SQLite

      ** 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.  */
      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
        ** 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
        ){
          memcpy(&pWal->hdr, pSnapshot, sizeof(WalIndexHdr));
          *pChanged = bChanged;
        }else{
          rc = SQLITE_BUSY_SNAPSHOT;
        }

        /* Release the shared CKPT lock obtained above. */
        walUnlockShared(pWal, WAL_CKPT_LOCK);
      }


      if( rc!=SQLITE_OK ){
        sqlite3WalEndReadTransaction(pWal);
      }
    }
  }
#endif

    BEGIN;
  }
  list [catch {sqlite3_snapshot_open db main $snapshot} msg] $msg
} {1 SQLITE_BUSY_SNAPSHOT}
do_test 4.2.4 {
  sqlite3_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 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 5.2 {
  set ::snapshot [sqlite3_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} {
breakpoint
  do_test 5.3.[incr ::t53] {
    execsql BEGIN
    list [catch { sqlite3_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
sqlite3_snapshot_free $snapshot

finish_test