int sqlite3_snapshot_get(
  sqlite3 *db, 
  const char *zDb,
  sqlite3_snapshot **ppSnapshot
){
  int rc = SQLITE_ERROR;
#ifndef SQLITE_OMIT_WAL


#ifdef SQLITE_ENABLE_API_ARMOR
  if( !sqlite3SafetyCheckOk(db) ){
    return SQLITE_MISUSE_BKPT;
  }
#endif
  sqlite3_mutex_enter(db->mutex);

  if( db->autoCommit==0 ){
    int iDb = sqlite3FindDbName(db, zDb);
    if( iDb==0 || iDb>1 ){
      Btree *pBt = db->aDb[iDb].pBt;
      if( 0==sqlite3BtreeIsInTrans(pBt) ){
        rc = sqlite3BtreeBeginTrans(pBt, 0);
        if( rc==SQLITE_OK ){
          rc = sqlite3PagerSnapshotGet(sqlite3BtreePager(pBt), ppSnapshot);
        }
      }
    }
  }

  sqlite3_mutex_leave(db->mutex);
#endif   /* SQLITE_OMIT_WAL */
  return rc;

          rc = sqlite3BtreeBeginTrans(pBt, 0);
          sqlite3PagerSnapshotOpen(sqlite3BtreePager(pBt), 0);
        }
      }
    }
  }

  sqlite3_mutex_leave(db->mutex);
#endif   /* SQLITE_OMIT_WAL */
  return rc;
}

/*
** Recover as many snapshots as possible from the wal file associated with
** schema zDb of database db.
*/
int sqlite3_snapshot_recover(sqlite3 *db, const char *zDb){
  int rc = SQLITE_ERROR;
  int iDb;
#ifndef SQLITE_OMIT_WAL

#ifdef SQLITE_ENABLE_API_ARMOR
  if( !sqlite3SafetyCheckOk(db) ){
    return SQLITE_MISUSE_BKPT;
  }
#endif

  sqlite3_mutex_enter(db->mutex);
  iDb = sqlite3FindDbName(db, zDb);
  if( iDb==0 || iDb>1 ){
    Btree *pBt = db->aDb[iDb].pBt;
    if( 0==sqlite3BtreeIsInReadTrans(pBt) ){
      rc = sqlite3BtreeBeginTrans(pBt, 0);
      if( rc==SQLITE_OK ){
        rc = sqlite3PagerSnapshotRecover(sqlite3BtreePager(pBt));
        sqlite3BtreeCommit(pBt);
      }
    }
  }
  sqlite3_mutex_leave(db->mutex);
#endif   /* SQLITE_OMIT_WAL */
  return rc;
}

/*
** Free a snapshot handle obtained from sqlite3_snapshot_get().
*/
void sqlite3_snapshot_free(sqlite3_snapshot *pSnapshot){
  sqlite3_free(pSnapshot);
}
#endif /* SQLITE_ENABLE_SNAPSHOT */

  if( pPager->pWal ){
    sqlite3WalSnapshotOpen(pPager->pWal, pSnapshot);
  }else{
    rc = SQLITE_ERROR;
  }
  return rc;
}

/*
** If this is a WAL database, call sqlite3WalSnapshotRecover(). If this 
** is not a WAL database, return an error.
*/
int sqlite3PagerSnapshotRecover(Pager *pPager){
  int rc;
  if( pPager->pWal ){
    rc = sqlite3WalSnapshotRecover(pPager->pWal);
  }else{
    rc = SQLITE_ERROR;
  }
  return rc;
}
#endif /* SQLITE_ENABLE_SNAPSHOT */
#endif /* !SQLITE_OMIT_WAL */

#ifdef SQLITE_ENABLE_ZIPVFS
/*
** A read-lock must be held on the pager when this function is called. If
** the pager is in WAL mode and the WAL file currently contains one or more

  int sqlite3PagerWalCallback(Pager *pPager);
  int sqlite3PagerOpenWal(Pager *pPager, int *pisOpen);
  int sqlite3PagerCloseWal(Pager *pPager, sqlite3*);
  int sqlite3PagerUseWal(Pager *pPager);
# ifdef SQLITE_ENABLE_SNAPSHOT
  int sqlite3PagerSnapshotGet(Pager *pPager, sqlite3_snapshot **ppSnapshot);
  int sqlite3PagerSnapshotOpen(Pager *pPager, sqlite3_snapshot *pSnapshot);
  int sqlite3PagerSnapshotRecover(Pager *pPager);
# endif
#else
# define sqlite3PagerUseWal(x) 0
#endif

#ifdef SQLITE_ENABLE_ZIPVFS
  int sqlite3PagerWalFramesize(Pager *pPager);

** the most recent version.
**
** The constructor for this object is [sqlite3_snapshot_get()].  The
** [sqlite3_snapshot_open()] method causes a fresh read transaction to refer
** to an historical snapshot (if possible).  The destructor for 
** sqlite3_snapshot objects is [sqlite3_snapshot_free()].
*/
typedef struct sqlite3_snapshot {
  unsigned char hidden[48];
} sqlite3_snapshot;

/*
** CAPI3REF: Record A Database Snapshot
** EXPERIMENTAL
**
** ^The [sqlite3_snapshot_get(D,S,P)] interface attempts to make a
** new [sqlite3_snapshot] object that records the current state of
** schema S in database connection D.  ^On success, the
** [sqlite3_snapshot_get(D,S,P)] interface writes a pointer to the newly
** created [sqlite3_snapshot] object into *P and returns SQLITE_OK.
** If there is not already a read-transaction open on schema S when
** this function is called, one is opened automatically. 
**
** The following must be true for this function to succeed. If any of
** the following statements are false when sqlite3_snapshot_get() is
** called, SQLITE_ERROR is returned. The final value of *P is undefined
** in this case. 
**
** <ul>
**   <li> The database handle must be in [autocommit mode].
**
**   <li> Schema S of [database connection] D must be a [WAL mode] database.
**
**   <li> There must not be a write transaction open on schema S of database
**        connection D.
**
**   <li> One or more transactions must have been written to the current wal
**        file since it was created on disk (by any connection). This means
**        that a snapshot cannot be taken on a wal mode database with no wal 
**        file immediately after it is first opened. At least one transaction
**        must be written to it first.
** </ul>
**
** This function may also return SQLITE_NOMEM.  If it is called with the
** database handle in autocommit mode but fails for some other reason, 
** whether or not a read transaction is opened on schema S is undefined.

**
** The [sqlite3_snapshot] object returned from a successful call to
** [sqlite3_snapshot_get()] must be freed using [sqlite3_snapshot_free()]
** to avoid a memory leak.
**
** The [sqlite3_snapshot_get()] interface is only available when the
** SQLITE_ENABLE_SNAPSHOT compile-time option is used.

** snapshot, and a positive value if P1 is a newer snapshot than P2.
*/
SQLITE_EXPERIMENTAL int sqlite3_snapshot_cmp(
  sqlite3_snapshot *p1,
  sqlite3_snapshot *p2
);

/*
** CAPI3REF: Recover snapshots from a wal file
** EXPERIMENTAL
**
** If all connections disconnect from a database file but do not perform
** a checkpoint, the existing wal file is opened along with the database
** file the next time the database is opened. At this point it is only
** possible to successfully call sqlite3_snapshot_open() to open the most
** recent snapshot of the database (the one at the head of the wal file),
** even though the wal file may contain other valid snapshots for which
** clients have sqlite3_snapshot handles.
**
** This function attempts to scan the wal file associated with database zDb
** of database handle db and make all valid snapshots available to
** sqlite3_snapshot_open(). It is an error if there is already a read
** transaction open on the database, or if the database is not a wal mode
** database.
**
** SQLITE_OK is returned if successful, or an SQLite error code otherwise.
*/
SQLITE_EXPERIMENTAL int sqlite3_snapshot_recover(sqlite3 *db, const char *zDb);

/*
** Undo the hack that converts floating point types to integer for
** builds on processors without floating point support.
*/
#ifdef SQLITE_OMIT_FLOATING_POINT
# undef double
#endif

#ifdef __cplusplus
}  /* End of the 'extern "C"' block */
#endif
#endif /* SQLITE3_H */

    if( sqlite3TestMakePointerStr(interp, zBuf, pSnapshot) ) return TCL_ERROR;
    Tcl_SetObjResult(interp, Tcl_NewStringObj(zBuf, -1));
  }
  return TCL_OK;
}
#endif /* SQLITE_ENABLE_SNAPSHOT */

#ifdef SQLITE_ENABLE_SNAPSHOT
/*
** Usage: sqlite3_snapshot_recover DB DBNAME
*/
static int SQLITE_TCLAPI test_snapshot_recover(
  void * clientData,
  Tcl_Interp *interp,
  int objc,
  Tcl_Obj *CONST objv[]
){
  int rc;
  sqlite3 *db;
  char *zName;

  if( objc!=3 ){
    Tcl_WrongNumArgs(interp, 1, objv, "DB DBNAME");
    return TCL_ERROR;
  }
  if( getDbPointer(interp, Tcl_GetString(objv[1]), &db) ) return TCL_ERROR;
  zName = Tcl_GetString(objv[2]);

  rc = sqlite3_snapshot_recover(db, zName);
  if( rc!=SQLITE_OK ){
    Tcl_SetObjResult(interp, Tcl_NewStringObj(sqlite3ErrName(rc), -1));
    return TCL_ERROR;
  }else{
    Tcl_ResetResult(interp);
  }
  return TCL_OK;
}
#endif /* SQLITE_ENABLE_SNAPSHOT */

#ifdef SQLITE_ENABLE_SNAPSHOT
/*
** Usage: sqlite3_snapshot_open DB DBNAME SNAPSHOT
*/
static int SQLITE_TCLAPI test_snapshot_open(
  void * clientData,
  Tcl_Interp *interp,

    return TCL_ERROR;
  }
  p1 = (sqlite3_snapshot*)sqlite3TestTextToPtr(Tcl_GetString(objv[1]));
  p2 = (sqlite3_snapshot*)sqlite3TestTextToPtr(Tcl_GetString(objv[2]));
  res = sqlite3_snapshot_cmp(p1, p2);
  Tcl_SetObjResult(interp, Tcl_NewIntObj(res));
  return TCL_OK;
}
#endif /* SQLITE_ENABLE_SNAPSHOT */

#ifdef SQLITE_ENABLE_SNAPSHOT
/*
** Usage: sqlite3_snapshot_get_blob DB DBNAME
*/
static int SQLITE_TCLAPI test_snapshot_get_blob(
  void * clientData,
  Tcl_Interp *interp,
  int objc,
  Tcl_Obj *CONST objv[]
){
  int rc;
  sqlite3 *db;
  char *zName;
  sqlite3_snapshot *pSnapshot = 0;

  if( objc!=3 ){
    Tcl_WrongNumArgs(interp, 1, objv, "DB DBNAME");
    return TCL_ERROR;
  }
  if( getDbPointer(interp, Tcl_GetString(objv[1]), &db) ) return TCL_ERROR;
  zName = Tcl_GetString(objv[2]);

  rc = sqlite3_snapshot_get(db, zName, &pSnapshot);
  if( rc!=SQLITE_OK ){
    Tcl_SetObjResult(interp, Tcl_NewStringObj(sqlite3ErrName(rc), -1));
    return TCL_ERROR;
  }else{
    Tcl_SetObjResult(interp, 
        Tcl_NewByteArrayObj((unsigned char*)pSnapshot, sizeof(sqlite3_snapshot))
    );
    sqlite3_snapshot_free(pSnapshot);
  }
  return TCL_OK;
}
#endif /* SQLITE_ENABLE_SNAPSHOT */

#ifdef SQLITE_ENABLE_SNAPSHOT
  /*
  ** Usage: sqlite3_snapshot_open_blob DB DBNAME SNAPSHOT
*/
static int SQLITE_TCLAPI test_snapshot_open_blob(
  void * clientData,
  Tcl_Interp *interp,
  int objc,
  Tcl_Obj *CONST objv[]
){
  int rc;
  sqlite3 *db;
  char *zName;
  unsigned char *pBlob;
  int nBlob;

  if( objc!=4 ){
    Tcl_WrongNumArgs(interp, 1, objv, "DB DBNAME SNAPSHOT");
    return TCL_ERROR;
  }
  if( getDbPointer(interp, Tcl_GetString(objv[1]), &db) ) return TCL_ERROR;
  zName = Tcl_GetString(objv[2]);
  pBlob = Tcl_GetByteArrayFromObj(objv[3], &nBlob);
  if( nBlob!=sizeof(sqlite3_snapshot) ){
    Tcl_AppendResult(interp, "bad SNAPSHOT", 0);
    return TCL_ERROR;
  }
  rc = sqlite3_snapshot_open(db, zName, (sqlite3_snapshot*)pBlob);
  if( rc!=SQLITE_OK ){
    Tcl_SetObjResult(interp, Tcl_NewStringObj(sqlite3ErrName(rc), -1));
    return TCL_ERROR;
  }
  return TCL_OK;
}
#endif /* SQLITE_ENABLE_SNAPSHOT */

#ifdef SQLITE_ENABLE_SNAPSHOT
/*
** Usage: sqlite3_snapshot_cmp_blob SNAPSHOT1 SNAPSHOT2
*/
static int SQLITE_TCLAPI test_snapshot_cmp_blob(
  void * clientData,
  Tcl_Interp *interp,
  int objc,
  Tcl_Obj *CONST objv[]
){
  int res;
  unsigned char *p1;
  unsigned char *p2;
  int n1;
  int n2;

  if( objc!=3 ){
    Tcl_WrongNumArgs(interp, 1, objv, "SNAPSHOT1 SNAPSHOT2");
    return TCL_ERROR;
  }

  p1 = Tcl_GetByteArrayFromObj(objv[1], &n1);
  p2 = Tcl_GetByteArrayFromObj(objv[2], &n2);

  if( n1!=sizeof(sqlite3_snapshot) || n1!=n2 ){
    Tcl_AppendResult(interp, "bad SNAPSHOT", 0);
    return TCL_ERROR;
  }

  res = sqlite3_snapshot_cmp((sqlite3_snapshot*)p1, (sqlite3_snapshot*)p2);
  Tcl_SetObjResult(interp, Tcl_NewIntObj(res));
  return TCL_OK;
}
#endif /* SQLITE_ENABLE_SNAPSHOT */

/*
** Usage: sqlite3_delete_database FILENAME
*/
int sqlite3_delete_database(const char*);   /* in test_delete.c */

#endif
     { "vfs_current_time_int64",           vfsCurrentTimeInt64,   0 },
#ifdef SQLITE_ENABLE_SNAPSHOT
     { "sqlite3_snapshot_get", test_snapshot_get, 0 },
     { "sqlite3_snapshot_open", test_snapshot_open, 0 },
     { "sqlite3_snapshot_free", test_snapshot_free, 0 },
     { "sqlite3_snapshot_cmp", test_snapshot_cmp, 0 },
     { "sqlite3_snapshot_recover", test_snapshot_recover, 0 },
     { "sqlite3_snapshot_get_blob", test_snapshot_get_blob, 0 },
     { "sqlite3_snapshot_open_blob", test_snapshot_open_blob, 0 },
     { "sqlite3_snapshot_cmp_blob", test_snapshot_cmp_blob, 0 },
#endif
     { "sqlite3_delete_database", test_delete_database, 0 },
  };
  static int bitmask_size = sizeof(Bitmask)*8;
  static int longdouble_size = sizeof(LONGDOUBLE_TYPE);
  int i;
  extern int sqlite3_sync_count, sqlite3_fullsync_count;

  }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 
** variable so that older snapshots can be accessed. To do this, loop
** through all wal frames from nBackfillAttempted to (nBackfill+1), 
** comparing their content to the corresponding page with the database
** file, if any. Set nBackfillAttempted to the frame number of the
** first frame for which the wal file content matches the db file.
**
** This is only really safe if the file-system is such that any page 
** writes made by earlier checkpointers were atomic operations, which 
** is not always true. It is also possible that nBackfillAttempted
** may be left set to a value larger than expected, if a wal frame
** contains content that duplicate of an earlier version of the same
** page.
**
** SQLITE_OK is returned if successful, or an SQLite error code if an
** error occurs. It is not an error if nBackfillAttempted cannot be
** decreased at all.
*/
int sqlite3WalSnapshotRecover(Wal *pWal){
  int rc;

  assert( pWal->readLock>=0 );
  rc = walLockExclusive(pWal, WAL_CKPT_LOCK, 1);
  if( rc==SQLITE_OK ){
    volatile WalCkptInfo *pInfo = walCkptInfo(pWal);
    int szPage = (int)pWal->szPage;
    i64 szDb;                   /* Size of db file in bytes */

    rc = sqlite3OsFileSize(pWal->pDbFd, &szDb);
    if( rc==SQLITE_OK ){
      void *pBuf1 = sqlite3_malloc(szPage);
      void *pBuf2 = sqlite3_malloc(szPage);
      if( pBuf1==0 || pBuf2==0 ){
        rc = SQLITE_NOMEM;
      }else{
        u32 i = pInfo->nBackfillAttempted;
        for(i=pInfo->nBackfillAttempted; i>pInfo->nBackfill; i--){
          volatile ht_slot *dummy;
          volatile u32 *aPgno;      /* Array of page numbers */
          u32 iZero;                /* Frame corresponding to aPgno[0] */
          u32 pgno;                 /* Page number in db file */
          i64 iDbOff;               /* Offset of db file entry */
          i64 iWalOff;              /* Offset of wal file entry */

          rc = walHashGet(pWal, walFramePage(i), &dummy, &aPgno, &iZero);
          if( rc!=SQLITE_OK ) break;
          pgno = aPgno[i-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;
            }
          }

          pInfo->nBackfillAttempted = i-1;
        }
      }

      sqlite3_free(pBuf1);
      sqlite3_free(pBuf2);
    }
    walUnlockExclusive(pWal, WAL_CKPT_LOCK, 1);
  }

  return rc;
}
#endif /* SQLITE_ENABLE_SNAPSHOT */

/*
** Begin a read transaction on the database.
**
** This routine used to be called sqlite3OpenSnapshot() and with good reason:
** it takes a snapshot of the state of the WAL and wal-index for the current
** instant in time.  The current thread will continue to use this snapshot.
** Other threads might append new content to the WAL and wal-index but

      /* It is possible that there is a checkpointer thread running 
      ** concurrent with this code. If this is the case, it may be that the
      ** checkpointer has already determined that it will checkpoint 
      ** snapshot X, where X is later in the wal file than pSnapshot, but 
      ** has not yet set the pInfo->nBackfillAttempted variable to indicate 
      ** its intent. To avoid the race condition this leads to, ensure that
      ** there is no checkpointer process by taking a shared CKPT lock 
      ** before checking pInfo->nBackfillAttempted.  
      **
      ** TODO: Does the aReadMark[] lock prevent a checkpointer from doing
      **       this already?
      */
      rc = walLockShared(pWal, WAL_CKPT_LOCK);

      if( rc==SQLITE_OK ){
        /* Check that the wal file has not been wrapped. Assuming that it has
        ** not, also check that no checkpointer has attempted to checkpoint any
        ** frames beyond pSnapshot->mxFrame. If either of these conditions are
        ** true, return SQLITE_BUSY_SNAPSHOT. Otherwise, overwrite pWal->hdr

/* Create a snapshot object.  The content of a snapshot is opaque to
** every other subsystem, so the WAL module can put whatever it needs
** in the object.
*/
int sqlite3WalSnapshotGet(Wal *pWal, sqlite3_snapshot **ppSnapshot){
  int rc = SQLITE_OK;
  WalIndexHdr *pRet;
  static const u32 aZero[4] = { 0, 0, 0, 0 };

  assert( pWal->readLock>=0 && pWal->writeLock==0 );

  if( memcmp(&pWal->hdr.aFrameCksum[0],aZero,16)==0 ){
    *ppSnapshot = 0;
    return SQLITE_ERROR;
  }
  pRet = (WalIndexHdr*)sqlite3_malloc(sizeof(WalIndexHdr));
  if( pRet==0 ){
    rc = SQLITE_NOMEM_BKPT;
  }else{
    memcpy(pRet, &pWal->hdr, sizeof(WalIndexHdr));
    *ppSnapshot = (sqlite3_snapshot*)pRet;
  }

** WAL module is using shared-memory, return false. 
*/
int sqlite3WalHeapMemory(Wal *pWal);

#ifdef SQLITE_ENABLE_SNAPSHOT
int sqlite3WalSnapshotGet(Wal *pWal, sqlite3_snapshot **ppSnapshot);
void sqlite3WalSnapshotOpen(Wal *pWal, sqlite3_snapshot *pSnapshot);
int sqlite3WalSnapshotRecover(Wal *pWal);
#endif

#ifdef SQLITE_ENABLE_ZIPVFS
/* If the WAL file is not empty, return the number of bytes of content
** stored in each frame (i.e. the db page-size when the WAL was created).
*/
int sqlite3WalFramesize(Wal *pWal);

# "PRAGMA journal_mode=memory", which fails if the database is in wal mode
# and there are one or more existing connections.
if {[permutation]=="inmemory_journal"} {
  finish_test
  return
}

foreach {tn tcl} {
  1 {
    proc snapshot_get {DB DBNAME} {
      uplevel [list sqlite3_snapshot_get $DB $DBNAME]
    }
    proc snapshot_open {DB DBNAME SNAPSHOT} {
      uplevel [list sqlite3_snapshot_open $DB $DBNAME $SNAPSHOT]
    }
    proc snapshot_free {SNAPSHOT} {
      uplevel [list sqlite3_snapshot_free $SNAPSHOT]
    }
    proc snapshot_cmp {SNAPSHOT1 SNAPSHOT2} {
      uplevel [list sqlite3_snapshot_cmp $SNAPSHOT1 $SNAPSHOT2]
    }
  }

  2 {
    proc snapshot_get {DB DBNAME} {
      uplevel [list sqlite3_snapshot_get_blob $DB $DBNAME]
    }
    proc snapshot_open {DB DBNAME SNAPSHOT} {
      uplevel [list sqlite3_snapshot_open_blob $DB $DBNAME $SNAPSHOT]
    }
    proc snapshot_free {SNAPSHOT} {
    }
    proc snapshot_cmp {SNAPSHOT1 SNAPSHOT2} {
      uplevel [list sqlite3_snapshot_cmp_blob $SNAPSHOT1 $SNAPSHOT2]
    }
  }
} {

  reset_db
  eval $tcl

  #-------------------------------------------------------------------------
  # Check some error conditions in snapshot_get(). It is an error if:
  #
  #  1) snapshot_get() is called on a non-WAL database, or
  #  2) there is an open write transaction on the database.
  #  3) the database handle is in auto-commit mode
  #
  do_execsql_test $tn.1.0 {
    CREATE TABLE t1(a, b);
    INSERT INTO t1 VALUES(1, 2);
    INSERT INTO t1 VALUES(3, 4);
  }

  do_test $tn.1.1.1 {
    execsql { BEGIN; SELECT * FROM t1; }
    list [catch { snapshot_get db main } msg] $msg
  } {1 SQLITE_ERROR}
  do_execsql_test $tn.1.1.2 COMMIT

  do_test $tn.1.2.1 {
    execsql {
      PRAGMA journal_mode = WAL;
      BEGIN;
        INSERT INTO t1 VALUES(5, 6);
        INSERT INTO t1 VALUES(7, 8);
    }
    list [catch { snapshot_get db main } msg] $msg
  } {1 SQLITE_ERROR}
  do_execsql_test $tn.1.2.2 COMMIT

  do_test $tn.1.3.1 {
    list [catch { snapshot_get db main } msg] $msg
  } {1 SQLITE_ERROR}
  do_test $tn.1.3.2 {
    db trans { set snap [snapshot_get db main] }
    snapshot_free $snap
  } {}

  #-------------------------------------------------------------------------
  # Check that a simple case works. Reuse the database created by the
  # block of tests above.
  #
  do_execsql_test $tn.2.1.0 {
    BEGIN;
      SELECT * FROM t1;
  } {1 2 3 4 5 6 7 8}

  do_test $tn.2.1.1 {
    set snapshot [snapshot_get db main]
    execsql {
      COMMIT;
      INSERT INTO t1 VALUES(9, 10);
      SELECT * FROM t1;
    }
  } {1 2 3 4 5 6 7 8 9 10}

  do_test $tn.2.1.2 {
    execsql BEGIN
    snapshot_open db main $snapshot
    execsql {
      SELECT * FROM t1;
    }
  } {1 2 3 4 5 6 7 8}

  do_test $tn.2.1.3 {
    snapshot_free $snapshot
    execsql COMMIT
  } {}

  do_test $tn.2.2.0 {
    sqlite3 db2 test.db
    execsql {
      BEGIN;
        SELECT * FROM t1;
    } db2
  } {1 2 3 4 5 6 7 8 9 10}

  do_test $tn.2.2.1 {
    set snapshot [snapshot_get db2 main]
    execsql {
      INSERT INTO t1 VALUES(11, 12);
      SELECT * FROM t1;
    }
  } {1 2 3 4 5 6 7 8 9 10 11 12}

  do_test $tn.2.2.2 {
    execsql BEGIN
    snapshot_open db main $snapshot
    execsql {
      SELECT * FROM t1;
    }
  } {1 2 3 4 5 6 7 8 9 10}

  do_test $tn.2.2.3 {
    snapshot_free $snapshot
    execsql COMMIT
    execsql COMMIT db2
    db2 close
  } {}

  do_test $tn.2.3.1 {
    execsql { DELETE FROM t1 WHERE a>6 }
    db trans { set snapshot [snapshot_get db main] }
    execsql {
      INSERT INTO t1 VALUES('a', 'b');
      INSERT INTO t1 VALUES('c', 'd');
      SELECT * FROM t1;
    }
  } {1 2 3 4 5 6 a b c d}
  do_test $tn.2.3.2 {
    execsql BEGIN
    snapshot_open db main $snapshot
    execsql { SELECT * FROM t1 }
  } {1 2 3 4 5 6}

  do_test $tn.2.3.3 {
    catchsql {
      INSERT INTO t1 VALUES('x','y')
    }
  } {1 {database is locked}}
  do_test $tn.2.3.4 {
    execsql COMMIT
    snapshot_free $snapshot
  } {}

  #-------------------------------------------------------------------------
  # Check some errors in snapshot_open(). It is an error if:
  #
  #   1) the db is in auto-commit mode,
  #   2) the db has an open (read or write) transaction,
  #   3) the db is not a wal database,
  #
  # Reuse the database created by earlier tests.
  #
  do_execsql_test $tn.3.0.0 {
    CREATE TABLE t2(x, y);
    INSERT INTO t2 VALUES('a', 'b');
    INSERT INTO t2 VALUES('c', 'd');
    BEGIN;
      SELECT * FROM t2;
  } {a b c d}
  do_test $tn.3.0.1 {
    set snapshot [snapshot_get db main]
    execsql { COMMIT }
    execsql { INSERT INTO t2 VALUES('e', 'f'); }
  } {}

  do_test $tn.3.1 {
    list [catch {snapshot_open db main $snapshot } msg] $msg
  } {1 SQLITE_ERROR}

  do_test $tn.3.2.1 {
    execsql {
      BEGIN;
        SELECT * FROM t2;
    }
  } {a b c d e f}
  do_test $tn.3.2.2 {
    list [catch {snapshot_open db main $snapshot } msg] $msg
  } {1 SQLITE_ERROR}

  do_test $tn.3.2.3 {
    execsql {
      COMMIT;
      BEGIN;
        INSERT INTO t2 VALUES('g', 'h');
    }
    list [catch {snapshot_open db main $snapshot } msg] $msg
  } {1 SQLITE_ERROR}
  do_execsql_test $tn.3.2.4 COMMIT

  do_test $tn.3.3.1 {
    execsql { PRAGMA journal_mode = DELETE }
    execsql { BEGIN }
    list [catch {snapshot_open db main $snapshot } msg] $msg
  } {1 SQLITE_ERROR}

  do_test $tn.$tn.3.3.2 {
    snapshot_free $snapshot
    execsql COMMIT
  } {}

  #-------------------------------------------------------------------------
  # Check that SQLITE_BUSY_SNAPSHOT is returned if the specified snapshot
  # no longer exists because the wal file has been checkpointed.
  #
  #   1. Reading a snapshot from the middle of a wal file is not possible
  #      after the wal file has been checkpointed.
  #
  #   2. That a snapshot from the end of a wal file can not be read once
  #      the wal file has been wrapped.
  #
  do_execsql_test $tn.4.1.0 {
    PRAGMA journal_mode = wal;
    CREATE TABLE t3(i, j);
    INSERT INTO t3 VALUES('o', 't');
    INSERT INTO t3 VALUES('t', 'f');
    BEGIN;
      SELECT * FROM t3;
  } {wal o t t f}

  do_test $tn.4.1.1 {
    set snapshot [snapshot_get db main]
    execsql COMMIT
  } {}
  do_test $tn.4.1.2 {
    execsql { 
      INSERT INTO t3 VALUES('f', 's'); 
      BEGIN;
    }
    snapshot_open db main $snapshot
    execsql { SELECT * FROM t3 }
  } {o t t f}

  do_test $tn.4.1.3 {
    execsql { 
      COMMIT;
      PRAGMA wal_checkpoint;
      BEGIN;
    }
    list [catch {snapshot_open db main $snapshot} msg] $msg
  } {1 SQLITE_BUSY_SNAPSHOT}
  do_test $tn.4.1.4 {
    snapshot_free $snapshot
    execsql COMMIT
  } {}

  do_test $tn.4.2.1 {
    execsql {
      INSERT INTO t3 VALUES('s', 'e');
      INSERT INTO t3 VALUES('n', 't');
      BEGIN;
        SELECT * FROM t3;
    }
  } {o t t f f s s e n t}
  do_test $tn.4.2.2 {
    set snapshot [snapshot_get db main]
    execsql {
      COMMIT;
      PRAGMA wal_checkpoint;
      BEGIN;
    }
    snapshot_open db main $snapshot
    execsql { SELECT * FROM t3 }
  } {o t t f f s s e n t}
  do_test $tn.4.2.3 {
    execsql {
      COMMIT;
      INSERT INTO t3 VALUES('e', 't');
      BEGIN;
    }
    list [catch {snapshot_open db main $snapshot} msg] $msg
  } {1 SQLITE_BUSY_SNAPSHOT}
  do_test $tn.4.2.4 {
    snapshot_free $snapshot
  } {}

  #-------------------------------------------------------------------------
  # Check that SQLITE_BUSY is returned if a checkpoint is running when
  # sqlite3_snapshot_open() is called.
  #
  reset_db
  db close
  testvfs tvfs
  sqlite3 db test.db -vfs tvfs

  do_execsql_test $tn.5.1 {
    PRAGMA journal_mode = wal;
    CREATE TABLE x1(x, xx, xxx);
    INSERT INTO x1 VALUES('z', 'zz', 'zzz');
    BEGIN;
      SELECT * FROM x1;
  } {wal z zz zzz}

  do_test $tn.5.2 {
    set ::snapshot [snapshot_get db main]
    sqlite3 db2 test.db -vfs tvfs
    execsql {
      INSERT INTO x1 VALUES('a', 'aa', 'aaa');
      COMMIT;
    }
  } {}

  set t53 0
  proc write_callback {args} {
    do_test $tn.5.3.[incr ::t53] {
      execsql BEGIN
      list [catch { snapshot_open db main $::snapshot } msg] $msg
    } {1 SQLITE_BUSY}
    catchsql COMMIT
  }

  tvfs filter xWrite
  tvfs script write_callback
  db2 eval { PRAGMA wal_checkpoint }
  db close
  db2 close
  tvfs delete
  snapshot_free $snapshot

  #-------------------------------------------------------------------------
  # Test that sqlite3_snapshot_get() may be called immediately after
  # "BEGIN; PRAGMA user_version;". And that sqlite3_snapshot_open() may
  # be called after opening the db handle and running the script
  # "PRAGMA user_version; BEGIN".
  reset_db
  do_execsql_test $tn.6.1 {
    PRAGMA journal_mode = wal;
    CREATE TABLE x1(x, xx, xxx);
    INSERT INTO x1 VALUES('z', 'zz', 'zzz');
    BEGIN;
      PRAGMA user_version;
  } {wal 0}
  do_test $tn.6.2 {
    set ::snapshot [snapshot_get db main]
    execsql {
      INSERT INTO x1 VALUES('a', 'aa', 'aaa');
      COMMIT;
    }
  } {}
  do_test $tn.6.3 {
    sqlite3 db2 test.db 
    db2 eval "PRAGMA user_version ; BEGIN"
    snapshot_open db2 main $::snapshot
    db2 eval { SELECT * FROM x1 }
  } {z zz zzz}
  do_test $tn.6.4 {
    db2 close
    sqlite3 db2 test.db 
    db2 eval "PRAGMA application_id"
    db2 eval "BEGIN"
    snapshot_open db2 main $::snapshot
    db2 eval { SELECT * FROM x1 }
  } {z zz zzz}

  do_test $tn.6.5 {
    db2 close
    sqlite3 db2 test.db 
    db2 eval "BEGIN"
    list [catch {snapshot_open db2 main $::snapshot} msg] $msg
  } {1 SQLITE_ERROR}

  snapshot_free $snapshot

  #-------------------------------------------------------------------------
  # The following tests investigate the sqlite3_snapshot_cmp() API.
  #

  # Compare snapshots $p1 and $p2, checking that the result is $r.
  #
  proc do_snapshot_cmp_test {tn p1 p2 r} {
    uplevel [list do_test $tn.1 [list snapshot_cmp $p1 $p2] $r]
    uplevel [list do_test $tn.2 [list snapshot_cmp $p2 $p1] [expr $r*-1]]
    uplevel [list do_test $tn.3 [list snapshot_cmp $p1 $p1] 0]
    uplevel [list do_test $tn.4 [list snapshot_cmp $p2 $p2] 0]
  }

  catch { db2 close }
  reset_db

  do_execsql_test $tn.7.1 {
    PRAGMA journal_mode = wal;
    CREATE TABLE t1(x);
  } wal

  do_test $tn.7.1.2 {
    execsql { BEGIN ; PRAGMA application_id }
    set p1 [snapshot_get db main]
    execsql {
      INSERT INTO t1 VALUES(10);
      COMMIT;
    }
    execsql { BEGIN ; PRAGMA application_id }
    set p2 [snapshot_get db main]
    execsql COMMIT
  } {}

  do_snapshot_cmp_test $tn.7.1.3 $p1 $p2 -1
  snapshot_free $p1
  snapshot_free $p2

  do_execsql_test $tn.7.2.1 {
    INSERT INTO t1 VALUES(11);
    INSERT INTO t1 VALUES(12);
    INSERT INTO t1 VALUES(13);
    BEGIN; 
      PRAGMA application_id;
  } {0}
  do_test $tn.7.2.2 {
    set p1 [snapshot_get db main]
    execsql {
      COMMIT;
      INSERT INTO t1 VALUES(14);
      PRAGMA wal_checkpoint;
      BEGIN;
        PRAGMA application_id;
    }
    set p2 [snapshot_get db main]
    execsql COMMIT
  } {}

  do_snapshot_cmp_test $tn.7.2.3 $p1 $p2 -1
  snapshot_free $p2

  do_test $tn.7.3.1 {
    execsql {
      INSERT INTO t1 VALUES(14);
      BEGIN;
        PRAGMA application_id;
    }
    set p2 [snapshot_get db main]
    execsql COMMIT
  } {}

  do_snapshot_cmp_test $tn.7.3.2 $p1 $p2 -1
  snapshot_free $p1
  snapshot_free $p2
}

finish_test

# 2016 November 18
#
# 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 the sqlite3_snapshot_xxx() APIs.
#

set testdir [file dirname $argv0]
source $testdir/tester.tcl
ifcapable !snapshot {finish_test; return}
set testprefix snapshot2

# This test does not work with the inmemory_journal permutation. The reason
# is that each connection opened as part of this permutation executes
# "PRAGMA journal_mode=memory", which fails if the database is in wal mode
# and there are one or more existing connections.
if {[permutation]=="inmemory_journal"} {
  finish_test
  return
}

#-------------------------------------------------------------------------
# Check that it is not possible to obtain a snapshot immediately after
# a wal mode database with an empty wal file is opened. But it is after
# the file has been written, even by some other connection.
#
do_execsql_test 1.0 {
  PRAGMA journal_mode = wal;
  CREATE TABLE t1(a, b, c);
  INSERT INTO t1 VALUES(1, 2, 3);
  INSERT INTO t1 VALUES(4, 5, 6);
} {wal}

db close
do_test 1.1.1 { list [file exists test.db] [file exists test.db-wal] } {1 0}

sqlite3 db test.db
do_execsql_test 1.1.2 { SELECT * FROM t1 } {1 2 3 4 5 6}

do_test 1.1.3 {
  execsql BEGIN
  list [catch { sqlite3_snapshot_get_blob db main } msg] $msg
} {1 SQLITE_ERROR}
execsql COMMIT

do_test 1.1.4 {
  execsql { INSERT INTO t1 VALUES(7, 8, 9) }
  execsql BEGIN
  string length [sqlite3_snapshot_get_blob db main]
} 48
execsql COMMIT

db close
do_test 1.2.1 { list [file exists test.db] [file exists test.db-wal] } {1 0}

sqlite3 db test.db
do_execsql_test 1.2.2 { SELECT * FROM t1 } {1 2 3 4 5 6 7 8 9}

do_test 1.2.3 {
  execsql BEGIN
  list [catch { sqlite3_snapshot_get_blob db main } msg] $msg
} {1 SQLITE_ERROR}
execsql COMMIT

do_test 1.2.4 {
  sqlite3 db2 test.db
  execsql { INSERT INTO t1 VALUES(10, 11, 12) } db2
  execsql BEGIN
  string length [sqlite3_snapshot_get_blob db main]
} 48
execsql COMMIT
db2 close

#-------------------------------------------------------------------------
# Simple tests for sqlite3_snapshot_recover().
#
reset_db
do_execsql_test 2.0 {
  CREATE TABLE t1(x);
  PRAGMA journal_mode = wal;
  INSERT INTO t1 VALUES(1);
  INSERT INTO t1 VALUES(2);
} {wal}

do_test 2.1 {
  db trans { set snap [sqlite3_snapshot_get_blob db main] }
  sqlite3_db_config db NO_CKPT_ON_CLOSE 1
  db close
  sqlite3 db test.db

  execsql {SELECT * FROM sqlite_master}
  execsql BEGIN
  sqlite3_snapshot_open_blob db main $snap
  execsql COMMIT;
  execsql { INSERT INTO t1 VALUES(3); }
} {}

do_test 2.2 {
  sqlite3_db_config db NO_CKPT_ON_CLOSE 1
  db close
  sqlite3 db test.db

  execsql {SELECT * FROM sqlite_master}
  execsql BEGIN
  list [catch { sqlite3_snapshot_open_blob db main $snap } msg] $msg
} {1 SQLITE_BUSY_SNAPSHOT}

do_test 2.3 {
  execsql COMMIT
  sqlite3_snapshot_recover db main
  execsql BEGIN
  sqlite3_snapshot_open_blob db main $snap
  execsql { SELECT * FROM t1 }
} {1 2}

do_test 2.4 {
  execsql COMMIT
  execsql { SELECT * FROM t1 }
} {1 2 3}

do_test 2.5 {
  execsql { PRAGMA wal_checkpoint }
  sqlite3_db_config db NO_CKPT_ON_CLOSE 1
  db close
  sqlite3 db test.db

  sqlite3_snapshot_recover db main
  execsql BEGIN
  list [catch { sqlite3_snapshot_open_blob db main $snap } msg] $msg
} {1 SQLITE_BUSY_SNAPSHOT}

#-------------------------------------------------------------------------
# Check that calling sqlite3_snapshot_recover() does not confuse the
# pager cache.
reset_db
do_execsql_test 3.0 {
  PRAGMA journal_mode = wal;
  CREATE TABLE t1(x, y);
  INSERT INTO t1 VALUES('a', 'b');
  INSERT INTO t1 VALUES('c', 'd');
} {wal}
do_test 3.1 {
  sqlite3 db2 test.db
  execsql { INSERT INTO t1 VALUES('e', 'f') } db2
  db2 close
  sqlite3_snapshot_recover db main
} {}
do_execsql_test 3.2 {
  SELECT * FROM t1;
} {a b c d e f}

#-------------------------------------------------------------------------
# Check that sqlite3_snapshot_recover() returns an error if it is called
# with an open read-transaction. Or on a database that does not exist. Or
# on the temp database. Or on a db that is not in wal mode.
#
do_test 4.1 {
  sqlite3_snapshot_recover db main
} {}
do_test 4.2 {
  execsql {
    BEGIN;
      SELECT * FROM sqlite_master;
  }
  list [catch { sqlite3_snapshot_recover db main } msg] $msg
} {1 SQLITE_ERROR}
do_test 4.3 {
  execsql COMMIT
  sqlite3_snapshot_recover db main
} {}
do_test 4.4 {
  list [catch { sqlite3_snapshot_recover db aux } msg] $msg
} {1 SQLITE_ERROR}
do_test 4.5 {
  forcedelete test.db2
  execsql {
    ATTACH 'test.db2' AS aux;
    PRAGMA aux.journal_mode = wal;
    CREATE TABLE aux.t2(x, y);
  }
  list [catch { sqlite3_snapshot_recover db aux } msg] $msg
} {0 {}}
do_test 4.6 {
  list [catch { sqlite3_snapshot_recover db temp } msg] $msg
} {1 SQLITE_ERROR}
do_test 4.7 {
  execsql {
    PRAGMA aux.journal_mode = delete;
  }
  list [catch { sqlite3_snapshot_recover db aux } msg] $msg
} {1 SQLITE_ERROR}

finish_test

      PRAGMA integrity_check;
    }]
    if {$res != "1 2 3 ok"} { error "res is $res" }
  }

  sqlite3_snapshot_free $::snapshot
}

#-------------------------------------------------------------------------
# Test the handling of faults that occur within sqlite3_snapshot_recover().
#
reset_db
do_execsql_test 4.0 {
  PRAGMA journal_mode = wal;
  CREATE TABLE t1(zzz);
  INSERT INTO t1 VALUES('abc');
  INSERT INTO t1 VALUES('def');
} {wal}
faultsim_save_and_close

do_test 4.0.1 {
  faultsim_restore_and_reopen
  db eval { SELECT * FROM sqlite_master } 
  sqlite3_snapshot_recover db main
} {}
db close

do_faultsim_test 4.0 -faults oom* -prep {
  faultsim_restore_and_reopen
  db eval { SELECT * FROM sqlite_master } 
} -body {
  sqlite3_snapshot_recover db main
} -test {
  faultsim_test_result {0 {}} {1 SQLITE_NOMEM} {1 SQLITE_IOERR_NOMEM}
}

# The following test cases contrive to call sqlite3_snapshot_recover()
# before all pages of the *-shm file have been mapped. This tests an
# extra branch of error handling logic in snapshot_recover().
#
reset_db
do_execsql_test 4.1.0 {
  PRAGMA page_size = 512;
  PRAGMA journal_mode = wal;
  PRAGMA wal_autocheckpoint = 0;
  CREATE TABLE t1(zzz);
  INSERT INTO t1 VALUES(randomblob( 500 * 9500 ));
  PRAGMA user_version = 211;
} {wal 0}

do_test 4.1.1 {
  list [file size test.db-shm] [file size test.db]
} {98304 512}

faultsim_save_and_close
do_faultsim_test 4.1 -faults shm* -prep {
  catch { db2 close } 
  catch { db close } 
  faultsim_restore_and_reopen
  sqlite3 db2 test.db
  db2 eval { SELECT * FROM sqlite_master } 
  db eval BEGIN
  sqlite3_snapshot_get_blob db main
  db eval COMMIT
} -body {
  sqlite3_snapshot_recover db main
} -test {
  faultsim_test_result {0 {}} {1 SQLITE_IOERR}
}



finish_test