SQLite

const char *sqlite3ErrName(int rc){
  const char *zName = 0;
  int i, origRc = rc;
  for(i=0; i<2 && zName==0; i++, rc &= 0xff){
    switch( rc ){
      case SQLITE_OK:                 zName = "SQLITE_OK";                break;
      case SQLITE_ERROR:              zName = "SQLITE_ERROR";             break;
      case SQLITE_ERROR_SNAPSHOT:     zName = "SQLITE_ERROR_SNAPSHOT";    break;
      case SQLITE_INTERNAL:           zName = "SQLITE_INTERNAL";          break;
      case SQLITE_PERM:               zName = "SQLITE_PERM";              break;
      case SQLITE_ABORT:              zName = "SQLITE_ABORT";             break;
      case SQLITE_ABORT_ROLLBACK:     zName = "SQLITE_ABORT_ROLLBACK";    break;
      case SQLITE_BUSY:               zName = "SQLITE_BUSY";              break;
      case SQLITE_BUSY_RECOVERY:      zName = "SQLITE_BUSY_RECOVERY";     break;
      case SQLITE_BUSY_SNAPSHOT:      zName = "SQLITE_BUSY_SNAPSHOT";     break;

** on a per database connection basis using the
** [sqlite3_extended_result_codes()] API.  Or, the extended code for
** the most recent error can be obtained using
** [sqlite3_extended_errcode()].
*/
#define SQLITE_ERROR_MISSING_COLLSEQ   (SQLITE_ERROR | (1<<8))
#define SQLITE_ERROR_RETRY             (SQLITE_ERROR | (2<<8))
#define SQLITE_ERROR_SNAPSHOT          (SQLITE_ERROR | (3<<8))
#define SQLITE_IOERR_READ              (SQLITE_IOERR | (1<<8))
#define SQLITE_IOERR_SHORT_READ        (SQLITE_IOERR | (2<<8))
#define SQLITE_IOERR_WRITE             (SQLITE_IOERR | (3<<8))
#define SQLITE_IOERR_FSYNC             (SQLITE_IOERR | (4<<8))
#define SQLITE_IOERR_DIR_FSYNC         (SQLITE_IOERR | (5<<8))
#define SQLITE_IOERR_TRUNCATE          (SQLITE_IOERR | (6<<8))
#define SQLITE_IOERR_FSTAT             (SQLITE_IOERR | (7<<8))

** must have no active statements (SELECT statements that have been passed
** to sqlite3_step() but not sqlite3_reset() or sqlite3_finalize()). 
** SQLITE_ERROR is returned if either of these conditions is violated, or
** if schema S does not exist, or if the snapshot object is invalid.
**
** ^A call to sqlite3_snapshot_open() will fail to open if the specified
** snapshot has been overwritten by a [checkpoint]. In this case 
** SQLITE_ERROR_SNAPSHOT is returned.
**
** If there is already a read transaction open when this function is 
** invoked, then the same read transaction remains open (on the same
** database snapshot) if SQLITE_ERROR, SQLITE_BUSY or SQLITE_ERROR_SNAPSHOT
** is returned. If another error code - for example SQLITE_PROTOCOL or an
** SQLITE_IOERR error code - is returned, then the final state of the
** read transaction is undefined. If SQLITE_OK is returned, then the 
** read transaction is now open on database snapshot P.
**
** ^(A call to [sqlite3_snapshot_open(D,S,P)] will fail if the
** database connection D does not know that the database file for

      */
      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_ERROR_SNAPSHOT. Otherwise, overwrite pWal->hdr
        ** with *pSnapshot and set *pChanged as appropriate for opening the
        ** snapshot.  */
        if( !memcmp(pSnapshot->aSalt, pWal->hdr.aSalt, sizeof(pWal->hdr.aSalt))
         && pSnapshot->mxFrame>=pInfo->nBackfillAttempted
        ){
          assert( pWal->readLock>0 );
          memcpy(&pWal->hdr, pSnapshot, sizeof(WalIndexHdr));
          *pChanged = bChanged;
        }else{
          rc = SQLITE_ERROR_SNAPSHOT;
        }

        /* Release the shared CKPT lock obtained above. */
        walUnlockShared(pWal, WAL_CKPT_LOCK);
        pWal->minFrame = 1;
      }

  int rc;
  rc = walLockShared(pWal, WAL_CKPT_LOCK);
  if( rc==SQLITE_OK ){
    WalIndexHdr *pNew = (WalIndexHdr*)pSnapshot;
    if( memcmp(pNew->aSalt, pWal->hdr.aSalt, sizeof(pWal->hdr.aSalt))
     || pNew->mxFrame<walCkptInfo(pWal)->nBackfillAttempted
    ){
      rc = SQLITE_ERROR_SNAPSHOT;
      walUnlockShared(pWal, WAL_CKPT_LOCK);
    }
  }
  return rc;
}

/*

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

  #-------------------------------------------------------------------------
  # Check that SQLITE_ERROR_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_test $tn.4.1.3 {
    execsql { 
      COMMIT;
      PRAGMA wal_checkpoint;
      BEGIN;
    }
    list [catch {snapshot_open db main $snapshot} msg] $msg
  } {1 SQLITE_ERROR_SNAPSHOT}
  do_test $tn.4.1.4 {
    snapshot_free $snapshot
    execsql COMMIT
  } {}

  do_test $tn.4.2.1 {
    execsql {

  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_ERROR_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.

  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_ERROR_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 }

  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_ERROR_SNAPSHOT}

#-------------------------------------------------------------------------
# Check that calling sqlite3_snapshot_recover() does not confuse the
# pager cache.
reset_db
do_execsql_test 3.0 {
  PRAGMA journal_mode = wal;

  db2 eval { SELECT * FROM t2 ; END }
} {abc def ghi}

do_test 5.4 {
  execsql { INSERT INTO t2 VALUES('jkl') } 
  execsql BEGIN db2
  list [catch { sqlite3_snapshot_open_blob db2 main $snap } msg] $msg
} {1 SQLITE_ERROR_SNAPSHOT}


finish_test

  execsql { PRAGMA wal_checkpoint = truncate }
  file size test.db-wal
} 0

do_test 1.8 {
  execsql BEGIN db3
  list [catch { sqlite3_snapshot_open_blob db3 main $snap } msg] $msg
} {1 SQLITE_ERROR_SNAPSHOT}

finish_test

do_execsql_test 1.6 {
  SELECT * FROM t1
} {1 2 3 4 5 6 7 8 9 10 11 12}

do_test 1.7 {
  list [catch { sqlite3_snapshot_open db main $::snap1 } msg] $msg
} {1 SQLITE_ERROR_SNAPSHOT}

do_execsql_test 1.8 {
  SELECT * FROM t1
} {1 2 3 4 5 6 7 8 9 10 11 12}

do_test 1.9 {
  execsql { COMMIT ; BEGIN }
  list [catch { sqlite3_snapshot_open db main $::snap1 } msg] $msg
} {1 SQLITE_ERROR_SNAPSHOT}

do_test 1.10 {
  execsql { COMMIT }
  execsql {
    PRAGMA wal_checkpoint;
    DELETE FROM t1 WHERE a = 1;
  } db2

do_test 1.12 {
  sqlite3_snapshot_open db main $::snap4
  execsql { SELECT * FROM t1 }
} {4 5 6 7 8 9 10 11 12 13 14 15}

do_test 1.13 {
  list [catch { sqlite3_snapshot_open db main $::snap3 } msg] $msg
} {1 SQLITE_ERROR_SNAPSHOT}
do_test 1.14 {
  execsql { SELECT * FROM t1 }
} {4 5 6 7 8 9 10 11 12 13 14 15}

db close
db2 close
sqlite3 db test.db
do_execsql_test 1.15 {
  BEGIN;
    SELECT * FROM t1
} {7 8 9 10 11 12 13 14 15}
do_test 1.16 {
  list [catch { sqlite3_snapshot_open db main $::snap4 } msg] $msg
} {1 SQLITE_ERROR_SNAPSHOT}
do_execsql_test 1.17 { COMMIT }

sqlite3_snapshot_free $::snap1
sqlite3_snapshot_free $::snap2
sqlite3_snapshot_free $::snap3
sqlite3_snapshot_free $::snap4