SQLite

    ** 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->doNotUseWAL==0 ){
      int isOpen = 0;
      rc = sqlite3PagerOpenLog(pBt->pPager, &isOpen);
      if( rc!=SQLITE_OK ){
        goto page1_init_failed;
      }else if( isOpen==0 ){
        releasePage(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( pBtree->inTrans==TRANS_NONE );

  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->doNotUseWAL = (iVersion==1);

  rc = sqlite3BtreeBeginTrans(pBtree, 0);
  if( rc==SQLITE_OK ){
    u8 *aData = pBt->pPage1->aData;
    if( aData[18]!=(u8)iVersion || aData[19]!=(u8)iVersion ){
      rc = sqlite3BtreeBeginTrans(pBtree, 1);
      if( rc==SQLITE_OK ){
        rc = sqlite3PagerWrite(pBt->pPage1->pDbPage);
        if( rc==SQLITE_OK ){
          aData[18] = (u8)iVersion;
          aData[19] = (u8)iVersion;
        }
      }
    }
  }

  pBt->doNotUseWAL = 0;
  return rc;
}

  u16 pageSize;         /* Total number of bytes on a page */
  u16 usableSize;       /* Number of usable bytes on each page */
  u16 maxLocal;         /* Maximum local payload in non-LEAFDATA tables */
  u16 minLocal;         /* Minimum local payload in non-LEAFDATA tables */
  u16 maxLeaf;          /* Maximum local payload in a LEAFDATA table */
  u16 minLeaf;          /* Minimum local payload in a LEAFDATA table */
  u8 inTransaction;     /* Transaction state */
  u8 doNotUseWAL;       /* If true, do not open write-ahead-log file */
  int nTransaction;     /* Number of open transactions (read + write) */
  u32 nPage;            /* Number of pages in the database */
  void *pSchema;        /* Pointer to space allocated by sqlite3BtreeSchema() */
  void (*xFreeSchema)(void*);  /* Destructor for BtShared.pSchema */
  sqlite3_mutex *mutex; /* Non-recursive mutex required to access this struct */
  Bitvec *pHasContent;  /* Set of pages moved to free-list this transaction */
#ifndef SQLITE_OMIT_SHARED_CACHE

        logSummaryUnmap(pSummary, 1);
      }else{
        if( rc==SQLITE_BUSY ){
          rc = SQLITE_OK;
        }
        logSummaryUnmap(pSummary, 0);
      }


      sqlite3_mutex_free(pSummary->mutex);
      sqlite3_free(pSummary);
    }else{
      sqlite3_mutex_leave(mutex);
    }

  /* The changeCountDone flag is always set for temp-files */
  assert( pPager->tempFile==0 || pPager->changeCountDone );

  return 1;
}
#endif












/*
** Return true if it is necessary to write page *pPg into the sub-journal.
** A page needs to be written into the sub-journal if there exists one
** or more open savepoints for which:
**
**   * The page-number is less than or equal to PagerSavepoint.nOrig, and

        ** Variable iNextHdrOffset is set to the offset at which this
        ** problematic header will occur, if it exists. aMagic is used 
        ** as a temporary buffer to inspect the first couple of bytes of
        ** the potential journal header.
        */
        i64 iNextHdrOffset;
        u8 aMagic[8];
        u8 zHeader[sizeof(aJournalMagic)+4];

        memcpy(zHeader, aJournalMagic, sizeof(aJournalMagic));
        put32bits(&zHeader[sizeof(aJournalMagic)], pPager->nRec);

        iNextHdrOffset = journalHdrOffset(pPager);
        rc = sqlite3OsRead(pPager->jfd, aMagic, 8, iNextHdrOffset);
        if( rc==SQLITE_OK && 0==memcmp(aMagic, aJournalMagic, 8) ){
          static const u8 zerobyte = 0;
          rc = sqlite3OsWrite(pPager->jfd, &zerobyte, 1, iNextHdrOffset);
        }

          IOTRACE(("JSYNC %p\n", pPager))
          rc = sqlite3OsSync(pPager->jfd, pPager->sync_flags);
          if( rc!=SQLITE_OK ) return rc;
        }
        IOTRACE(("JHDR %p %lld\n", pPager, pPager->journalHdr));
        rc = sqlite3OsWrite(
            pPager->jfd, zHeader, sizeof(zHeader), pPager->journalHdr
        );
        if( rc!=SQLITE_OK ) return rc;
      }
      if( 0==(iDc&SQLITE_IOCAP_SEQUENTIAL) ){
        PAGERTRACE(("SYNC journal of %d\n", PAGERID(pPager)));
        IOTRACE(("JSYNC %p\n", pPager))
        rc = sqlite3OsSync(pPager->jfd, pPager->sync_flags| 
          (pPager->sync_flags==SQLITE_SYNC_FULL?SQLITE_SYNC_DATAONLY:0)

  int rc;                         /* Return code */

  /* Check that a SHARED lock is held on the database file. Because an
  ** EXCLUSIVE lock on the db file is required to delete a WAL, this
  ** ensures there is no race condition between the xAccess() below and
  ** an xDelete() being executed by some other connection.
  */
  assert( pPager->state>=PAGER_SHARED );

  if( !pPager->tempFile ){
    char *zLog = sqlite3_mprintf("%s-wal", pPager->zFilename);
    if( !zLog ){
      rc = SQLITE_NOMEM;
    }else{
      rc = sqlite3OsAccess(pPager->pVfs, zLog, SQLITE_ACCESS_EXISTS, pExists);

**
** The caller must be holding a SHARED lock on the database file to call
** this function.
*/
int sqlite3PagerOpenLog(Pager *pPager, int *pisOpen){
  int rc = SQLITE_OK;             /* Return code */

  assert( pPager->state>=PAGER_SHARED );
  if( !pPager->pLog ){

    /* Open the connection to the log file. If this operation fails, 
    ** (e.g. due to malloc() failure), unlock the database file and 
    ** return an error code.
    */
    rc = sqlite3LogOpen(pPager->pVfs, pPager->zFilename, &pPager->pLog);

** If successful, the EXCLUSIVE lock is not released before returning.
*/
int sqlite3PagerCloseLog(Pager *pPager){
  int rc = SQLITE_OK;
  if( pPager->pLog ){

    /* Try to obtain an EXCLUSIVE lock on the database file. */
    rc = sqlite3OsLock(pPager->fd, SQLITE_LOCK_EXCLUSIVE);

    /* If the EXCLUSIVE lock was obtained, checkpoint and close the log. */
    if( rc==SQLITE_OK ){
      rc = sqlite3LogClose(pPager->pLog, pPager->fd,
        (pPager->noSync ? 0 : pPager->sync_flags), 
        (u8*)pPager->pTmpSpace
      );
      pPager->pLog = 0;
    }


  }
  return rc;
}

#endif /* SQLITE_OMIT_DISKIO */

          sqlite3PagerJournalMode(pPager, PAGER_JOURNALMODE_DELETE);
        }
  
        /* Open a transaction on the database file. Regardless of the journal
        ** mode, this transaction always uses a rollback journal.
        */
        assert( sqlite3BtreeIsInTrans(pBt)==0 );



        rc = sqlite3BtreeSetVersion(pBt, (eNew==PAGER_JOURNALMODE_WAL ? 2 : 1));
        if( rc!=SQLITE_OK ) goto abort_due_to_error;
      }
    }
  }

  eNew = sqlite3PagerJournalMode(pPager, eNew);
  pOut = &aMem[pOp->p2];
  pOut->flags = MEM_Str|MEM_Static|MEM_Term;
  pOut->z = (char *)sqlite3JournalModename(eNew);
  pOut->n = sqlite3Strlen30(pOut->z);
  pOut->enc = SQLITE_UTF8;
  sqlite3VdbeChangeEncoding(pOut, encoding);
  break;
};  

#if !defined(SQLITE_OMIT_VACUUM) && !defined(SQLITE_OMIT_ATTACH)

  execsql {
    PRAGMA cache_size = 10;
    BEGIN;
      INSERT INTO t1 SELECT blob(900) FROM t1;   -- 32
      SELECT count(*) FROM t1;
  }
  list [expr [file size test.db]/1024] [file size test.db-wal]
} [list 37 [log_file_size 35 1024]]
do_test wal-11.11 {
  execsql {
      SELECT count(*) FROM t1;
    ROLLBACK;
    SELECT count(*) FROM t1;
  }
} {32 16}
do_test wal-11.12 {
  list [expr [file size test.db]/1024] [file size test.db-wal]
} [list 37 [log_file_size 35 1024]]
do_test wal-11.13 {
  execsql {
    INSERT INTO t1 VALUES( blob(900) );
    SELECT count(*) FROM t1;
    PRAGMA integrity_check;
  }
} {17 ok}
do_test wal-11.14 {
  list [expr [file size test.db]/1024] [file size test.db-wal]
} [list 37 [log_file_size 35 1024]]


#-------------------------------------------------------------------------
# This block of tests, wal-12.*, tests the fix for a problem that 
# could occur if a log that is a prefix of an older log is written 
# into a reused log file.
#

# "PRAGMA journal_mode=WAL" mode.
#

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

do_test walmode-1.1 {
  set sqlite_sync_count 0
  execsql { PRAGMA page_size = 1024 }
  execsql { PRAGMA journal_mode = wal }
} {wal}
do_test walmode-1.2 {
  file size test.db
} {1024}
do_test walmode-1.3 {
  set sqlite_sync_count
} {4}
do_test walmode-1.4 {
  file exists test.db-wal
} {0}
do_test walmode-1.5 {
  execsql { CREATE TABLE t1(a, b) }
  file size test.db
} {1024}
do_test walmode-1.6 {
  file exists test.db-wal
} {1}
do_test walmode-1.7 {
  db close
  file exists test.db-wal
} {0}

# There is now a database file with the read and write versions set to 2
# in the file system. This file should default to WAL mode.
#
do_test walmode-2.1 {
  sqlite3 db test.db
  file exists test.db-wal
} {0}
do_test walmode-2.2 {
  execsql { SELECT * FROM sqlite_master }
  file exists test.db-wal
} {1}
do_test walmode-2.3 {
  db close
  file exists test.db-wal
} {0}

# If the first statement executed is "PRAGMA journal_mode = wal", and
# the file is already configured for WAL (read and write versions set
# to 2), then there should be no need to write the database. The 
# statement should cause the client to connect to the log file.
#
set sqlite_sync_count 0
do_test walmode-3.1 {
  sqlite3 db test.db
  execsql { PRAGMA journal_mode = wal }
} {wal}
do_test walmode-3.2 {
  list $sqlite_sync_count [file exists test.db-wal] [file size test.db-wal]
} {0 1 0}

do_test walmode-4.1 {
  execsql { INSERT INTO t1 VALUES(1, 2) }
  execsql { PRAGMA journal_mode = persist }
} {persist}
do_test walmode-4.2 {
  list [file exists test.db-journal] [file exists test.db-wal]
} {1 0}
do_test walmode-4.3 {
  execsql { SELECT * FROM t1 }
} {1 2}
do_test walmode-4.4 {
  db close
  sqlite3 db test.db
  execsql { SELECT * FROM t1 }
} {1 2}
do_test walmode-4.5 {
  list [file exists test.db-journal] [file exists test.db-wal]
} {1 0}

finish_test