SQLite

  const char *zFile;
  char *zPath = 0;
  char *zErr = 0;
  unsigned int flags;
  Db *aNew;
  char *zErrDyn = 0;
  sqlite3_vfs *pVfs;
  const char *zVfs = db->pVfs->zName;       /* Name of default (main) VFS */
  int btflags = 0;

  UNUSED_PARAMETER(NotUsed);

  zFile = (const char *)sqlite3_value_text(argv[0]);
  zName = (const char *)sqlite3_value_text(argv[1]);
  if( zFile==0 ) zFile = "";
  if( zName==0 ) zName = "";

  memset(aNew, 0, sizeof(*aNew));

  /* Open the database file. If the btree is successfully opened, use
  ** it to obtain the database schema. At this point the schema may
  ** or may not be initialised.
  */
  flags = db->openFlags;
  rc = sqlite3ParseUri(zVfs, zFile, &flags, &btflags, &pVfs, &zPath, &zErr);
  if( rc!=SQLITE_OK ){
    if( rc==SQLITE_NOMEM ) db->mallocFailed = 1;
    sqlite3_result_error(context, zErr, -1);
    sqlite3_free(zErr);
    return;
  }
  assert( pVfs );
  flags |= SQLITE_OPEN_MAIN_DB;
  rc = sqlite3BtreeOpen(pVfs, zPath, db, &aNew->pBt, btflags, flags);
  sqlite3_free( zPath );
  db->nDb++;
  if( rc==SQLITE_CONSTRAINT ){
    rc = SQLITE_ERROR;
    zErrDyn = sqlite3MPrintf(db, "database is already attached");
  }else if( rc==SQLITE_OK ){
    Pager *pPager;

** pager.h.
*/
#define BTREE_OMIT_JOURNAL  1  /* Do not create or use a rollback journal */
#define BTREE_NO_READLOCK   2  /* Omit readlocks on readonly files */
#define BTREE_MEMORY        4  /* This is an in-memory DB */
#define BTREE_SINGLE        8  /* The file contains at most 1 b-tree */
#define BTREE_UNORDERED    16  /* Use of a hash implementation is OK */
#define BTREE_READONLYSHM  32  /* Read-only SHM access is acceptable */

int sqlite3BtreeClose(Btree*);
int sqlite3BtreeSetCacheSize(Btree*,int);
int sqlite3BtreeSetSafetyLevel(Btree*,int,int,int);
int sqlite3BtreeSyncDisabled(Btree*);
int sqlite3BtreeSetPageSize(Btree *p, int nPagesize, int nReserve, int eFix);
int sqlite3BtreeGetPageSize(Btree*);

** message. It is the responsibility of the caller to eventually release
** this buffer by calling sqlite3_free().
*/
int sqlite3ParseUri(
  const char *zDefaultVfs,        /* VFS to use if no "vfs=xxx" query option */
  const char *zUri,               /* Nul-terminated URI to parse */
  unsigned int *pFlags,           /* IN/OUT: SQLITE_OPEN_XXX flags */
  int *pBtflags,                  /* IN/OUT: BTREE_XXX flags */
  sqlite3_vfs **ppVfs,            /* OUT: VFS to use */ 
  char **pzFile,                  /* OUT: Filename component of URI */
  char **pzErrMsg                 /* OUT: Error message (if rc!=SQLITE_OK) */
){
  int rc = SQLITE_OK;
  unsigned int flags = *pFlags;
  const char *zVfs = zDefaultVfs;

    while( zOpt[0] ){
      int nOpt = sqlite3Strlen30(zOpt);
      char *zVal = &zOpt[nOpt+1];
      int nVal = sqlite3Strlen30(zVal);

      if( nOpt==3 && memcmp("vfs", zOpt, 3)==0 ){
        zVfs = zVal;
      }else if( nOpt==12 && memcmp("readonly_shm", zOpt, 12)==0 ){
        if( sqlite3Atoi(zVal) ){
          *pBtflags |= BTREE_READONLYSHM;
        }else{
          *pBtflags &= ~BTREE_READONLYSHM;
        }
      }else{
        struct OpenMode {
          const char *z;
          int mode;
        } *aMode = 0;
        char *zModeType;
        int mask;

  const char *zVfs       /* Name of the VFS to use */
){
  sqlite3 *db;                    /* Store allocated handle here */
  int rc;                         /* Return code */
  int isThreadsafe;               /* True for threadsafe connections */
  char *zOpen = 0;                /* Filename argument to pass to BtreeOpen() */
  char *zErrMsg = 0;              /* Error message from sqlite3ParseUri() */
  int btflags = 0;                /* Mask of BTREE_XXX flags */

  *ppDb = 0;
#ifndef SQLITE_OMIT_AUTOINIT
  rc = sqlite3_initialize();
  if( rc ) return rc;
#endif

  assert( db->pDfltColl!=0 );

  /* Also add a UTF-8 case-insensitive collation sequence. */
  createCollation(db, "NOCASE", SQLITE_UTF8, SQLITE_COLL_NOCASE, 0,
                  nocaseCollatingFunc, 0);

  /* Parse the filename/URI argument. */
  rc = sqlite3ParseUri(
      zVfs, zFilename, &flags, &btflags, &db->pVfs, &zOpen, &zErrMsg);
  if( rc!=SQLITE_OK ){
    if( rc==SQLITE_NOMEM ) db->mallocFailed = 1;
    sqlite3Error(db, rc, zErrMsg ? "%s" : 0, zErrMsg);
    sqlite3_free(zErrMsg);
    goto opendb_out;
  }

  /* Open the backend database driver */
  db->openFlags = flags;
  rc = sqlite3BtreeOpen(db->pVfs, zOpen, db, &db->aDb[0].pBt, btflags,
                        flags | SQLITE_OPEN_MAIN_DB);
  if( rc!=SQLITE_OK ){
    if( rc==SQLITE_IOERR_NOMEM ){
      rc = SQLITE_NOMEM;
    }
    sqlite3Error(db, rc, 0);
    goto opendb_out;

  u8 noSync;                  /* Do not sync the journal if true */
  u8 fullSync;                /* Do extra syncs of the journal for robustness */
  u8 ckptSyncFlags;           /* SYNC_NORMAL or SYNC_FULL for checkpoint */
  u8 syncFlags;               /* SYNC_NORMAL or SYNC_FULL otherwise */
  u8 tempFile;                /* zFilename is a temporary file */
  u8 readOnly;                /* True for a read-only database */
  u8 memDb;                   /* True to inhibit all file I/O */
  u8 readOnlyShm;             /* True if read-only shm access is Ok */

  /**************************************************************************
  ** The following block contains those class members that change during
  ** routine opertion.  Class members not in this block are either fixed
  ** when the pager is first created or else only change when there is a
  ** significant mode change (such as changing the page_size, locking_mode,
  ** or the journal_mode).  From another view, these class members describe

** makes a snapshot of the database at the current point in time and preserves
** that snapshot for use by the reader in spite of concurrently changes by
** other writers or checkpointers.
*/
static int pagerBeginReadTransaction(Pager *pPager){
  int rc;                         /* Return code */
  int changed = 0;                /* True if cache must be reset */
  Wal *pWal = pPager->pWal;

  assert( pagerUseWal(pPager) );
  assert( pPager->eState==PAGER_OPEN || pPager->eState==PAGER_READER );

  /* sqlite3WalEndReadTransaction() was not called for the previous
  ** transaction in locking_mode=EXCLUSIVE.  So call it now.  If we
  ** are in locking_mode=NORMAL and EndRead() was previously called,
  ** the duplicate call is harmless.
  */
  sqlite3WalEndReadTransaction(pWal);

  rc = sqlite3WalBeginReadTransaction(pWal, pPager->readOnlyShm, &changed);
  if( rc!=SQLITE_OK || changed ){
    pager_reset(pPager);
  }

  return rc;
}
#endif

    memcpy(pPager->zWal, zPathname, nPathname);
    memcpy(&pPager->zWal[nPathname], "-wal", 4);
#endif
    sqlite3_free(zPathname);
  }
  pPager->pVfs = pVfs;
  pPager->vfsFlags = vfsFlags;
  pPager->readOnlyShm = (flags & PAGER_READONLYSHM)!=0;

  /* Open the pager file.
  */
  if( zFilename && zFilename[0] ){
    int fout = 0;                    /* VFS flags returned by xOpen() */
    rc = sqlite3OsOpen(pVfs, pPager->zFilename, pPager->fd, vfsFlags, &fout);
    assert( !memDb );

** Allowed values for the flags parameter to sqlite3PagerOpen().
**
** NOTE: These values must match the corresponding BTREE_ values in btree.h.
*/
#define PAGER_OMIT_JOURNAL  0x0001    /* Do not use a rollback journal */
#define PAGER_NO_READLOCK   0x0002    /* Omit readlocks on readonly files */
#define PAGER_MEMORY        0x0004    /* In-memory database */
#define PAGER_READONLYSHM   0x0020    /* Read-only SHM access is acceptable */

/*
** Valid values for the second argument to sqlite3PagerLockingMode().
*/
#define PAGER_LOCKINGMODE_QUERY      -1
#define PAGER_LOCKINGMODE_NORMAL      0
#define PAGER_LOCKINGMODE_EXCLUSIVE   1

void sqlite3AddNotNull(Parse*, int);
void sqlite3AddPrimaryKey(Parse*, ExprList*, int, int, int);
void sqlite3AddCheckConstraint(Parse*, Expr*);
void sqlite3AddColumnType(Parse*,Token*);
void sqlite3AddDefaultValue(Parse*,ExprSpan*);
void sqlite3AddCollateType(Parse*, Token*);
void sqlite3EndTable(Parse*,Token*,Token*,Select*);
int sqlite3ParseUri(const char*,const char*,unsigned int*,int*,
                    sqlite3_vfs**,char**,char **);

Bitvec *sqlite3BitvecCreate(u32);
int sqlite3BitvecTest(Bitvec*, u32);
int sqlite3BitvecSet(Bitvec*, u32);
void sqlite3BitvecClear(Bitvec*, u32, void*);
void sqlite3BitvecDestroy(Bitvec*);

    if( pWal->exclusiveMode==WAL_HEAPMEMORY_MODE ){
      pWal->apWiData[iPage] = (u32 volatile *)sqlite3MallocZero(WALINDEX_PGSZ);
      if( !pWal->apWiData[iPage] ) rc = SQLITE_NOMEM;
    }else{
      rc = sqlite3OsShmMap(pWal->pDbFd, iPage, WALINDEX_PGSZ, 
          pWal->writeLock, (void volatile **)&pWal->apWiData[iPage]
      );
      if( rc==SQLITE_CANTOPEN && pWal->readOnlyShm>1 ){
        assert( iPage==0 );
        sqlite3OsFileControl(pWal->pDbFd, SQLITE_FCNTL_READONLY_SHM, (void*)1);
        rc = sqlite3OsShmMap(pWal->pDbFd, iPage, WALINDEX_PGSZ, 
            pWal->writeLock, (void volatile **)&pWal->apWiData[iPage]
        );
        if( rc==SQLITE_OK ){
          pWal->readOnly = pWal->readOnlyShm = 1;
        }
        sqlite3OsFileControl(pWal->pDbFd, SQLITE_FCNTL_READONLY_SHM, (void*)0);
      }
    }
  }

  *ppPage = pWal->apWiData[iPage];

  assert( iPage==0 || *ppPage || rc!=SQLITE_OK );
  if( pWal->readOnlyShm>1 ) pWal->readOnlyShm = 0;
  return rc;
}

/*
** Return a pointer to the WalCkptInfo structure in the wal-index.
*/
static volatile WalCkptInfo *walCkptInfo(Wal *pWal){

  */
  assert( pChanged );
  rc = walIndexPage(pWal, 0, &page0);
  if( rc!=SQLITE_OK ){
    return rc;
  };
  assert( page0 || pWal->writeLock==0 );
  assert( pWal->readOnlyShm==0 || pWal->readOnlyShm==1 );

  /* If the first page of the wal-index has been mapped, try to read the
  ** wal-index header immediately, without holding any lock. This usually
  ** works, but may fail if the wal-index header is corrupt or currently 
  ** being modified by another thread or process.
  */
  badHdr = (page0 ? walIndexTryHdr(pWal, pChanged) : 1);

** that extra content is ignored by the current thread.
**
** If the database contents have changes since the previous read
** transaction, then *pChanged is set to 1 before returning.  The
** Pager layer will use this to know that is cache is stale and
** needs to be flushed.
*/
int sqlite3WalBeginReadTransaction(Wal *pWal, int readOnlyShm, int *pChanged){
  int rc;                         /* Return code */
  int cnt = 0;                    /* Number of TryBeginRead attempts */

  if( pWal->nWiData==0 || pWal->apWiData[0]==0 ){
    assert( readOnlyShm==0 || readOnlyShm==1 );
    pWal->readOnlyShm = readOnlyShm*2;
  }
  do{
    rc = walTryBeginRead(pWal, pChanged, 0, ++cnt);
  }while( rc==WAL_RETRY );
  assert( rc || pWal->readOnlyShm==0 || (readOnlyShm && pWal->readOnlyShm==1) );
  testcase( (rc&0xff)==SQLITE_BUSY );
  testcase( (rc&0xff)==SQLITE_IOERR );
  testcase( rc==SQLITE_PROTOCOL );
  testcase( rc==SQLITE_OK );
  return rc;
}

#define _WAL_H_

#include "sqliteInt.h"

#ifdef SQLITE_OMIT_WAL
# define sqlite3WalOpen(x,y,z)                   0
# define sqlite3WalClose(w,x,y,z)                0
# define sqlite3WalBeginReadTransaction(x,y,z)   0
# define sqlite3WalEndReadTransaction(z)
# define sqlite3WalRead(v,w,x,y,z)               0
# define sqlite3WalDbsize(y)                     0
# define sqlite3WalBeginWriteTransaction(y)      0
# define sqlite3WalEndWriteTransaction(x)        0
# define sqlite3WalUndo(x,y,z)                   0
# define sqlite3WalSavepoint(y,z)

/* 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
** at an instant in time.  sqlite3WalOpenSnapshot gets a read lock and
** preserves the current state even if the other threads or processes
** write to or checkpoint the WAL.  sqlite3WalCloseSnapshot() closes the
** transaction and releases the lock.
*/
int sqlite3WalBeginReadTransaction(Wal *pWal, int, int *);
void sqlite3WalEndReadTransaction(Wal *pWal);

/* Read a page from the write-ahead log, if it is present. */
int sqlite3WalRead(Wal *pWal, Pgno pgno, int *pInWal, int nOut, u8 *pOut);

/* If the WAL is not empty, return the size of the database. */
Pgno sqlite3WalDbsize(Wal *pWal);

  # Close all connections and delete the database.
  #
  code1 { db close  }
  code2 { db2 close }
  code3 { db3 close }
  forcedelete test.db
  forcedelete walro

  foreach c {code1 code2 code3} {
    $c {
      sqlite3_shutdown
      sqlite3_config_uri 1
    }
  }

  file mkdir walro

  do_test 1.1.1 {
    code2 { sqlite3 db2 test.db }
    sql2 { 
      PRAGMA journal_mode = WAL;
      CREATE TABLE t1(x, y);
      INSERT INTO t1 VALUES('a', 'b');
    }
    file exists test.db-shm
  } {1}

  do_test 1.1.2 {
    file attributes test.db-shm -permissions r--r--r--
    code1 { sqlite3 db file:test.db?readonly_shm=1 }
  } {}

  do_test 1.1.3 { sql1 "SELECT * FROM t1" }                {a b}
  do_test 1.1.4 { sql2 "INSERT INTO t1 VALUES('c', 'd')" } {}
  do_test 1.1.5 { sql1 "SELECT * FROM t1" }                {a b c d}

  # Check that the read-only connection cannot write or checkpoint the db.

  do_test 1.2.1 {
    code2 { db2 close }
    code1 { db close }
    list [file exists test.db-wal] [file exists test.db-shm]
  } {1 1}
  do_test 1.2.2 {
    code1 { sqlite3 db file:test.db?readonly_shm=1 }
    sql1 { SELECT * FROM t1 }
  } {a b c d e f g h i j}

  do_test 1.2.3 {
    code1 { db close }
    file attributes test.db-shm -permissions rw-r--r--
    hexio_write test.db-shm 0 01020304 
    file attributes test.db-shm -permissions r--r--r--
    code1 { sqlite3 db file:test.db?readonly_shm=1 }
    csql1 { SELECT * FROM t1 }
  } {1 {attempt to write a readonly database}}
  do_test 1.2.4 {
    code1 { sqlite3_extended_errcode db } 
  } {SQLITE_READONLY_RECOVERY}

  do_test 1.2.5 {