to open wal and wal-index */
  sqlite3_file *pDbFd,            /* The open database file */
  const char *zWalName,           /* Name of the WAL file */
  int bNoShm,                     /* True to run in heap-memory mode */
  i64 mxWalSize,                  /* Truncate WAL to this size on reset */
  int bWal2,                      /* True to open in wal2 mode */
  Wal **ppWal                     /* OUT: Allocated Wal handle */
){
  int rc;                         /* Return Code */
  Wal *pRet;                      /* Object to allocate and return */
  int flags;                      /* Flags passed to OsOpen() */
  int nWalName;                   /* Length of zWalName in bytes */
  int nByte;                      /* Bytes of space to allocate */

  assert( zWalName && zWalName[0] );
  assert( pDbFd );

  /* In the amalgamation, the os_unix.c and os_win.c source files come before
  ** this source file.  Verify that the #defines of the locking byte offsets
  ** in os_unix.c and os_win.c agree with the WALINDEX_LOCK_OFFSET value.
................................................................................
#ifdef WIN_SHM_BASE
  assert( WIN_SHM_BASE==WALINDEX_LOCK_OFFSET );
#endif
#ifdef UNIX_SHM_BASE
  assert( UNIX_SHM_BASE==WALINDEX_LOCK_OFFSET );
#endif

  nWalName = sqlite3Strlen30(zWalName);
  nByte = sizeof(Wal) + pVfs->szOsFile*2 + nWalName+2;

  /* Allocate an instance of struct Wal to return. */
  *ppWal = 0;
  pRet = (Wal*)sqlite3MallocZero(nByte);
  if( !pRet ){
    return SQLITE_NOMEM_BKPT;
  }

  pRet->pVfs = pVfs;
  pRet->apWalFd[0] = (sqlite3_file*)((char*)pRet+sizeof(Wal));
  pRet->apWalFd[1] = (sqlite3_file*)((char*)pRet+sizeof(Wal)+pVfs->szOsFile);
  pRet->pDbFd = pDbFd;
  pRet->readLock = WAL_LOCK_NONE;
  pRet->mxWalSize = mxWalSize;
  pRet->zWalName = zWalName;
  pRet->syncHeader = 1;
  pRet->padToSectorBoundary = 1;
  pRet->exclusiveMode = (bNoShm ? WAL_HEAPMEMORY_MODE: WAL_NORMAL_MODE);
  pRet->bWal2 = bWal2;

  pRet->zWalName2 = (char*)pRet + sizeof(Wal) + 2*pVfs->szOsFile;
  memcpy(pRet->zWalName2, zWalName, nWalName);
  pRet->zWalName2[nWalName] = '2';
  pRet->zWalName2[nWalName+1] = '\0';

  /* Open a file handle on the first write-ahead log file. */
  flags = (SQLITE_OPEN_READWRITE|SQLITE_OPEN_CREATE|SQLITE_OPEN_WAL);
  rc = sqlite3OsOpen(pVfs, zWalName, pRet->apWalFd[0], flags, &flags);
  if( rc==SQLITE_OK && flags&SQLITE_OPEN_READONLY ){
    pRet->readOnly = WAL_RDONLY;
  }

  if( rc!=SQLITE_OK ){
    walIndexClose(pRet, 0);

    sqlite3_free(pRet);
  }else{
    int iDC = sqlite3OsDeviceCharacteristics(pDbFd);
    if( iDC & SQLITE_IOCAP_SEQUENTIAL ){ pRet->syncHeader = 0; }
    if( iDC & SQLITE_IOCAP_POWERSAFE_OVERWRITE ){
      pRet->padToSectorBoundary = 0;
    }
................................................................................
*/
static void walIteratorFree(WalIterator *p){
  sqlite3_free(p);
}

/*
** Construct a WalInterator object that can be used to loop over all 
** pages in wal file iWal following frame nBackfill in ascending order. Frames
** nBackfill or earlier may be included - excluding them is an optimization
** only. The caller must hold the checkpoint lock.
**
** On success, make *pp point to the newly allocated WalIterator object
** and return SQLITE_OK. Otherwise, return an error code. If this routine
** returns an error, the final value of *pp is undefined.
**
** The calling routine should invoke walIteratorFree() to destroy the
** WalIterator object when it has finished with it.
*/
static int walIteratorInit(
  Wal *pWal, 
  int iWal, 
  u32 nBackfill, 
  WalIterator **pp
){
  WalIterator *p;                 /* Return value */
  int nSegment;                   /* Number of segments to merge */
  u32 iLast;                      /* Last frame in log */
  int nByte;                      /* Number of bytes to allocate */
  int i;                          /* Iterator variable */
  int iLastSeg;                   /* Last hash table to iterate though */
  ht_slot *aTmp;                  /* Temp space used by merge-sort */
  int rc = SQLITE_OK;             /* Return Code */
  int iMode = isWalMode2(pWal) ? 2 : 1;

  assert( isWalMode2(pWal) || iWal==0 );
  assert( 0==isWalMode2(pWal) || nBackfill==0 );

  /* This routine only runs while holding the checkpoint lock. And
  ** it only runs if there is actually content in the log (mxFrame>0).
  */
  iLast = walidxGetMxFrame(&pWal->hdr, iWal);
  assert( pWal->ckptLock && iLast>0 );


  if( iMode==2 ){
    iLastSeg = walFramePage2(iWal, iLast);
  }else{
    iLastSeg = walFramePage(iLast);
  }
  nSegment = 1 + (iLastSeg/iMode);

  /* Allocate space for the WalIterator object. */

  nByte = sizeof(WalIterator) 
        + (nSegment-1)*sizeof(struct WalSegment)
        + iLast*sizeof(ht_slot);
  p = (WalIterator *)sqlite3_malloc64(nByte);
  if( !p ){
    return SQLITE_NOMEM_BKPT;
  }
................................................................................
  aTmp = (ht_slot *)sqlite3_malloc64(
      sizeof(ht_slot) * (iLast>HASHTABLE_NPAGE?HASHTABLE_NPAGE:iLast)
  );
  if( !aTmp ){
    rc = SQLITE_NOMEM_BKPT;
  }

  i = iMode==2 ? iWal : walFramePage(nBackfill+1);
  for(; rc==SQLITE_OK && i<=iLastSeg; i+=iMode){
    WalHashLoc sLoc;

    rc = walHashGet(pWal, i, &sLoc);
    if( rc==SQLITE_OK ){
      int j;                      /* Counter variable */
      int nEntry;                 /* Number of entries in this segment */
      ht_slot *aIndex;            /* Sorted index for this segment */
      u32 iZero;

      if( iMode==2 ){
        walExternalDecode(sLoc.iZero+1, &iZero);
        iZero--;
        assert( iZero==0 || i>=2 );
      }else{
        iZero = sLoc.iZero;
      }

      sLoc.aPgno++;

      if( i==iLastSeg ){
        nEntry = (int)(iLast - iZero);
      }else{
        nEntry = (int)((u32*)sLoc.aHash - (u32*)sLoc.aPgno);
      }
      aIndex = &((ht_slot *)&p->aSegment[p->nSegment])[iZero];
      iZero++;
  
      for(j=0; j<nEntry; j++){
        aIndex[j] = (ht_slot)j;
      }
      walMergesort((u32*)sLoc.aPgno, aTmp, aIndex, &nEntry);
      p->aSegment[i/iMode].iZero = iZero;
      p->aSegment[i/iMode].nEntry = nEntry;
      p->aSegment[i/iMode].aIndex = aIndex;
      p->aSegment[i/iMode].aPgno = (u32*)sLoc.aPgno;
    }
  }
  sqlite3_free(aTmp);

  if( rc!=SQLITE_OK ){
    walIteratorFree(p);
    p = 0;
................................................................................
** new wal-index header. It should be passed a pseudo-random value (i.e. 
** one obtained from sqlite3_randomness()).
*/
static void walRestartHdr(Wal *pWal, u32 salt1){
  volatile WalCkptInfo *pInfo = walCkptInfo(pWal);
  int i;                          /* Loop counter */
  u32 *aSalt = pWal->hdr.aSalt;   /* Big-endian salt values */
  assert( isWalMode2(pWal)==0 );
  pWal->nCkpt++;
  pWal->hdr.mxFrame = 0;
  sqlite3Put4byte((u8*)&aSalt[0], 1 + sqlite3Get4byte((u8*)&aSalt[0]));
  memcpy(&pWal->hdr.aSalt[1], &salt1, 4);
  walIndexWriteHdr(pWal);
  pInfo->nBackfill = 0;
  pInfo->nBackfillAttempted = 0;
................................................................................
  WalIterator *pIter = 0;         /* Wal iterator context */
  u32 iDbpage = 0;                /* Next database page to write */
  u32 iFrame = 0;                 /* Wal frame containing data for iDbpage */
  u32 mxSafeFrame;                /* Max frame that can be backfilled */
  u32 mxPage;                     /* Max database page to write */
  int i;                          /* Loop counter */
  volatile WalCkptInfo *pInfo;    /* The checkpoint status information */
  int bWal2 = isWalMode2(pWal);   /* True for wal2 connections */
  int iCkpt = bWal2 ? !walidxGetFile(&pWal->hdr) : 0;

  mxSafeFrame = walidxGetMxFrame(&pWal->hdr, iCkpt);
  szPage = walPagesize(pWal);
  testcase( szPage<=32768 );
  testcase( szPage>=65536 );
  pInfo = walCkptInfo(pWal);
  if( (bWal2==1 && pInfo->nBackfill==0 && mxSafeFrame) 
   || (bWal2==0 && pInfo->nBackfill<mxSafeFrame)
  ){
    sqlite3_file *pWalFd = pWal->apWalFd[iCkpt];
    mxPage = pWal->hdr.nPage;

    /* If this is a wal2 system, check for a reader holding a lock 
    ** preventing this checkpoint operation. If one is found, return
    ** early.  */
    if( bWal2 ){
      rc = walLockExclusive(pWal, WAL_READ_LOCK(1 + iCkpt*2), 1);
      if( rc!=SQLITE_OK ) return rc;
    }

    /* EVIDENCE-OF: R-62920-47450 The busy-handler callback is never invoked
    ** in the SQLITE_CHECKPOINT_PASSIVE mode. */
    assert( eMode!=SQLITE_CHECKPOINT_PASSIVE || xBusy==0 );

    /* If this is a wal system (not wal2), compute in mxSafeFrame the index 
    ** of the last frame of the WAL that is safe to write into the database.
    ** Frames beyond mxSafeFrame might overwrite database pages that are in 
    ** use by active readers and thus cannot be backfilled from the WAL.
    */
    if( bWal2==0 ){
      mxSafeFrame = pWal->hdr.mxFrame;
      mxPage = pWal->hdr.nPage;
      for(i=1; i<WAL_NREADER; i++){
        /* Thread-sanitizer reports that the following is an unsafe read,
        ** as some other thread may be in the process of updating the value
        ** of the aReadMark[] slot. The assumption here is that if that is
        ** happening, the other client may only be increasing the value,
        ** not decreasing it. So assuming either that either the "old" or
        ** "new" version of the value is read, and not some arbitrary value
        ** that would never be written by a real client, things are still 
        ** safe.  */
        u32 y = pInfo->aReadMark[i];
        if( mxSafeFrame>y ){
          assert( y<=pWal->hdr.mxFrame );
          rc = walBusyLock(pWal, xBusy, pBusyArg, WAL_READ_LOCK(i), 1);
          if( rc==SQLITE_OK ){
            pInfo->aReadMark[i] = (i==1 ? mxSafeFrame : READMARK_NOT_USED);
            walUnlockExclusive(pWal, WAL_READ_LOCK(i), 1);
          }else if( rc==SQLITE_BUSY ){
            mxSafeFrame = y;
            xBusy = 0;
          }else{
            goto walcheckpoint_out;
          }
        }
      }
    }

    /* Allocate the iterator */
    if( bWal2 || pInfo->nBackfill<mxSafeFrame ){
      assert( bWal2==0 || pInfo->nBackfill==0 );
      rc = walIteratorInit(pWal, iCkpt, pInfo->nBackfill, &pIter);
      assert( rc==SQLITE_OK || pIter==0 );
    }

    if( pIter
     && (rc = walBusyLock(pWal, xBusy, pBusyArg, WAL_READ_LOCK(0),1))==SQLITE_OK
    ){
      u32 nBackfill = pInfo->nBackfill;

      assert( bWal2==0 || nBackfill==0 );
      pInfo->nBackfillAttempted = mxSafeFrame;


      /* Sync the wal file being checkpointed to disk */
      rc = sqlite3OsSync(pWalFd, CKPT_SYNC_FLAGS(sync_flags));

      /* If the database may grow as a result of this checkpoint, hint
      ** about the eventual size of the db file to the VFS layer.  */

      if( rc==SQLITE_OK ){
        i64 nReq = ((i64)mxPage * szPage);
        i64 nSize;                    /* Current size of database file */
        rc = sqlite3OsFileSize(pWal->pDbFd, &nSize);
        if( rc==SQLITE_OK && nSize<nReq ){
          sqlite3OsFileControlHint(pWal->pDbFd, SQLITE_FCNTL_SIZE_HINT, &nReq);
        }
      }


      /* Iterate through the contents of the WAL, copying data to the db file */
      while( rc==SQLITE_OK && 0==walIteratorNext(pIter, &iDbpage, &iFrame) ){
        i64 iOffset;

        assert( bWal2==1 || walFramePgno(pWal, iFrame)==iDbpage );
        assert( bWal2==0 || walFramePgno2(pWal, iCkpt, iFrame)==iDbpage );

        if( db->u1.isInterrupted ){
          rc = db->mallocFailed ? SQLITE_NOMEM_BKPT : SQLITE_INTERRUPT;
          break;
        }
        if( iFrame<=nBackfill || iFrame>mxSafeFrame || iDbpage>mxPage ){
          assert( bWal2==0 || iDbpage>mxPage );
          continue;
        }
        iOffset = walFrameOffset(iFrame, szPage) + WAL_FRAME_HDRSIZE;
        WALTRACE(("WAL%p: checkpoint frame %d of wal %d to db page %d\n",
              pWal, (int)iFrame, iCkpt, (int)iDbpage
        ));
        /* testcase( IS_BIG_INT(iOffset) ); // requires a 4GiB WAL file */
        rc = sqlite3OsRead(pWalFd, zBuf, szPage, iOffset);
        if( rc!=SQLITE_OK ) break;
        iOffset = (iDbpage-1)*(i64)szPage;
        testcase( IS_BIG_INT(iOffset) );
        rc = sqlite3OsWrite(pWal->pDbFd, zBuf, szPage, iOffset);
        if( rc!=SQLITE_OK ) break;
      }

      /* If work was actually accomplished, truncate the db file, sync the wal
      ** file and set WalCkptInfo.nBackfill to indicate so. */
      if( rc==SQLITE_OK && (bWal2 || mxSafeFrame==walIndexHdr(pWal)->mxFrame) ){
        if( !bWal2 ){
          i64 szDb = pWal->hdr.nPage*(i64)szPage;
          testcase( IS_BIG_INT(szDb) );
          rc = sqlite3OsTruncate(pWal->pDbFd, szDb);
        }
        if( rc==SQLITE_OK ){
          rc = sqlite3OsSync(pWal->pDbFd, CKPT_SYNC_FLAGS(sync_flags));
        }
      }
      if( rc==SQLITE_OK ){
        pInfo->nBackfill = bWal2 ? 1 : mxSafeFrame;
      }


      /* Release the reader lock held while backfilling */
      walUnlockExclusive(pWal, WAL_READ_LOCK(bWal2 ? 1 + iCkpt*2 : 0), 1);
    }

    if( rc==SQLITE_BUSY ){
      /* Reset the return code so as not to report a checkpoint failure
      ** just because there are active readers.  */
      rc = SQLITE_OK;
    }
................................................................................
  }

  /* If this is an SQLITE_CHECKPOINT_RESTART or TRUNCATE operation, and the
  ** entire wal file has been copied into the database file, then block 
  ** until all readers have finished using the wal file. This ensures that 
  ** the next process to write to the database restarts the wal file.
  */
  if( bWal2==0 && rc==SQLITE_OK && eMode!=SQLITE_CHECKPOINT_PASSIVE ){
    assert( pWal->writeLock );
    if( pInfo->nBackfill<pWal->hdr.mxFrame ){
      rc = SQLITE_BUSY;
    }else if( eMode>=SQLITE_CHECKPOINT_RESTART ){
      u32 salt1;
      sqlite3_randomness(4, &salt1);
      assert( pInfo->nBackfill==pWal->hdr.mxFrame );
................................................................................
          ** writer clients should see that the entire log file has been
          ** checkpointed and behave accordingly. This seems unsafe though,
          ** as it would leave the system in a state where the contents of
          ** the wal-index header do not match the contents of the 
          ** file-system. To avoid this, update the wal-index header to
          ** indicate that the log file contains zero valid frames.  */
          walRestartHdr(pWal, salt1);
          rc = sqlite3OsTruncate(pWal->apWalFd[0], 0);
        }
        walUnlockExclusive(pWal, WAL_READ_LOCK(1), WAL_NREADER-1);
      }
    }
  }

 walcheckpoint_out:
................................................................................
}

/*
** If the WAL file is currently larger than nMax bytes in size, truncate
** it to exactly nMax bytes. If an error occurs while doing so, ignore it.
*/
static void walLimitSize(Wal *pWal, i64 nMax){
  if( isWalMode2(pWal)==0 ){
    i64 sz;
    int rx;
    sqlite3BeginBenignMalloc();
    rx = sqlite3OsFileSize(pWal->apWalFd[0], &sz);
    if( rx==SQLITE_OK && (sz > nMax ) ){
      rx = sqlite3OsTruncate(pWal->apWalFd[0], nMax);
    }
    sqlite3EndBenignMalloc();
    if( rx ){
      sqlite3_log(rx, "cannot limit WAL size: %s", pWal->zWalName);
    }
  }
}

/*
** Close a connection to a log file.
*/
int sqlite3WalClose(
................................................................................
    ** the wal and wal-index files.
    **
    ** The EXCLUSIVE lock is not released before returning.
    */
    if( zBuf!=0
     && SQLITE_OK==(rc = sqlite3OsLock(pWal->pDbFd, SQLITE_LOCK_EXCLUSIVE))
    ){
      int i;
      if( pWal->exclusiveMode==WAL_NORMAL_MODE ){
        pWal->exclusiveMode = WAL_EXCLUSIVE_MODE;
      }
      for(i=0; rc==SQLITE_OK && i<2; i++){
        rc = sqlite3WalCheckpoint(pWal, db, 
            SQLITE_CHECKPOINT_PASSIVE, 0, 0, sync_flags, nBuf, zBuf, 0, 0
        );
        if( rc==SQLITE_OK ){
          int bPersist = -1;
          sqlite3OsFileControlHint(
              pWal->pDbFd, SQLITE_FCNTL_PERSIST_WAL, &bPersist
          );
          if( bPersist!=1 ){
            /* Try to delete the WAL file if the checkpoint completed and
            ** fsyned (rc==SQLITE_OK) and if we are not in persistent-wal
            ** mode (!bPersist) */
            isDelete = 1;
          }else if( pWal->mxWalSize>=0 ){
            /* Try to truncate the WAL file to zero bytes if the checkpoint
            ** completed and fsynced (rc==SQLITE_OK) and we are in persistent
            ** WAL mode (bPersist) and if the PRAGMA journal_size_limit is a
            ** non-negative value (pWal->mxWalSize>=0).  Note that we truncate
            ** to zero bytes as truncating to the journal_size_limit might
            ** leave a corrupt WAL file on disk. */
            walLimitSize(pWal, 0);
          }
        }

        if( isWalMode2(pWal)==0 ) break;

        walCkptInfo(pWal)->nBackfill = 0;
        walidxSetFile(&pWal->hdr, !walidxGetFile(&pWal->hdr));
        pWal->writeLock = 1;
        walIndexWriteHdr(pWal);
        pWal->writeLock = 0;
      }
    }

    walIndexClose(pWal, isDelete);

    if( isDelete ){
      sqlite3BeginBenignMalloc();
      sqlite3OsDelete(pWal->pVfs, pWal->zWalName, 0);
      sqlite3OsDelete(pWal->pVfs, pWal->zWalName2, 0);
      sqlite3EndBenignMalloc();
    }
    WALTRACE(("WAL%p: closed\n", pWal));
    sqlite3_free((void *)pWal->apWiData);
    sqlite3_free(pWal);
  }
  return rc;
................................................................................
    }
  }

  /* If the header is read successfully, check the version number to make
  ** sure the wal-index was not constructed with some future format that
  ** this version of SQLite cannot understand.
  */
  if( badHdr==0 
   && pWal->hdr.iVersion!=WAL_VERSION1 && pWal->hdr.iVersion!=WAL_VERSION2
  ){
    rc = SQLITE_CANTOPEN_BKPT;
  }
  if( pWal->bShmUnreliable ){
    if( rc!=SQLITE_OK ){
      walIndexClose(pWal, 0);
      pWal->bShmUnreliable = 0;
      assert( pWal->nWiData>0 && pWal->apWiData[0]==0 );
................................................................................
  ** into pWal->hdr.
  */
  memcpy(&pWal->hdr, (void*)walIndexHdr(pWal), sizeof(WalIndexHdr));

  /* Make sure some writer hasn't come in and changed the WAL file out
  ** from under us, then disconnected, while we were not looking.
  */
  rc = sqlite3OsFileSize(pWal->apWalFd[0], &szWal);
  if( rc!=SQLITE_OK ){
    goto begin_unreliable_shm_out;
  }
  if( szWal<WAL_HDRSIZE ){
    /* If the wal file is too small to contain a wal-header and the
    ** wal-index header has mxFrame==0, then it must be safe to proceed
    ** reading the database file only. However, the page cache cannot
................................................................................
    ** since this client's last read transaction.  */
    *pChanged = 1;
    rc = (pWal->hdr.mxFrame==0 ? SQLITE_OK : WAL_RETRY);
    goto begin_unreliable_shm_out;
  }

  /* Check the salt keys at the start of the wal file still match. */
  rc = sqlite3OsRead(pWal->apWalFd[0], aBuf, WAL_HDRSIZE, 0);
  if( rc!=SQLITE_OK ){
    goto begin_unreliable_shm_out;
  }
  if( memcmp(&pWal->hdr.aSalt, &aBuf[16], 8) ){
    /* Some writer has wrapped the WAL file while we were not looking.
    ** Return WAL_RETRY which will cause the in-memory WAL-index to be
    ** rebuilt. */
................................................................................
      iOffset+szFrame<=szWal; 
      iOffset+=szFrame
  ){
    u32 pgno;                   /* Database page number for frame */
    u32 nTruncate;              /* dbsize field from frame header */

    /* Read and decode the next log frame. */
    rc = sqlite3OsRead(pWal->apWalFd[0], aFrame, szFrame, iOffset);
    if( rc!=SQLITE_OK ) break;
    if( !walDecodeFrame(pWal, &pgno, &nTruncate, aData, aFrame) ) break;

    /* If nTruncate is non-zero, then a complete transaction has been
    ** appended to this wal file. Set rc to WAL_RETRY and break out of
    ** the loop.  */
    if( nTruncate ){
................................................................................
** checkpoint process do as much work as possible.  This routine might
** update values of the aReadMark[] array in the header, but if it does
** so it takes care to hold an exclusive lock on the corresponding
** WAL_READ_LOCK() while changing values.
*/
static int walTryBeginRead(Wal *pWal, int *pChanged, int useWal, int cnt){
  volatile WalCkptInfo *pInfo;    /* Checkpoint information in wal-index */



  int rc = SQLITE_OK;             /* Return code  */


  assert( pWal->readLock==WAL_LOCK_NONE );     /* Not currently locked */

  /* useWal may only be set for read/write connections */
  assert( (pWal->readOnly & WAL_SHM_RDONLY)==0 || useWal==0 );

  /* Take steps to avoid spinning forever if there is a protocol error.
  **
  ** Circumstances that cause a RETRY should only last for the briefest
................................................................................
      return walBeginShmUnreliable(pWal, pChanged);
    }
  }

  assert( pWal->nWiData>0 );
  assert( pWal->apWiData[0]!=0 );
  pInfo = walCkptInfo(pWal);
  if( isWalMode2(pWal) ){
    int eLock = 1 + (walidxGetFile(&pWal->hdr)*2);
    if( pInfo->nBackfill==0 ){
      eLock += walidxGetMxFrame(&pWal->hdr, !walidxGetFile(&pWal->hdr))>0;
    }
    rc = walLockReader(pWal, eLock, 1);
    if( rc!=SQLITE_OK ){
      return rc;
    }

    walShmBarrier(pWal);
    if( memcmp((void *)walIndexHdr(pWal), &pWal->hdr, sizeof(WalIndexHdr)) ){
      walLockReader(pWal, eLock, 0);
      return WAL_RETRY;
    }else{
      pWal->readLock = eLock;
    }
    assert( pWal->minFrame==0 && walFramePage(pWal->minFrame)==0 );
  }else{
    u32 mxReadMark;               /* Largest aReadMark[] value */
    int mxI;                      /* Index of largest aReadMark[] value */
    int i;                        /* Loop counter */
    u32 mxFrame;                  /* Wal frame to lock to */
    if( !useWal && pInfo->nBackfill==pWal->hdr.mxFrame
  #ifdef SQLITE_ENABLE_SNAPSHOT
     && (pWal->pSnapshot==0 || pWal->hdr.mxFrame==0)
  #endif
    ){
      /* The WAL has been completely backfilled (or it is empty).
      ** and can be safely ignored.
      */
      rc = walLockShared(pWal, WAL_READ_LOCK(0));
      walShmBarrier(pWal);
      if( rc==SQLITE_OK ){
        if( memcmp((void *)walIndexHdr(pWal), &pWal->hdr,sizeof(WalIndexHdr)) ){
          /* It is not safe to allow the reader to continue here if frames
          ** may have been appended to the log before READ_LOCK(0) was obtained.
          ** When holding READ_LOCK(0), the reader ignores the entire log file,
          ** which implies that the database file contains a trustworthy
          ** snapshot. Since holding READ_LOCK(0) prevents a checkpoint from
          ** happening, this is usually correct.
          **
          ** However, if frames have been appended to the log (or if the log 
          ** is wrapped and written for that matter) before the READ_LOCK(0)
          ** is obtained, that is not necessarily true. A checkpointer may
          ** have started to backfill the appended frames but crashed before
          ** it finished. Leaving a corrupt image in the database file.
          */
          walUnlockShared(pWal, WAL_READ_LOCK(0));
          return WAL_RETRY;
        }
        pWal->readLock = 0;
        return SQLITE_OK;
      }else if( rc!=SQLITE_BUSY ){
        return rc;
      }
    }
  
    /* If we get this far, it means that the reader will want to use
    ** the WAL to get at content from recent commits.  The job now is
    ** to select one of the aReadMark[] entries that is closest to
    ** but not exceeding pWal->hdr.mxFrame and lock that entry.
    */
    mxReadMark = 0;
    mxI = 0;
    mxFrame = pWal->hdr.mxFrame;
  #ifdef SQLITE_ENABLE_SNAPSHOT
    if( pWal->pSnapshot && pWal->pSnapshot->mxFrame<mxFrame ){
      mxFrame = pWal->pSnapshot->mxFrame;
    }
  #endif
    for(i=1; i<WAL_NREADER; i++){
      u32 thisMark = AtomicLoad(pInfo->aReadMark+i);
      if( mxReadMark<=thisMark && thisMark<=mxFrame ){
        assert( thisMark!=READMARK_NOT_USED );
        mxReadMark = thisMark;
        mxI = i;
      }
    }
    if( (pWal->readOnly & WAL_SHM_RDONLY)==0
     && (mxReadMark<mxFrame || mxI==0)
    ){
      for(i=1; i<WAL_NREADER; i++){
        rc = walLockExclusive(pWal, WAL_READ_LOCK(i), 1);
        if( rc==SQLITE_OK ){
          mxReadMark = AtomicStore(pInfo->aReadMark+i,mxFrame);
          mxI = i;
          walUnlockExclusive(pWal, WAL_READ_LOCK(i), 1);
          break;
        }else if( rc!=SQLITE_BUSY ){
          return rc;
        }
      }
    }
    if( mxI==0 ){
      assert( rc==SQLITE_BUSY || (pWal->readOnly & WAL_SHM_RDONLY)!=0 );
      return rc==SQLITE_BUSY ? WAL_RETRY : SQLITE_READONLY_CANTINIT;
    }
  
    rc = walLockShared(pWal, WAL_READ_LOCK(mxI));
    if( rc ){
      return rc==SQLITE_BUSY ? WAL_RETRY : rc;
    }
    /* Now that the read-lock has been obtained, check that neither the
    ** value in the aReadMark[] array or the contents of the wal-index
    ** header have changed.
    **
    ** It is necessary to check that the wal-index header did not change
    ** between the time it was read and when the shared-lock was obtained
    ** on WAL_READ_LOCK(mxI) was obtained to account for the possibility
    ** that the log file may have been wrapped by a writer, or that frames
    ** that occur later in the log than pWal->hdr.mxFrame may have been
    ** copied into the database by a checkpointer. If either of these things
    ** happened, then reading the database with the current value of
    ** pWal->hdr.mxFrame risks reading a corrupted snapshot. So, retry
    ** instead.
    **
    ** Before checking that the live wal-index header has not changed
    ** since it was read, set Wal.minFrame to the first frame in the wal
    ** file that has not yet been checkpointed. This client will not need
    ** to read any frames earlier than minFrame from the wal file - they
    ** can be safely read directly from the database file.
    **
    ** Because a ShmBarrier() call is made between taking the copy of 
    ** nBackfill and checking that the wal-header in shared-memory still
    ** matches the one cached in pWal->hdr, it is guaranteed that the 
    ** checkpointer that set nBackfill was not working with a wal-index
    ** header newer than that cached in pWal->hdr. If it were, that could
    ** cause a problem. The checkpointer could omit to checkpoint
    ** a version of page X that lies before pWal->minFrame (call that version
    ** A) on the basis that there is a newer version (version B) of the same
    ** page later in the wal file. But if version B happens to like past
    ** frame pWal->hdr.mxFrame - then the client would incorrectly assume
    ** that it can read version A from the database file. However, since
    ** we can guarantee that the checkpointer that set nBackfill could not
    ** see any pages past pWal->hdr.mxFrame, this problem does not come up.
    */
    pWal->minFrame = AtomicLoad(&pInfo->nBackfill)+1;
    walShmBarrier(pWal);
    if( AtomicLoad(pInfo->aReadMark+mxI)!=mxReadMark
     || memcmp((void *)walIndexHdr(pWal), &pWal->hdr, sizeof(WalIndexHdr))
    ){
      walUnlockShared(pWal, WAL_READ_LOCK(mxI));
      return WAL_RETRY;
    }else{
      assert( mxReadMark<=pWal->hdr.mxFrame );
      pWal->readLock = (i16)mxI;
    }
  }
  return rc;
}

#ifdef SQLITE_ENABLE_SNAPSHOT
/*
** Attempt to reduce the value of the WalCkptInfo.nBackfillAttempted 
................................................................................
          rc = walHashGet(pWal, walFramePage(i), &sLoc);
          if( rc!=SQLITE_OK ) break;
          pgno = sLoc.aPgno[i-sLoc.iZero];
          iDbOff = (i64)(pgno-1) * szPage;

          if( iDbOff+szPage<=szDb ){
            iWalOff = walFrameOffset(i, szPage) + WAL_FRAME_HDRSIZE;
            rc = sqlite3OsRead(pWal->apWalFd[0], pBuf1, szPage, iWalOff);

            if( rc==SQLITE_OK ){
              rc = sqlite3OsRead(pWal->pDbFd, pBuf2, szPage, iDbOff);
            }

            if( rc!=SQLITE_OK || 0==memcmp(pBuf1, pBuf2, szPage) ){
              break;
................................................................................
  do{
    rc = walTryBeginRead(pWal, pChanged, 0, ++cnt);
  }while( rc==WAL_RETRY );
  testcase( (rc&0xff)==SQLITE_BUSY );
  testcase( (rc&0xff)==SQLITE_IOERR );
  testcase( rc==SQLITE_PROTOCOL );
  testcase( rc==SQLITE_OK );
  
  if( rc==SQLITE_OK && pWal->hdr.iVersion==WAL_VERSION2 ){
    rc = walOpenWal2(pWal);
  }

  pWal->nPriorFrame = pWal->hdr.mxFrame;
#ifdef SQLITE_ENABLE_SNAPSHOT
  if( rc==SQLITE_OK ){
    if( pSnapshot && memcmp(pSnapshot, &pWal->hdr, sizeof(WalIndexHdr))!=0 ){
      /* At this point the client has a lock on an aReadMark[] slot holding
      ** a value equal to or smaller than pSnapshot->mxFrame, but pWal->hdr
................................................................................

/*
** Finish with a read transaction.  All this does is release the
** read-lock.
*/
void sqlite3WalEndReadTransaction(Wal *pWal){
  sqlite3WalEndWriteTransaction(pWal);
  if( pWal->readLock!=WAL_LOCK_NONE ){
    if( isWalMode2(pWal) ){
      (void)walLockReader(pWal, pWal->readLock, 0);
    }else{
      walUnlockShared(pWal, WAL_READ_LOCK(pWal->readLock));
    }
    pWal->readLock = WAL_LOCK_NONE;
  }
}

/* Search hash table iHash for an entry matching page number
** pgno. Each call to this function searches a single hash table
** (each hash table indexes up to HASHTABLE_NPAGE frames).
**
** This code might run concurrently to the code in walIndexAppend()
** that adds entries to the wal-index (and possibly to this hash 
** table). This means the value just read from the hash 
** slot (aHash[iKey]) may have been added before or after the 
** current read transaction was opened. Values added after the
** read transaction was opened may have been written incorrectly -
** i.e. these slots may contain garbage data. However, we assume
** that any slots written before the current read transaction was
** opened remain unmodified.
**
** For the reasons above, the if(...) condition featured in the inner
** loop of the following block is more stringent that would be required 
** if we had exclusive access to the hash-table:
**
**   (aPgno[iFrame]==pgno): 
**     This condition filters out normal hash-table collisions.
**
**   (iFrame<=iLast): 
**     This condition filters out entries that were added to the hash
**     table after the current read-transaction had started.
*/
static int walSearchHash(
  Wal *pWal, 
  u32 iLast,
  int iHash, 
  Pgno pgno, 
  u32 *piRead
){
  WalHashLoc sLoc;                /* Hash table location */
  int iKey;                       /* Hash slot index */
  int nCollide;                   /* Number of hash collisions remaining */
  int rc;                         /* Error code */

  rc = walHashGet(pWal, iHash, &sLoc);
  if( rc!=SQLITE_OK ){
    return rc;
  }
  nCollide = HASHTABLE_NSLOT;
  for(iKey=walHash(pgno); sLoc.aHash[iKey]; iKey=walNextHash(iKey)){
    u32 iFrame = sLoc.aHash[iKey] + sLoc.iZero;
    if( iFrame<=iLast 
     && iFrame>=pWal->minFrame 
     && sLoc.aPgno[sLoc.aHash[iKey]]==pgno 
    ){
      assert( iFrame>*piRead || CORRUPT_DB );
      *piRead = iFrame;
    }
    if( (nCollide--)==0 ){
      return SQLITE_CORRUPT_BKPT;
    }
  }

  return SQLITE_OK;
}

static int walSearchWal(
  Wal *pWal, 
  int iWal, 
  Pgno pgno, 
  u32 *piRead
){
  int rc = SQLITE_OK;
  int bWal2 = isWalMode2(pWal);
  u32 iLast = walidxGetMxFrame(&pWal->hdr, iWal);
  if( iLast ){
    int iHash;
    int iMinHash = walFramePage(pWal->minFrame);
    u32 iExternal = bWal2 ? walExternalEncode(iWal, iLast) : iLast;
    assert( bWal2==0 || pWal->minFrame==0 );
    for(iHash=walFramePage(iExternal); 
        iHash>=iMinHash && *piRead==0; 
        iHash-=(1+bWal2)
    ){
      rc = walSearchHash(pWal, iExternal, iHash, pgno, piRead);
      if( rc!=SQLITE_OK ) break;
    }
  }
  return rc;
}

/*
** Search the wal file for page pgno. If found, set *piRead to the frame that
** contains the page. Otherwise, if pgno is not in the wal file, set *piRead
** to zero.
**
** Return SQLITE_OK if successful, or an error code if an error occurs. If an
................................................................................
** error does occur, the final value of *piRead is undefined.
*/
int sqlite3WalFindFrame(
  Wal *pWal,                      /* WAL handle */
  Pgno pgno,                      /* Database page number to read data for */
  u32 *piRead                     /* OUT: Frame number (or zero) */
){
  int bWal2 = isWalMode2(pWal);
  int iApp = walidxGetFile(&pWal->hdr);
  int rc = SQLITE_OK;
  u32 iRead = 0;                  /* If !=0, WAL frame to return data from */




  /* This routine is only be called from within a read transaction. */
  assert( pWal->readLock!=WAL_LOCK_NONE );

  /* If this is a wal2 system, the client must have a partial-wal lock 
  ** on wal file iApp. Or if it is a wal system, iApp==0 must be true.  */
  assert( bWal2==0 || iApp==1
       || pWal->readLock==WAL_LOCK_PART1 || pWal->readLock==WAL_LOCK_PART1_FULL2


  );
  assert( bWal2==0 || iApp==0
       || pWal->readLock==WAL_LOCK_PART2 || pWal->readLock==WAL_LOCK_PART2_FULL1
  );
  assert( bWal2 || iApp==0 );

  /* Return early if read-lock 0 is held. */
  if( (pWal->readLock==0 && pWal->bShmUnreliable==0) ){
    *piRead = 0;
    return SQLITE_OK;
  }






  /* Search the wal file that the client holds a partial lock on first */
  rc = walSearchWal(pWal, iApp, pgno, &iRead);



























  /* If the requested page was not found, no error has occured, and 
  ** the client holds a full-wal lock on the other wal file, search it
  ** too.  */
  if( rc==SQLITE_OK && bWal2 && iRead==0 && (
        pWal->readLock==WAL_LOCK_PART1_FULL2 
     || pWal->readLock==WAL_LOCK_PART2_FULL1
  )){
    rc = walSearchWal(pWal, !iApp, pgno, &iRead);
  }












#if defined(SQLITE_TEST) && defined(SQLITE_DEBUG)
  if( iRead ){ 
    u32 iFrame;
    int iWal = walExternalDecode(iRead, &iFrame);
    WALTRACE(("WAL%p: page %d @ frame %d wal %d\n",pWal,(int)pgno,iFrame,iWal));
  }else{
    WALTRACE(("WAL%p: page %d not found\n", pWal, (int)pgno));
  }
#endif

#ifdef SQLITE_ENABLE_EXPENSIVE_ASSERT
  /* If expensive assert() statements are available, do a linear search
  ** of the wal-index file content. Make sure the results agree with the
  ** result obtained using the hash indexes above.  
  **
  ** TODO: This is broken for wal2.
  */
  if( rc==SQLITE_OK ){
    u32 iRead2 = 0;
    u32 iTest;
    assert( pWal->bShmUnreliable || pWal->minFrame>0 );
    for(iTest=iLast; iTest>=pWal->minFrame && iTest>0; iTest--){
      if( walFramePgno(pWal, iTest)==pgno ){
        iRead2 = iTest;
................................................................................
/*
** Read the contents of frame iRead from the wal file into buffer pOut
** (which is nOut bytes in size). Return SQLITE_OK if successful, or an
** error code otherwise.
*/
int sqlite3WalReadFrame(
  Wal *pWal,                      /* WAL handle */
  u32 iExternal,                  /* Frame to read */
  int nOut,                       /* Size of buffer pOut in bytes */
  u8 *pOut                        /* Buffer to write page data to */
){
  int sz;
  int iWal = 0;
  u32 iRead;
  i64 iOffset;

  /* Figure out the page size */
  sz = pWal->hdr.szPage;
  sz = (sz&0xfe00) + ((sz&0x0001)<<16);
  testcase( sz<=32768 );
  testcase( sz>=65536 );

  if( isWalMode2(pWal) ){
    /* Figure out which of the two wal files, and the frame within, that 
    ** iExternal refers to.  */
    iWal = walExternalDecode(iExternal, &iRead);
  }else{
    iRead = iExternal;
  }

  WALTRACE(("WAL%p: reading frame %d wal %d\n", pWal, iRead, iWal));
  iOffset = walFrameOffset(iRead, sz) + WAL_FRAME_HDRSIZE;
  /* testcase( IS_BIG_INT(iOffset) ); // requires a 4GiB WAL */
  return sqlite3OsRead(pWal->apWalFd[iWal], pOut, (nOut>sz?sz:nOut), iOffset);
}

/* 
** Return the size of the database in pages (or zero, if unknown).
*/
Pgno sqlite3WalDbsize(Wal *pWal){
  if( pWal && ALWAYS(pWal->readLock!=WAL_LOCK_NONE) ){
    return pWal->hdr.nPage;
  }
  return 0;
}

/*
** Take the WRITER lock on the WAL file. Return SQLITE_OK if successful,
................................................................................
              u8 aNew[4];
              u8 *aOld = &((u8*)pPage1->pData)[40];
              int sz;
              i64 iOffset;
              sz = pWal->hdr.szPage;
              sz = (sz&0xfe00) + ((sz&0x0001)<<16);
              iOffset = walFrameOffset(i+sLoc.iZero, sz) + WAL_FRAME_HDRSIZE+40;
              rc = sqlite3OsRead(pWal->apWalFd[0], aNew, sizeof(aNew), iOffset);
              if( rc==SQLITE_OK && memcmp(aOld, aNew, sizeof(aNew)) ){
                rc = SQLITE_BUSY_SNAPSHOT;
              }
            }else if( sqlite3BitvecTestNotNull(pAllRead, sLoc.aPgno[i]) ){
              *piConflict = sLoc.aPgno[i];
              rc = SQLITE_BUSY_SNAPSHOT;
            }else if( (pPg = sqlite3PagerLookup(pPager, sLoc.aPgno[i])) ){
................................................................................
**
** Otherwise, if the callback function does not return an error, this
** function returns SQLITE_OK.
*/
int sqlite3WalUndo(Wal *pWal, int (*xUndo)(void *, Pgno), void *pUndoCtx){
  int rc = SQLITE_OK;
  if( pWal->writeLock ){
    int iWal = walidxGetFile(&pWal->hdr);
    Pgno iMax = walidxGetMxFrame(&pWal->hdr, iWal);
    Pgno iNew;
    Pgno iFrame;

    assert( isWalMode2(pWal) || iWal==0 );

    /* Restore the clients cache of the wal-index header to the state it
    ** was in before the client began writing to the database. 
    */
    memcpy(&pWal->hdr, (void *)walIndexHdr(pWal), sizeof(WalIndexHdr));
    assert( walidxGetFile(&pWal->hdr)==iWal );
    iNew = walidxGetMxFrame(&pWal->hdr, walidxGetFile(&pWal->hdr));


    for(iFrame=iNew+1; ALWAYS(rc==SQLITE_OK) && iFrame<=iMax; iFrame++){


      /* This call cannot fail. Unless the page for which the page number
      ** is passed as the second argument is (a) in the cache and 
      ** (b) has an outstanding reference, then xUndo is either a no-op
      ** (if (a) is false) or simply expels the page from the cache (if (b)
      ** is false).
      **
      ** If the upper layer is doing a rollback, it is guaranteed that there
      ** are no outstanding references to any page other than page 1. And
      ** page 1 is never written to the log until the transaction is
      ** committed. As a result, the call to xUndo may not fail.
      */
      Pgno pgno;
      if( isWalMode2(pWal) ){
        pgno = walFramePgno2(pWal, iWal, iFrame);
      }else{
        pgno = walFramePgno(pWal, iFrame);
      }
      assert( pgno!=1 );
      rc = xUndo(pUndoCtx, pgno);
    }
    if( iMax!=iNew ) walCleanupHash(pWal);
  }
  return rc;
}

/* 
** Argument aWalData must point to an array of WAL_SAVEPOINT_NDATA u32 
** values. This function populates the array with values required to 
** "rollback" the write position of the WAL handle back to the current 
** point in the event of a savepoint rollback (via WalSavepointUndo()).
*/
void sqlite3WalSavepoint(Wal *pWal, u32 *aWalData){
  int iWal = walidxGetFile(&pWal->hdr);
  assert( pWal->writeLock );
  assert( isWalMode2(pWal) || iWal==0 );
  aWalData[0] = walidxGetMxFrame(&pWal->hdr, iWal);
  aWalData[1] = pWal->hdr.aFrameCksum[0];
  aWalData[2] = pWal->hdr.aFrameCksum[1];

  aWalData[3] = isWalMode2(pWal) ? iWal : pWal->nCkpt;
}

/* 
** Move the write position of the WAL back to the point identified by
** the values in the aWalData[] array. aWalData must point to an array
** of WAL_SAVEPOINT_NDATA u32 values that has been previously populated
** by a call to WalSavepoint().
*/
int sqlite3WalSavepointUndo(Wal *pWal, u32 *aWalData){
  int rc = SQLITE_OK;
  int iWal = walidxGetFile(&pWal->hdr);
  int iCmp = isWalMode2(pWal) ? iWal : pWal->nCkpt;

  assert( pWal->writeLock || aWalData[0]==pWal->hdr.mxFrame );

  assert( isWalMode2(pWal) || iWal==0 );
  assert( aWalData[3]!=iCmp || aWalData[0]<=walidxGetMxFrame(&pWal->hdr,iWal) );

  if( aWalData[3]!=iCmp ){
    /* This savepoint was opened immediately after the write-transaction
    ** was started. Right after that, the writer decided to wrap around
    ** to the start of the log. Update the savepoint values to match.
    */
    aWalData[0] = 0;
    aWalData[3] = iCmp;
  }

  if( aWalData[0]<walidxGetMxFrame(&pWal->hdr, iWal) ){
    walidxSetMxFrame(&pWal->hdr, iWal, aWalData[0]);
    pWal->hdr.aFrameCksum[0] = aWalData[1];
    pWal->hdr.aFrameCksum[1] = aWalData[2];
    walCleanupHash(pWal);
  }

  return rc;
}

/*
** This function is used in wal2 mode.
**
** This function is called when writer pWal is just about to start 
** writing out frames. The "other" wal file (wal file !pWal->hdr.iAppend)
** has been fully checkpointed. This function returns SQLITE_OK if there
** are no readers preventing the writer from switching to the other wal
** file. Or SQLITE_BUSY if there are.
*/
static int walRestartOk(Wal *pWal){
  int rc;                                        /* Return code */
  int iApp = walidxGetFile(&pWal->hdr);          /* Current WAL file */

  /* No reader can be doing a "partial" read of wal file !iApp - in that
  ** case it would not have been possible to checkpoint the file. So
  ** it is only necessary to test for "full" readers. See the comment
  ** above walLockReader() function for exactly what this means in terms
  ** of locks.  */
  int i = (iApp==0) ? 2 : 4;

  rc = walLockExclusive(pWal, WAL_READ_LOCK(i), 1);
  if( rc==SQLITE_OK ){
    walUnlockExclusive(pWal, WAL_READ_LOCK(i), 1);
  }
  return rc;
}

/*
** This function is called just before writing a set of frames to the log
** file (see sqlite3WalFrames()). It checks to see if, instead of appending
** to the current log file, it is possible and desirable to switch to the



** other log file and write the new transaction to the start of it.
** If so, the wal-index header is updated accordingly - both in heap memory
** and in the *-shm file.
**
** SQLITE_OK is returned if no error is encountered (regardless of whether
** or not the wal-index header is modified). An SQLite error code is returned
** if an error occurs.
*/
static int walRestartLog(Wal *pWal){
  int rc = SQLITE_OK;

  if( isWalMode2(pWal) ){
    int iApp = walidxGetFile(&pWal->hdr);
    int nWalSize = WAL_DEFAULT_WALSIZE;
    if( pWal->mxWalSize>0 ){
      nWalSize = (pWal->mxWalSize-WAL_HDRSIZE+pWal->szPage+WAL_FRAME_HDRSIZE-1) 
        / (pWal->szPage+WAL_FRAME_HDRSIZE);
      nWalSize = MAX(nWalSize, 1);
    }

    if( walidxGetMxFrame(&pWal->hdr, iApp)>=nWalSize ){
      volatile WalCkptInfo *pInfo = walCkptInfo(pWal);
      if( walidxGetMxFrame(&pWal->hdr, !iApp)==0 || pInfo->nBackfill ){
        rc = walRestartOk(pWal);
        if( rc==SQLITE_OK ){
          iApp = !iApp;
          pWal->nCkpt++;
          walidxSetFile(&pWal->hdr, iApp);
          walidxSetMxFrame(&pWal->hdr, iApp, 0);
          sqlite3Put4byte((u8*)&pWal->hdr.aSalt[0], pWal->hdr.aFrameCksum[0]);
          sqlite3Put4byte((u8*)&pWal->hdr.aSalt[1], pWal->hdr.aFrameCksum[1]);
          walIndexWriteHdr(pWal);
          pInfo->nBackfill = 0;
          walLockReader(pWal, pWal->readLock, 0);
          pWal->readLock = iApp ? WAL_LOCK_PART2_FULL1 : WAL_LOCK_PART1_FULL2;
          rc = walLockReader(pWal, pWal->readLock, 1);
        }else if( rc==SQLITE_BUSY ){
          rc = SQLITE_OK;
        }
      }
    }
  }else if( pWal->readLock==0 ){
    volatile WalCkptInfo *pInfo = walCkptInfo(pWal);
    assert( pInfo->nBackfill==pWal->hdr.mxFrame );
    if( pInfo->nBackfill>0 ){
      u32 salt1;
      sqlite3_randomness(4, &salt1);
      rc = walLockExclusive(pWal, WAL_READ_LOCK(1), WAL_NREADER-1);
      if( rc==SQLITE_OK ){
................................................................................
    ** only - never from the wal file. This means that if a writer holding
    ** a lock on aReadmark[0] were to commit a transaction but not close the
    ** read-transaction, subsequent read operations would read directly from
    ** the database file - ignoring the new pages just appended
    ** to the wal file. */
    rc = walUpgradeReadlock(pWal);
  }

  return rc;
}

/*
** Information about the current state of the WAL file and where
** the next fsync should occur - passed from sqlite3WalFrames() into
** walWriteToLog().
................................................................................
  PgHdr *pPage,               /* The page of the frame to be written */
  int nTruncate,              /* The commit flag.  Usually 0.  >0 for commit */
  sqlite3_int64 iOffset       /* Byte offset at which to write */
){
  int rc;                         /* Result code from subfunctions */
  void *pData;                    /* Data actually written */
  u8 aFrame[WAL_FRAME_HDRSIZE];   /* Buffer to assemble frame-header in */

#if defined(SQLITE_TEST) && defined(SQLITE_DEBUG)
  { 
    int iWal = walidxGetFile(&p->pWal->hdr);
    int iFrame = 1 + (iOffset / (WAL_FRAME_HDRSIZE + p->pWal->szPage));
    assert( p->pWal->apWalFd[iWal]==p->pFd );
    WALTRACE(("WAL%p: page %d written to frame %d of wal %d\n",
          p->pWal, (int)pPage->pgno, iFrame, iWal
    ));
  }
#endif

#if defined(SQLITE_HAS_CODEC)
  if( (pData = sqlite3PagerCodec(pPage))==0 ) return SQLITE_NOMEM_BKPT;
#else
  pData = pPage->pData;
#endif
  walEncodeFrame(p->pWal, pPage->pgno, nTruncate, pData, aFrame);
  rc = walWriteToLog(p, aFrame, sizeof(aFrame), iOffset);
................................................................................
** one or more frames have been overwritten. It updates the checksums for
** all frames written to the wal file by the current transaction starting
** with the earliest to have been overwritten.
**
** SQLITE_OK is returned if successful, or an SQLite error code otherwise.
*/
static int walRewriteChecksums(Wal *pWal, u32 iLast){

  int rc = SQLITE_OK;             /* Return code */
  const int szPage = pWal->szPage;/* Database page size */
  u8 *aBuf;                       /* Buffer to load data from wal file into */
  u8 aFrame[WAL_FRAME_HDRSIZE];   /* Buffer to assemble frame-headers in */
  u32 iRead;                      /* Next frame to read from wal file */
  i64 iCksumOff;
  sqlite3_file *pWalFd = pWal->apWalFd[walidxGetFile(&pWal->hdr)];

  aBuf = sqlite3_malloc(szPage + WAL_FRAME_HDRSIZE);
  if( aBuf==0 ) return SQLITE_NOMEM_BKPT;

  /* Find the checksum values to use as input for the recalculating the
  ** first checksum. If the first frame is frame 1 (implying that the current
  ** transaction restarted the wal file), these values must be read from the
................................................................................
  ** previous frame.  */
  assert( pWal->iReCksum>0 );
  if( pWal->iReCksum==1 ){
    iCksumOff = 24;
  }else{
    iCksumOff = walFrameOffset(pWal->iReCksum-1, szPage) + 16;
  }
  rc = sqlite3OsRead(pWalFd, aBuf, sizeof(u32)*2, iCksumOff);
  pWal->hdr.aFrameCksum[0] = sqlite3Get4byte(aBuf);
  pWal->hdr.aFrameCksum[1] = sqlite3Get4byte(&aBuf[sizeof(u32)]);

  iRead = pWal->iReCksum;
  pWal->iReCksum = 0;
  for(; rc==SQLITE_OK && iRead<=iLast; iRead++){
    i64 iOff = walFrameOffset(iRead, szPage);
    rc = sqlite3OsRead(pWalFd, aBuf, szPage+WAL_FRAME_HDRSIZE, iOff);
    if( rc==SQLITE_OK ){
      u32 iPgno, nDbSize;
      iPgno = sqlite3Get4byte(aBuf);
      nDbSize = sqlite3Get4byte(&aBuf[4]);

      walEncodeFrame(pWal, iPgno, nDbSize, &aBuf[WAL_FRAME_HDRSIZE], aFrame);
      rc = sqlite3OsWrite(pWalFd, aFrame, sizeof(aFrame), iOff);
    }
  }

  sqlite3_free(aBuf);
  return rc;
}

................................................................................
  PgHdr *pLast = 0;               /* Last frame in list */
  int nExtra = 0;                 /* Number of extra copies of last page */
  int szFrame;                    /* The size of a single frame */
  i64 iOffset;                    /* Next byte to write in WAL file */
  WalWriter w;                    /* The writer */
  u32 iFirst = 0;                 /* First frame that may be overwritten */
  WalIndexHdr *pLive;             /* Pointer to shared header */
  int iApp;
  int bWal2 = isWalMode2(pWal);

  assert( pList );
  assert( pWal->writeLock );

  /* If this frame set completes a transaction, then nTruncate>0.  If
  ** nTruncate==0 then this frame set does not complete the transaction. */
  assert( (isCommit!=0)==(nTruncate!=0) );








  pLive = (WalIndexHdr*)walIndexHdr(pWal);
  if( memcmp(&pWal->hdr, (void *)pLive, sizeof(WalIndexHdr))!=0 ){

    /* if( isWalMode2(pWal)==0 ) */
    iFirst = walidxGetMxFrame(pLive, walidxGetFile(pLive))+1;
  }

  /* See if it is possible to write these frames into the start of the
  ** log file, instead of appending to it at pWal->hdr.mxFrame.
  */
  else if( SQLITE_OK!=(rc = walRestartLog(pWal)) ){
    return rc;
  }

  /* If this is the first frame written into the log, write the WAL
  ** header to the start of the WAL file. See comments at the top of
  ** this source file for a description of the WAL header format.
  */
  iApp = walidxGetFile(&pWal->hdr);
  iFrame = walidxGetMxFrame(&pWal->hdr, iApp);
  assert( iApp==0 || bWal2 );

#if defined(SQLITE_TEST) && defined(SQLITE_DEBUG)
  { int cnt; for(cnt=0, p=pList; p; p=p->pDirty, cnt++){}
    WALTRACE(("WAL%p: frame write begin. %d frames. iWal=%d. mxFrame=%d. %s\n",
              pWal, cnt, iApp, iFrame, isCommit ? "Commit" : "Spill"));
  }
#endif

  if( iFrame==0 ){
    u32 iCkpt = 0;
    u8 aWalHdr[WAL_HDRSIZE];      /* Buffer to assemble wal-header in */
    u32 aCksum[2];                /* Checksum for wal-header */

    sqlite3Put4byte(&aWalHdr[0], (WAL_MAGIC | SQLITE_BIGENDIAN));
    sqlite3Put4byte(&aWalHdr[4], pWal->hdr.iVersion);
    sqlite3Put4byte(&aWalHdr[8], szPage);
    if( bWal2 ){
      if( walidxGetMxFrame(&pWal->hdr, !iApp)>0 ){
        u8 aPrev[4];
        rc = sqlite3OsRead(pWal->apWalFd[!iApp], aPrev, 4, 12);
        if( rc!=SQLITE_OK ){
          return rc;
        }
        iCkpt = (sqlite3Get4byte(aPrev) + 1) & 0x0F;
      }
    }else{
      iCkpt = pWal->nCkpt;
    }
    sqlite3Put4byte(&aWalHdr[12], iCkpt);

    memcpy(&aWalHdr[16], pWal->hdr.aSalt, 8);
    walChecksumBytes(1, aWalHdr, WAL_HDRSIZE-2*4, 0, aCksum);
    sqlite3Put4byte(&aWalHdr[24], aCksum[0]);
    sqlite3Put4byte(&aWalHdr[28], aCksum[1]);

    pWal->szPage = szPage;
    pWal->hdr.bigEndCksum = SQLITE_BIGENDIAN;
    pWal->hdr.aFrameCksum[0] = aCksum[0];
    pWal->hdr.aFrameCksum[1] = aCksum[1];
    pWal->truncateOnCommit = 1;

    rc = sqlite3OsWrite(pWal->apWalFd[iApp], aWalHdr, sizeof(aWalHdr), 0);
    WALTRACE(("WAL%p: wal-header write %s\n", pWal, rc ? "failed" : "ok"));
    if( rc!=SQLITE_OK ){
      return rc;
    }

    /* Sync the header (unless SQLITE_IOCAP_SEQUENTIAL is true or unless
    ** all syncing is turned off by PRAGMA synchronous=OFF).  Otherwise
    ** an out-of-order write following a WAL restart could result in
    ** database corruption.  See the ticket:
    **
    **     https://sqlite.org/src/info/ff5be73dee
    */
    if( pWal->syncHeader ){
      rc = sqlite3OsSync(pWal->apWalFd[iApp], CKPT_SYNC_FLAGS(sync_flags));
      if( rc ) return rc;
    }
  }
  assert( (int)pWal->szPage==szPage );

  /* Setup information needed to write frames into the WAL */
  w.pWal = pWal;
  w.pFd = pWal->apWalFd[iApp];
  w.iSyncPoint = 0;
  w.syncFlags = sync_flags;
  w.szPage = szPage;
  iOffset = walFrameOffset(iFrame+1, szPage);
  szFrame = szPage + WAL_FRAME_HDRSIZE;

  /* Write all frames into the log file exactly once */
................................................................................

    /* Check if this page has already been written into the wal file by
    ** the current transaction. If so, overwrite the existing frame and
    ** set Wal.writeLock to WAL_WRITELOCK_RECKSUM - indicating that 
    ** checksums must be recomputed when the transaction is committed.  */
    if( iFirst && (p->pDirty || isCommit==0) ){
      u32 iWrite = 0;
      VVA_ONLY(rc =) walSearchWal(pWal, iApp, p->pgno, &iWrite);
      assert( rc==SQLITE_OK || iWrite==0 );
      if( iWrite && bWal2 ){
        walExternalDecode(iWrite, &iWrite);
      }
      if( iWrite>=iFirst ){
        i64 iOff = walFrameOffset(iWrite, szPage) + WAL_FRAME_HDRSIZE;
        void *pData;
        if( pWal->iReCksum==0 || iWrite<pWal->iReCksum ){
          pWal->iReCksum = iWrite;
        }
#if defined(SQLITE_HAS_CODEC)
        if( (pData = sqlite3PagerCodec(p))==0 ) return SQLITE_NOMEM;
#else
        pData = p->pData;
#endif
        rc = sqlite3OsWrite(pWal->apWalFd[iApp], pData, szPage, iOff);
        if( rc ) return rc;
        p->flags &= ~PGHDR_WAL_APPEND;
        continue;
      }
    }

    iFrame++;
................................................................................
  ** boundary is crossed.  Only the part of the WAL prior to the last
  ** sector boundary is synced; the part of the last frame that extends
  ** past the sector boundary is written after the sync.
  */
  if( isCommit && WAL_SYNC_FLAGS(sync_flags)!=0 ){
    int bSync = 1;
    if( pWal->padToSectorBoundary ){
      int sectorSize = sqlite3SectorSize(w.pFd);
      w.iSyncPoint = ((iOffset+sectorSize-1)/sectorSize)*sectorSize;
      bSync = (w.iSyncPoint==iOffset);
      testcase( bSync );
      while( iOffset<w.iSyncPoint ){
        rc = walWriteOneFrame(&w, pLast, nTruncate, iOffset);
        if( rc ) return rc;
        iOffset += szFrame;
................................................................................
  }

  /* Append data to the wal-index. It is not necessary to lock the 
  ** wal-index to do this as the SQLITE_SHM_WRITE lock held on the wal-index
  ** guarantees that there are no other writers, and no data that may
  ** be in use by existing readers is being overwritten.
  */
  iFrame = walidxGetMxFrame(&pWal->hdr, iApp);
  for(p=pList; p && rc==SQLITE_OK; p=p->pDirty){
    if( (p->flags & PGHDR_WAL_APPEND)==0 ) continue;
    iFrame++;
    rc = walIndexAppend(pWal, iApp, iFrame, p->pgno);
  }
  while( rc==SQLITE_OK && nExtra>0 ){
    iFrame++;
    nExtra--;
    rc = walIndexAppend(pWal, iApp, iFrame, pLast->pgno);
  }

  if( rc==SQLITE_OK ){
    /* Update the private copy of the header. */
    pWal->hdr.szPage = (u16)((szPage&0xff00) | (szPage>>16));
    testcase( szPage<=32768 );
    testcase( szPage>=65536 );
    walidxSetMxFrame(&pWal->hdr, iApp, iFrame);
    if( isCommit ){
      pWal->hdr.iChange++;
      pWal->hdr.nPage = nTruncate;
    }
    /* If this is a commit, update the wal-index header too. */
    if( isCommit ){
      walIndexWriteHdr(pWal);
      if( bWal2 ){
        int iOther = !walidxGetFile(&pWal->hdr);
        if( walidxGetMxFrame(&pWal->hdr, iOther) 
            && !walCkptInfo(pWal)->nBackfill 
        ){
          pWal->iCallback = walidxGetMxFrame(&pWal->hdr, 0);
          pWal->iCallback += walidxGetMxFrame(&pWal->hdr, 1);
        }
      }else{
        pWal->iCallback = iFrame;
      }
    }
  }

  WALTRACE(("WAL%p: frame write %s\n", pWal, rc ? "failed" : "ok"));
  return rc;
}

................................................................................
    if( isChanged && pWal->pDbFd->pMethods->iVersion>=3 ){
      sqlite3OsUnfetch(pWal->pDbFd, 0, 0);
    }
  }

  /* Copy data from the log to the database file. */
  if( rc==SQLITE_OK ){
    if( (walPagesize(pWal)!=nBuf) 
     && (walidxGetMxFrame(&pWal->hdr, 0) || walidxGetMxFrame(&pWal->hdr, 1))
    ){
      rc = SQLITE_CORRUPT_BKPT;
    }else{
      rc = walCheckpoint(pWal, db, eMode2, xBusy2, pBusyArg, sync_flags, zBuf);
    }

    /* If no error occurred, set the output variables. */
    if( rc==SQLITE_OK || rc==SQLITE_BUSY ){
      if( pnLog ){
        *pnLog = walidxGetMxFrame(&pWal->hdr,0)+walidxGetMxFrame(&pWal->hdr,1);
      }
      if( pnCkpt ){
        if( isWalMode2(pWal) ){
          if( (int)(walCkptInfo(pWal)->nBackfill) ){
            *pnCkpt = walidxGetMxFrame(&pWal->hdr, !walidxGetFile(&pWal->hdr));
          }else{
            *pnCkpt = 0;
          }
        }else{
          *pnCkpt = walCkptInfo(pWal)->nBackfill;
        }
      }
    }
  }

  if( isChanged ){
    /* If a new wal-index header was loaded before the checkpoint was 
    ** performed, then the pager-cache associated with pWal is now
    ** out of date. So zero the cached wal-index header to ensure that
................................................................................
** If op is negative, then do a dry-run of the op==1 case but do
** not actually change anything. The pager uses this to see if it
** should acquire the database exclusive lock prior to invoking
** the op==1 case.
*/
int sqlite3WalExclusiveMode(Wal *pWal, int op){
  int rc;

  assert( pWal->writeLock==0 );
  assert( pWal->exclusiveMode!=WAL_HEAPMEMORY_MODE || op==-1 );

  /* pWal->readLock is usually set, but might be -1 if there was a 
  ** prior error while attempting to acquire are read-lock. This cannot 
  ** happen if the connection is actually in exclusive mode (as no xShmLock
  ** locks are taken in this case). Nor should the pager attempt to
  ** upgrade to exclusive-mode following such an error.
  */
  assert( pWal->readLock!=WAL_LOCK_NONE || pWal->lockError );
  assert( pWal->readLock!=WAL_LOCK_NONE || (op<=0 && pWal->exclusiveMode==0) );

  if( op==0 ){
    if( pWal->exclusiveMode ){
      pWal->exclusiveMode = WAL_NORMAL_MODE;
      if( isWalMode2(pWal) ){
        rc = walLockReader(pWal, pWal->readLock, 1);
      }else{
        rc = walLockShared(pWal, WAL_READ_LOCK(pWal->readLock));
      }
      if( rc!=SQLITE_OK ){
        pWal->exclusiveMode = WAL_EXCLUSIVE_MODE;
      }
      rc = pWal->exclusiveMode==WAL_NORMAL_MODE;
    }else{
      /* Already in locking_mode=NORMAL */
      rc = 0;
    }
  }else if( op>0 ){
    assert( pWal->exclusiveMode==WAL_NORMAL_MODE );
    assert( pWal->readLock>=0 );
    if( isWalMode2(pWal) ){
      walLockReader(pWal, pWal->readLock, 0);
    }else{
      walUnlockShared(pWal, WAL_READ_LOCK(pWal->readLock));
    }
    pWal->exclusiveMode = WAL_EXCLUSIVE_MODE;
    rc = 1;
  }else{
    rc = pWal->exclusiveMode==WAL_NORMAL_MODE;
  }
  return rc;
}
................................................................................
  return (pWal ? pWal->szPage : 0);
}
#endif

/* Return the sqlite3_file object for the WAL file
*/
sqlite3_file *sqlite3WalFile(Wal *pWal){
  return pWal->apWalFd[0];
}

/* 
** Return the values required by sqlite3_wal_info().
*/
int sqlite3WalInfo(Wal *pWal, u32 *pnPrior, u32 *pnFrame){
  int rc = SQLITE_OK;

Changes to src/wal.h.

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/* Macros for extracting appropriate sync flags for either transaction
** commits (WAL_SYNC_FLAGS(X)) or for checkpoint ops (CKPT_SYNC_FLAGS(X)):
*/
#define WAL_SYNC_FLAGS(X)   ((X)&0x03)
#define CKPT_SYNC_FLAGS(X)  (((X)>>2)&0x03)

#ifdef SQLITE_OMIT_WAL
# define sqlite3WalOpen(x,y,z)                   0
# define sqlite3WalLimit(x,y)
# define sqlite3WalClose(v,w,x,y,z)              0
# define sqlite3WalBeginReadTransaction(y,z)     0
# define sqlite3WalEndReadTransaction(z)
# define sqlite3WalDbsize(y)                     0
# define sqlite3WalBeginWriteTransaction(y)      0
# define sqlite3WalEndWriteTransaction(x)        0
................................................................................

/* Connection to a write-ahead log (WAL) file. 
** There is one object of this type for each pager. 
*/
typedef struct Wal Wal;

/* Open and close a connection to a write-ahead log. */
int sqlite3WalOpen(sqlite3_vfs*, sqlite3_file*, const char *, int, i64, Wal**);
int sqlite3WalClose(Wal *pWal, sqlite3*, int sync_flags, int, u8 *);

/* Set the limiting size of a WAL file. */
void sqlite3WalLimit(Wal*, i64);

/* Used by readers to open (lock) and close (unlock) a snapshot.  A 
** snapshot is like a read-transaction.  It is the state of the database







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/* Macros for extracting appropriate sync flags for either transaction
** commits (WAL_SYNC_FLAGS(X)) or for checkpoint ops (CKPT_SYNC_FLAGS(X)):
*/
#define WAL_SYNC_FLAGS(X)   ((X)&0x03)
#define CKPT_SYNC_FLAGS(X)  (((X)>>2)&0x03)

#ifdef SQLITE_OMIT_WAL
# define sqlite3WalOpen(w,x,y,z)                 0
# define sqlite3WalLimit(x,y)
# define sqlite3WalClose(v,w,x,y,z)              0
# define sqlite3WalBeginReadTransaction(y,z)     0
# define sqlite3WalEndReadTransaction(z)
# define sqlite3WalDbsize(y)                     0
# define sqlite3WalBeginWriteTransaction(y)      0
# define sqlite3WalEndWriteTransaction(x)        0
................................................................................

/* Connection to a write-ahead log (WAL) file. 
** There is one object of this type for each pager. 
*/
typedef struct Wal Wal;

/* Open and close a connection to a write-ahead log. */
int sqlite3WalOpen(sqlite3_vfs*, sqlite3_file*, const char *,int,i64,int,Wal**);
int sqlite3WalClose(Wal *pWal, sqlite3*, int sync_flags, int, u8 *);

/* Set the limiting size of a WAL file. */
void sqlite3WalLimit(Wal*, i64);

/* Used by readers to open (lock) and close (unlock) a snapshot.  A 
** snapshot is like a read-transaction.  It is the state of the database

Changes to test/corruptA.test.

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# Corrupt the file header in various ways and make sure the corruption
# is detected when opening the database file.
#
db close
forcecopy test.db test.db-template

set unreadable_version 02
ifcapable wal { set unreadable_version 03 }
do_test corruptA-2.1 {
  forcecopy test.db-template test.db
  hexio_write test.db 19 $unreadable_version   ;# the read format number
  sqlite3 db test.db
  catchsql {SELECT * FROM t1}  
} {1 {file is not a database}}
 







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# Corrupt the file header in various ways and make sure the corruption
# is detected when opening the database file.
#
db close
forcecopy test.db test.db-template

set unreadable_version 02
ifcapable wal { set unreadable_version 04 }
do_test corruptA-2.1 {
  forcecopy test.db-template test.db
  hexio_write test.db 19 $unreadable_version   ;# the read format number
  sqlite3 db test.db
  catchsql {SELECT * FROM t1}  
} {1 {file is not a database}}
 

Changes to test/permutations.test.

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    insert.test   insert2.test  insert3.test rollback.test 
    select1.test  select2.test  select3.test
  }
}

test_suite "wal" -description {
  Run tests with journal_mode=WAL

















} -initialize {
  set ::G(savepoint6_iterations) 100
} -shutdown {
  unset -nocomplain ::G(savepoint6_iterations)
} -files {
  savepoint.test     savepoint2.test     savepoint6.test
  trans.test         avtrans.test







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    insert.test   insert2.test  insert3.test rollback.test 
    select1.test  select2.test  select3.test
  }
}

test_suite "wal" -description {
  Run tests with journal_mode=WAL
} -initialize {
  set ::G(savepoint6_iterations) 100
} -shutdown {
  unset -nocomplain ::G(savepoint6_iterations)
} -files {
  savepoint.test     savepoint2.test     savepoint6.test
  trans.test         avtrans.test

  fts3aa.test  fts3ab.test  fts3ac.test  fts3ad.test
  fts3ae.test  fts3af.test  fts3ag.test  fts3ah.test
  fts3ai.test  fts3aj.test  fts3ak.test  fts3al.test
  fts3am.test  fts3an.test  fts3ao.test  fts3b.test
  fts3c.test   fts3d.test   fts3e.test   fts3query.test 
}

test_suite "wal2" -description {
  Run tests with journal_mode=WAL2
} -initialize {
  set ::G(savepoint6_iterations) 100
} -shutdown {
  unset -nocomplain ::G(savepoint6_iterations)
} -files {
  savepoint.test     savepoint2.test     savepoint6.test
  trans.test         avtrans.test

Changes to test/rdonly.test.

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# returns 1 if the database N of connection D is read-only, 0 if it is
# read/write, or -1 if N is not the name of a database on connection D.
#
do_test rdonly-1.1.1 {
  sqlite3_db_readonly db main
} {0}

# Changes the write version from 1 to 3.  Verify that the database
# can be read but not written.
#
do_test rdonly-1.2 {
  db close
  hexio_get_int [hexio_read test.db 18 1]
} 1
do_test rdonly-1.3 {
  hexio_write test.db 18 03
  sqlite3 db test.db
  execsql {
    SELECT * FROM t1;
  }
} {1}
do_test rdonly-1.3.1 {
  sqlite3_db_readonly db main
................................................................................

# Now, after connection [db] has loaded the database schema, modify the
# write-version of the file (and the change-counter, so that the 
# write-version is reloaded). This way, SQLite does not discover that
# the database is read-only until after it is locked.
#
set ro_version 02
ifcapable wal { set ro_version 03 }
do_test rdonly-1.6 {
  hexio_write test.db 18 $ro_version     ; # write-version
  hexio_write test.db 24 11223344        ; # change-counter
  catchsql {
    INSERT INTO t1 VALUES(2);
  }
} {1 {attempt to write a readonly database}}

finish_test







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# returns 1 if the database N of connection D is read-only, 0 if it is
# read/write, or -1 if N is not the name of a database on connection D.
#
do_test rdonly-1.1.1 {
  sqlite3_db_readonly db main
} {0}

# Changes the write version from 1 to 4.  Verify that the database
# can be read but not written.
#
do_test rdonly-1.2 {
  db close
  hexio_get_int [hexio_read test.db 18 1]
} 1
do_test rdonly-1.3 {
  hexio_write test.db 18 04
  sqlite3 db test.db
  execsql {
    SELECT * FROM t1;
  }
} {1}
do_test rdonly-1.3.1 {
  sqlite3_db_readonly db main
................................................................................

# Now, after connection [db] has loaded the database schema, modify the
# write-version of the file (and the change-counter, so that the 
# write-version is reloaded). This way, SQLite does not discover that
# the database is read-only until after it is locked.
#
set ro_version 02
ifcapable wal { set ro_version 04 }
do_test rdonly-1.6 {
  hexio_write test.db 18 $ro_version     ; # write-version
  hexio_write test.db 24 11223344        ; # change-counter
  catchsql {
    INSERT INTO t1 VALUES(2);
  }
} {1 {attempt to write a readonly database}}

finish_test

Changes to test/savepoint.test.

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do_test savepoint-1.1 {
  wal_set_journal_mode
  execsql {
    SAVEPOINT sp1;
    RELEASE sp1;
  }
} {}

do_test savepoint-1.2 {
  execsql {
    SAVEPOINT sp1;
    ROLLBACK TO sp1;
  }
} {}
do_test savepoint-1.3 {







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do_test savepoint-1.1 {
  wal_set_journal_mode
  execsql {
    SAVEPOINT sp1;
    RELEASE sp1;
  }
} {}
wal_check_journal_mode savepoint-1.1
do_test savepoint-1.2 {
  execsql {
    SAVEPOINT sp1;
    ROLLBACK TO sp1;
  }
} {}
do_test savepoint-1.3 {

Changes to test/savepoint6.test.

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#
# $Id: savepoint6.test,v 1.4 2009/06/05 17:09:12 drh Exp $

set testdir [file dirname $argv0]
source $testdir/tester.tcl

proc sql {zSql} {




  uplevel db eval [list $zSql]
  #puts stderr "$zSql ;"
}

set DATABASE_SCHEMA {
    PRAGMA auto_vacuum = incremental;
    CREATE TABLE t1(x, y);
................................................................................
#   rollback  NAME
#   release   NAME
#
#   insert_rows XVALUES
#   delete_rows XVALUES
#
proc savepoint {zName} {

  catch { sql "SAVEPOINT $zName" }
  lappend ::lSavepoint [list $zName [array get ::aEntry]]
}

proc rollback {zName} {

  catch { sql "ROLLBACK TO $zName" }
  for {set i [expr {[llength $::lSavepoint]-1}]} {$i>=0} {incr i -1} {
    set zSavepoint [lindex $::lSavepoint $i 0]
    if {$zSavepoint eq $zName} {
      unset -nocomplain ::aEntry
      array set ::aEntry [lindex $::lSavepoint $i 1]

................................................................................
      }
      break
    }
  }
}

proc release {zName} {

  catch { sql "RELEASE $zName" }
  for {set i [expr {[llength $::lSavepoint]-1}]} {$i>=0} {incr i -1} {
    set zSavepoint [lindex $::lSavepoint $i 0]
    if {$zSavepoint eq $zName} {
      set ::lSavepoint [lreplace $::lSavepoint $i end]
      break
    }
................................................................................

  if {[llength $::lSavepoint] == 0} {
    #puts stderr "-- End of transaction!!!!!!!!!!!!!"
  }
}

proc insert_rows {lX} {

  foreach x $lX {
    set y [x_to_y $x]

    # Update database [db]
    sql "INSERT OR REPLACE INTO t1 VALUES($x, '$y')"

    # Update the Tcl database.
    set ::aEntry($x) $y
  }
}

proc delete_rows {lX} {

  foreach x $lX {
    # Update database [db]
    sql "DELETE FROM t1 WHERE x = $x"

    # Update the Tcl database.
    unset -nocomplain ::aEntry($x)
  }
................................................................................
  set ret [list]
  for {set i 0} {$i<$nRes} {incr i} {
    lappend ret [expr int(rand()*$nRange)]
  }
  return $ret
} 
#-------------------------------------------------------------------------






proc database_op {} {
  set i [expr int(rand()*2)] 
  if {$i==0} {
    insert_rows [random_integers 100 1000]
  }
  if {$i==1} {
................................................................................
proc savepoint_op {} {
  set names {one two three four five}
  set cmds  {savepoint savepoint savepoint savepoint release rollback}

  set C [lindex $cmds [expr int(rand()*6)]]
  set N [lindex $names [expr int(rand()*5)]]

  #puts stderr "   $C $N ;  "
  #flush stderr

  $C $N
  return ok
}

expr srand(0)

############################################################################







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#
# $Id: savepoint6.test,v 1.4 2009/06/05 17:09:12 drh Exp $

set testdir [file dirname $argv0]
source $testdir/tester.tcl

proc sql {zSql} {
  if {0 && $::debug_op} {
    puts stderr "$zSql ;"
    flush stderr
  }
  uplevel db eval [list $zSql]
  #puts stderr "$zSql ;"
}

set DATABASE_SCHEMA {
    PRAGMA auto_vacuum = incremental;
    CREATE TABLE t1(x, y);
................................................................................
#   rollback  NAME
#   release   NAME
#
#   insert_rows XVALUES
#   delete_rows XVALUES
#
proc savepoint {zName} {
  if {$::debug_op} { puts stderr "savepoint $zName" ; flush stderr }
  catch { sql "SAVEPOINT $zName" }
  lappend ::lSavepoint [list $zName [array get ::aEntry]]
}

proc rollback {zName} {
  if {$::debug_op} { puts stderr "rollback $zName" ; flush stderr }
  catch { sql "ROLLBACK TO $zName" }
  for {set i [expr {[llength $::lSavepoint]-1}]} {$i>=0} {incr i -1} {
    set zSavepoint [lindex $::lSavepoint $i 0]
    if {$zSavepoint eq $zName} {
      unset -nocomplain ::aEntry
      array set ::aEntry [lindex $::lSavepoint $i 1]

................................................................................
      }
      break
    }
  }
}

proc release {zName} {
  if {$::debug_op} { puts stderr "release $zName" ; flush stderr }
  catch { sql "RELEASE $zName" }
  for {set i [expr {[llength $::lSavepoint]-1}]} {$i>=0} {incr i -1} {
    set zSavepoint [lindex $::lSavepoint $i 0]
    if {$zSavepoint eq $zName} {
      set ::lSavepoint [lreplace $::lSavepoint $i end]
      break
    }
................................................................................

  if {[llength $::lSavepoint] == 0} {
    #puts stderr "-- End of transaction!!!!!!!!!!!!!"
  }
}

proc insert_rows {lX} {
  if {$::debug_op} { puts stderr "insert_rows $lX" ; flush stderr }
  foreach x $lX {
    set y [x_to_y $x]

    # Update database [db]
    sql "INSERT OR REPLACE INTO t1 VALUES($x, '$y')"

    # Update the Tcl database.
    set ::aEntry($x) $y
  }
}

proc delete_rows {lX} {
  if {$::debug_op} { puts stderr "delete_rows $lX" ; flush stderr }
  foreach x $lX {
    # Update database [db]
    sql "DELETE FROM t1 WHERE x = $x"

    # Update the Tcl database.
    unset -nocomplain ::aEntry($x)
  }
................................................................................
  set ret [list]
  for {set i 0} {$i<$nRes} {incr i} {
    lappend ret [expr int(rand()*$nRange)]
  }
  return $ret
} 
#-------------------------------------------------------------------------

set ::debug_op 0
proc debug_ops {} {
  set ::debug_op 1
}

proc database_op {} {
  set i [expr int(rand()*2)] 
  if {$i==0} {
    insert_rows [random_integers 100 1000]
  }
  if {$i==1} {
................................................................................
proc savepoint_op {} {
  set names {one two three four five}
  set cmds  {savepoint savepoint savepoint savepoint release rollback}

  set C [lindex $cmds [expr int(rand()*6)]]
  set N [lindex $names [expr int(rand()*5)]]




  $C $N
  return ok
}

expr srand(0)

############################################################################

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# Create a test database
#
proc reset_db {} {
  catch {db close}
  forcedelete test.db
  forcedelete test.db-journal
  forcedelete test.db-wal

  sqlite3 db ./test.db
  set ::DB [sqlite3_connection_pointer db]
  if {[info exists ::SETUP_SQL]} {
    db eval $::SETUP_SQL
  }
}
reset_db
................................................................................
#     Otherwise (if not running a WAL permutation) this is a no-op.
#
#   wal_is_wal_mode
#
#     Returns true if this test should be run in WAL mode. False otherwise.
#
proc wal_is_wal_mode {} {
  expr {[permutation] eq "wal"}


}
proc wal_set_journal_mode {{db db}} {
  if { [wal_is_wal_mode] } {




    $db eval "PRAGMA journal_mode = WAL"





  }
}
proc wal_check_journal_mode {testname {db db}} {
  if { [wal_is_wal_mode] } {
    $db eval { SELECT * FROM sqlite_master }




    do_test $testname [list $db eval "PRAGMA main.journal_mode"] {wal}
  }
}

proc wal_is_capable {} {
  ifcapable !wal { return 0 }
  if {[permutation]=="journaltest"} { return 0 }
  return 1







>







 







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# Create a test database
#
proc reset_db {} {
  catch {db close}
  forcedelete test.db
  forcedelete test.db-journal
  forcedelete test.db-wal
  forcedelete test.db-wal2
  sqlite3 db ./test.db
  set ::DB [sqlite3_connection_pointer db]
  if {[info exists ::SETUP_SQL]} {
    db eval $::SETUP_SQL
  }
}
reset_db
................................................................................
#     Otherwise (if not running a WAL permutation) this is a no-op.
#
#   wal_is_wal_mode
#
#     Returns true if this test should be run in WAL mode. False otherwise.
#
proc wal_is_wal_mode {} {
  if {[permutation] eq "wal"} { return 1 }
  if {[permutation] eq "wal2"} { return 2 }
  return 0
}
proc wal_set_journal_mode {{db db}} {
  switch -- [wal_is_wal_mode] {
    0 {
    }

    1 {
      $db eval "PRAGMA journal_mode = WAL"
    }

    2 {
      $db eval "PRAGMA journal_mode = WAL2"
    }
  }
}
proc wal_check_journal_mode {testname {db db}} {
  if { [wal_is_wal_mode] } {
    $db eval { SELECT * FROM sqlite_master }
    set expected "wal"
    if {[wal_is_wal_mode]==2} {
      set expected "wal2"
    }
    do_test $testname [list $db eval "PRAGMA main.journal_mode"] $expected
  }
}

proc wal_is_capable {} {
  ifcapable !wal { return 0 }
  if {[permutation]=="journaltest"} { return 0 }
  return 1

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      CREATE TABLE aux.t2(a, b);
      PRAGMA main.journal_mode = WAL;
      PRAGMA aux.journal_mode = WAL;
      INSERT INTO t1 VALUES('x', 'y');
      INSERT INTO t2 VALUES('x', 'y');
    }
    lsort [array names ::T1]
  } {test.db1 test.db1-journal test.db1-wal}
  
  do_test 5.1.2 {
    lsort [array names ::T2]
  } {test.db2 test.db2-journal test.db2-wal}
  db close
  
  tvfs1 delete
  tvfs2 delete
}

#-------------------------------------------------------------------------







|



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      CREATE TABLE aux.t2(a, b);
      PRAGMA main.journal_mode = WAL;
      PRAGMA aux.journal_mode = WAL;
      INSERT INTO t1 VALUES('x', 'y');
      INSERT INTO t2 VALUES('x', 'y');
    }
    lsort [array names ::T1]
  } {test.db1 test.db1-journal test.db1-wal test.db1-wal2}
  
  do_test 5.1.2 {
    lsort [array names ::T2]
  } {test.db2 test.db2-journal test.db2-wal test.db2-wal2}
  db close
  
  tvfs1 delete
  tvfs2 delete
}

#-------------------------------------------------------------------------

Added test/wal2rewrite.test.

























































































































































































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# 2017 September 19
#
# The author disclaims copyright to this source code.  In place of
# a legal notice, here is a blessing:
#
#    May you do good and not evil.
#    May you find forgiveness for yourself and forgive others.
#    May you share freely, never taking more than you give.
#
#***********************************************************************
# This file implements regression tests for SQLite library.  The
# focus of this file is testing the operation of the library in
# "PRAGMA journal_mode=WAL2" mode.
#

set testdir [file dirname $argv0]
source $testdir/tester.tcl
source $testdir/lock_common.tcl
source $testdir/malloc_common.tcl
source $testdir/wal_common.tcl

set testprefix wal2rewrite
ifcapable !wal {finish_test ; return }

proc filesize {filename} {
  if {[file exists $filename]} {
    return [file size $filename]
  }
  return 0
}

foreach {tn jrnlmode} {
  1 wal
  2 wal2
} {
  reset_db
  execsql "PRAGMA journal_mode = $jrnlmode"
  do_execsql_test $tn.1 {
    PRAGMA journal_size_limit = 10000;
    PRAGMA cache_size = 5;
    PRAGMA wal_autocheckpoint = 10;
  
    CREATE TABLE t1(a INTEGER PRIMARY KEY, b INTEGER, c BLOB);
    CREATE INDEX t1b ON t1(b);
    CREATE INDEX t1c ON t1(c);
  
    WITH s(i) AS (
      SELECT 1 UNION SELECT i+1 FROM s WHERE i<10
    )
    INSERT INTO t1 SELECT i, i, randomblob(800) FROM s;
  } {10000 10}
  
  for {set i 0} {$i < 4} {incr i} {
    do_execsql_test $tn.$i.1 {
      UPDATE t1 SET c=randomblob(800) WHERE (b%10)==5 AND ($i%2)
    }
    do_execsql_test $tn.$i.2 {
      BEGIN;
      UPDATE t1 SET b=b+10, c=randomblob(800);
      UPDATE t1 SET b=b+10, c=randomblob(800);
      UPDATE t1 SET b=b+10, c=randomblob(800);
      UPDATE t1 SET b=b+10, c=randomblob(800);
      UPDATE t1 SET b=b+10, c=randomblob(800);
      UPDATE t1 SET b=b+10, c=randomblob(800);
      UPDATE t1 SET b=b+10, c=randomblob(800);
      UPDATE t1 SET b=b+10, c=randomblob(800);
      UPDATE t1 SET b=b+10, c=randomblob(800);
      UPDATE t1 SET b=b+10, c=randomblob(800);
    }
    execsql COMMIT

    do_test $tn.$i.3 { expr [filesize test.db-wal]  < 100000 } 1
    do_test $tn.$i.4 { expr [filesize test.db-wal2] < 100000 } 1

    set sum [db eval {SELECT sum(b), md5sum(c) FROM t1}]

    do_test $tn.$i.5 {
      foreach f [glob -nocomplain test.db2*] {forcedelete $f}
      foreach f [glob -nocomplain test.db*] {
        forcecopy $f [string map {test.db test.db2} $f]
      }

      sqlite3 db2 test.db2
      db2 eval {SELECT sum(b), md5sum(c) FROM t1}
    } $sum
    db2 close
  }
}
    


finish_test

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# 2017 September 19
#
# The author disclaims copyright to this source code.  In place of
# a legal notice, here is a blessing:
#
#    May you do good and not evil.
#    May you find forgiveness for yourself and forgive others.
#    May you share freely, never taking more than you give.
#
#***********************************************************************
# This file implements regression tests for SQLite library.  The
# focus of this file is testing the operation of the library in
# "PRAGMA journal_mode=WAL2" mode.
#

set testdir [file dirname $argv0]
source $testdir/tester.tcl
source $testdir/lock_common.tcl
source $testdir/malloc_common.tcl
source $testdir/wal_common.tcl

set testprefix wal2simple
ifcapable !wal {finish_test ; return }

#-------------------------------------------------------------------------
# The following tests verify that a client can switch in and out of wal
# and wal2 mode. But that it is not possible to change directly from wal
# to wal2, or from wal2 to wal mode.
#
do_execsql_test 1.1.0 {
  PRAGMA journal_mode = wal2
} {wal2}
execsql { SELECT * FROM sqlite_master} 
do_execsql_test 1.x {
  PRAGMA journal_mode;
  PRAGMA main.journal_mode;
} {wal2 wal2}
db close
do_test 1.1.1 { file size test.db } {1024}
do_test 1.1.2 { hexio_read test.db 18 2 } 0303

sqlite3 db test.db
do_execsql_test 1.2.0 {
  SELECT * FROM sqlite_master;
  PRAGMA journal_mode = delete;
} {delete}
db close
do_test 1.2.1 { file size test.db } {1024}
do_test 1.2.2 { hexio_read test.db 18 2 } 0101

sqlite3 db test.db
do_execsql_test 1.3.0 {
  SELECT * FROM sqlite_master;
  PRAGMA journal_mode = wal;
} {wal}
db close
do_test 1.3.1 { file size test.db } {1024}
do_test 1.3.2 { hexio_read test.db 18 2 } 0202

sqlite3 db test.db
do_catchsql_test 1.4.0 {
  PRAGMA journal_mode = wal2;
} {1 {cannot change from wal to wal2 mode}}
do_execsql_test 1.4.1 {
  PRAGMA journal_mode = wal;
  PRAGMA journal_mode = delete;
  PRAGMA journal_mode = wal2;
  PRAGMA journal_mode = wal2;
} {wal delete wal2 wal2}
do_catchsql_test 1.4.2 {
  PRAGMA journal_mode = wal;
} {1 {cannot change from wal2 to wal mode}}
db close
do_test 1.4.3 { hexio_read test.db 18 2 } 0303

#-------------------------------------------------------------------------
# Test that recovery in wal2 mode works.
#
forcedelete test.db test.db-wal test.db-wal2
reset_db
do_execsql_test 2.0 {
  CREATE TABLE t1(a INTEGER PRIMARY KEY, b);
  PRAGMA journal_mode = wal2;
  PRAGMA journal_size_limit = 5000;
} {wal2 5000}

proc wal_hook {DB nm nFrame} { $DB eval { PRAGMA wal_checkpoint } }
db wal_hook {wal_hook db}

for {set i 1} {$i <= 200} {incr i} {
  execsql { INSERT INTO t1 VALUES(NULL, randomblob(100)) }
  set res [db eval { SELECT sum(a), md5sum(b) FROM t1 }]

  do_test 2.1.$i {
    foreach f [glob -nocomplain test.db2*] { forcedelete $f }
    forcecopy test.db      test.db2
    forcecopy test.db-wal  test.db2-wal
    forcecopy test.db-wal2 test.db2-wal2

    sqlite3 db2 test.db2
    db2 eval { SELECT sum(a), md5sum(b) FROM t1 }
  } $res

  db2 close
}

#-------------------------------------------------------------------------

reset_db
do_execsql_test 3.0 {
  CREATE TABLE t1(x BLOB, y INTEGER PRIMARY KEY);
  CREATE INDEX i1 ON t1(x);
  PRAGMA cache_size = 5;
  PRAGMA journal_mode = wal2;
} {wal2}

breakpoint
do_test 3.1 {
  execsql BEGIN
  for {set i 1} {$i < 1000} {incr i} {
    execsql { INSERT INTO t1 VALUES(randomblob(800), $i) }
  }
  execsql COMMIT
} {}

do_execsql_test 3.2 {
  PRAGMA integrity_check;
} {ok}

#-------------------------------------------------------------------------
catch { db close }
foreach f [glob -nocomplain test.db*] { forcedelete $f }
reset_db
do_execsql_test 4.0 {
  CREATE TABLE t1(x, y);
  PRAGMA journal_mode = wal2;
} {wal2}

do_execsql_test 4.1 {
  SELECT * FROM t1;
} {}

do_execsql_test 4.2 {
  INSERT INTO t1 VALUES(1, 2);
} {}

do_execsql_test 4.3 {
  SELECT * FROM t1;
} {1 2}

do_test 4.4 {
  sqlite3 db2 test.db
  execsql { SELECT * FROM t1 } db2
} {1 2}

do_test 4.5 {
  lsort [glob test.db*]
} {test.db test.db-shm test.db-wal test.db-wal2}

do_test 4.6 {
  db close
  db2 close
  sqlite3 db test.db
  execsql { SELECT * FROM t1 }
} {1 2}

do_execsql_test 4.7 {
  PRAGMA journal_size_limit = 4000;
  INSERT INTO t1 VALUES(3, 4);
  INSERT INTO t1 VALUES(5, 6);
  INSERT INTO t1 VALUES(7, 8);
  INSERT INTO t1 VALUES(9, 10);
  INSERT INTO t1 VALUES(11, 12);
  INSERT INTO t1 VALUES(13, 14);
  INSERT INTO t1 VALUES(15, 16);
  INSERT INTO t1 VALUES(17, 18);
  SELECT * FROM t1;
} {4000 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18}

do_test 4.8 {
  sqlite3 db2 test.db
  execsql { SELECT * FROM t1 } db2
} {1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18}

do_test 4.9 {
  db close
  db2 close
  lsort [glob test.db*]
} {test.db}

#-------------------------------------------------------------------------
reset_db
do_execsql_test 5.0 {
  CREATE TABLE t1(a INTEGER PRIMARY KEY, b, c);
  CREATE INDEX i1 ON t1(b, c);
  PRAGMA journal_mode = wal2;
  PRAGMA journal_size_limit = 4000;
} {wal2 4000}

proc wal_hook {DB nm nFrame} {
  $DB eval { PRAGMA wal_checkpoint }
}
db wal_hook [list wal_hook db]


foreach js {4000 8000 12000} {
  foreach NROW [list 100 200 300 400 500 600 1000] {
    do_test 5.$js.$NROW.1 {
      db eval "DELETE FROM t1"
      db eval "PRAGMA journal_size_limit = $js"
      set nTotal 0
      for {set i 0} {$i < $NROW} {incr i} {
        db eval { INSERT INTO t1 VALUES($i, $i, randomblob(abs(random()%50))) }
        incr nTotal $i
      }
      set {} {}
    } {}

    do_test 5.$js.$NROW.2 {
      sqlite3 db2 test.db
      db2 eval { 
        PRAGMA integrity_check;
        SELECT count(*), sum(b) FROM t1;
      }
    } [list ok $NROW $nTotal]

    db2 close
  }
}


#-------------------------------------------------------------------------
reset_db
do_execsql_test 6.0 {
  CREATE TABLE tx(x);
  PRAGMA journal_mode = wal2;
  PRAGMA journal_size_limit = 3500;
} {wal2 3500}

do_test 6.1 {
  for {set i 0} {$i < 10} {incr i} {
    execsql "CREATE TABLE t$i (x);"
  }
} {}

puts "[file size test.db-wal] [file size test.db-wal2]"

do_test 6.2.1 {
  foreach f [glob -nocomplain test.db2*] { forcedelete $f }
  forcecopy test.db-wal2 test.db2-wal2
  sqlite3 db2 test.db2
  db2 eval { SELECT * FROM sqlite_master }
} {}
do_test 6.2.2 {
  db2 eval {
    PRAGMA journal_mode = wal2;
    SELECT * FROM sqlite_master;
  }
} {wal2}

do_test 6.3.1 {
  db2 close
  foreach f [glob -nocomplain test.db2*] { forcedelete $f }
  forcecopy test.db-wal2 test.db2-wal2
  forcecopy test.db test.db2
  sqlite3 db2 test.db2
  db2 eval { SELECT * FROM sqlite_master }
} {table tx tx 2 {CREATE TABLE tx(x)}}
do_test 6.3.2 {
  db2 eval {
    PRAGMA journal_mode = wal2;
    SELECT * FROM sqlite_master;
  }
} {wal2 table tx tx 2 {CREATE TABLE tx(x)}}

do_test 6.4.1 {
  db2 close
  foreach f [glob -nocomplain test.db2*] { forcedelete $f }
  forcecopy test.db-wal2 test.db2-wal2
  forcecopy test.db-wal test.db2-wal
  sqlite3 db2 test.db2
  db2 eval { SELECT * FROM sqlite_master }
} {}
do_test 6.4.2 {
  db2 eval {
    PRAGMA journal_mode = wal2;
    SELECT * FROM sqlite_master;
  }
} {wal2}
db2 close

finish_test

Changes to test/walprotocol2.test.

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#
proc lock_callback {method filename handle lock} {
  if {$lock=="0 1 lock exclusive"} {
    proc lock_callback {method filename handle lock} {}
    db2 eval { INSERT INTO x VALUES('x') }
  }
}
db timeout 10
do_catchsql_test 2.4 {
  BEGIN EXCLUSIVE;
} {0 {}}
do_execsql_test 2.5 {
  SELECT * FROM x;
  COMMIT;
} {z y x}

finish_test







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#
proc lock_callback {method filename handle lock} {
  if {$lock=="0 1 lock exclusive"} {
    proc lock_callback {method filename handle lock} {}
    db2 eval { INSERT INTO x VALUES('x') }
  }
}
db timeout 1100
do_catchsql_test 2.4 {
  BEGIN EXCLUSIVE;
} {0 {}}
do_execsql_test 2.5 {
  SELECT * FROM x;
  COMMIT;
} {z y x}

finish_test