SQLite

  if( !useWal && pInfo->nBackfill==pWal->hdr.mxFrame ){
    /* The WAL has been completely backfilled (or it is empty).
    ** and can be safely ignored.
    */
    rc = walLockShared(pWal, WAL_READ_LOCK(0));
    sqlite3OsShmBarrier(pWal->pDbFd);
    if( rc==SQLITE_OK ){
      if( memcmp((void *)pHdr, &pWal->hdr, sizeof(WalIndexHdr)) ){
        /* It is not safe to allow the reader to continue here if frames
        ** may have been appended to the log before READ_LOCK(0) was obtained.
        ** When holding READ_LOCK(0), the reader ignores the entire log file,
        ** which implies that the database file contains a trustworthy
        ** snapshoT. Since holding READ_LOCK(0) prevents a checkpoint from
        ** happening, this is usually correct.
        **

    ** that the log file may have been wrapped by a writer, or that frames
    ** that occur later in the log than pWal->hdr.mxFrame may have been
    ** copied into the database by a checkpointer. If either of these things
    ** happened, then reading the database with the current value of
    ** pWal->hdr.mxFrame risks reading a corrupted snapshot. So, retry
    ** instead.
    **
    ** This does not guarantee that the copy of the wal-index header is up to
    ** date before proceeding. That would not be possible without somehow
    ** blocking writers. It only guarantees that a dangerous checkpoint or 
    ** log-wrap (either of which would require an exclusive lock on
    ** WAL_READ_LOCK(mxI)) has not occurred since the snapshot was valid.
    */
    sqlite3OsShmBarrier(pWal->pDbFd);
    if( pInfo->aReadMark[mxI]!=mxReadMark
     || memcmp((void *)pHdr, &pWal->hdr, sizeof(WalIndexHdr))
    ){
      walUnlockShared(pWal, WAL_READ_LOCK(mxI));
      return WAL_RETRY;
    }else{
      pWal->readLock = mxI;
    }
  }

db close
T delete

#-------------------------------------------------------------------------
# When opening a read-transaction on a database, if the entire log has
# already been copied to the database file, the reader grabs a special
# kind of read lock (on aReadMark[0]). This set of test cases tests the 
# outcome of the following:
#
#   + The reader discovering that between the time when it determined 
#     that the log had been completely backfilled and the lock is obtained
#     that a writer has written to the log. In this case the reader should
#     acquire a different read-lock (not aReadMark[0]) and read the new
#     snapshot.
#

  expr {$sz2 == $sz1}
} {1}
 
db2 close
db close
T delete

#-------------------------------------------------------------------------
# When opening a read-transaction on a database, if the entire log has
# not yet been copied to the database file, the reader grabs a read
# lock on aReadMark[x], where x>0. The following test cases experiment
# with the outcome of the following:
#
#   + The reader discovering that between the time when it read the
#     wal-index header and the lock was obtained that a writer has 
#     written to the log. In this case the reader should re-read the 
#     wal-index header and lock a snapshot corresponding to the new 
#     header.
#
#   + The value in the aReadMark[x] slot has been modified since it was
#     read.
#
catch {db close}
testvfs T -default 1
do_test wal3-7.1.1 {
  file delete -force test.db test.db-journal test.db wal
  sqlite3 db test.db
  execsql {
    PRAGMA journal_mode = WAL;
    CREATE TABLE blue(red PRIMARY KEY, green);
  }
} {wal}

T script method_callback
T filter xOpen
proc method_callback {method args} {
  if {$method == "xOpen"} { return "reader" }
}
do_test wal3-7.1.2 {
  sqlite3 db2 test.db
  execsql { SELECT * FROM blue } db2
} {}

T filter xShmLock
set ::locks [list]
proc method_callback {method file handle spec} {
  if {$handle != "reader" } { return }
  if {$method == "xShmLock"} {
    catch { execsql { INSERT INTO blue VALUES(1, 2) } }
    catch { execsql { INSERT INTO blue VALUES(3, 4) } }
  }
  lappend ::locks $spec
}
do_test wal3-7.1.3 {
  execsql { SELECT * FROM blue } db2
} {1 2 3 4}
do_test wal3-7.1.4 {
  set ::locks
} {{4 1 lock shared} {4 1 unlock shared} {5 1 lock shared} {5 1 unlock shared}}

set ::locks [list]
proc method_callback {method file handle spec} {
  if {$handle != "reader" } { return }
  if {$method == "xShmLock"} {
    catch { execsql { INSERT INTO blue VALUES(5, 6) } }
  }
  lappend ::locks $spec
}
do_test wal3-7.2.1 {
  execsql { SELECT * FROM blue } db2
} {1 2 3 4 5 6}
do_test wal3-7.2.2 {
  set ::locks
} {{5 1 lock shared} {5 1 unlock shared} {4 1 lock shared} {4 1 unlock shared}}

db close
db2 close
T delete

finish_test