assert( pInfo==walCkptInfo(pWal) );
  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));
    if( rc==SQLITE_OK ){
      if( pHdr->mxFrame!=pWal->hdr.mxFrame ){













        walUnlockShared(pWal, WAL_READ_LOCK(0));
        return WAL_RETRY;
      }
      pWal->readLock = 0;
      return SQLITE_OK;
    }else if( rc!=SQLITE_BUSY ){
      return rc;
................................................................................
  **
  **   (aPgno[iFrame]==pgno): 
  **     This condition filters out normal hash-table collisions.
  **
  **   (iFrame<=iLast): 
  **     This condition filters out entries that were added to the hash
  **     table after the current read-transaction had started.
  **
  **   (iFrame>iRead): 
  **     This filters out a dangerous class of garbage data. The 
  **     garbage hash slot may refer to a frame with the correct page 
  **     number, but not the most recent version of the frame. For
  **     example, if at the start of the read-transaction the WAL
  **     contains three copies of the desired page in frames 2, 3 and 4,
  **     the hash table may contain the following:
  **
  **       { ..., 2, 3, 4, 99, 99, ..... }
  **
  **     The correct answer is to read data from frame 4. But a 
  **     dirty-read may potentially cause the hash-table to appear as 
  **     follows to the reader:
  **
  **       { ..., 2, 3, 4, 3, 99, ..... }
  **
  **     Without this part of the if(...) clause, the reader might
  **     incorrectly read data from frame 3 instead of 4. This would be
  **     an error.
  **
  ** It is not actually clear to the developers that such a dirty-read
  ** can occur. But if it does, it should not cause any problems.
  */
  for(iHash=iLast; iHash>0 && iRead==0; iHash-=HASHTABLE_NPAGE){
    volatile HASHTABLE_DATATYPE *aHash;  /* Pointer to hash table */
    volatile u32 *aPgno;                 /* Pointer to array of page numbers */
    u32 iZero;                    /* Frame number corresponding to aPgno[0] */
    int iKey;                     /* Hash slot index */
    int mxHash;                   /* upper bound on aHash[] values */

    walHashFind(pWal, iHash, &aHash, &aPgno, &iZero);
    mxHash = iLast - iZero;
    if( mxHash > HASHTABLE_NPAGE )  mxHash = HASHTABLE_NPAGE;
    for(iKey=walHash(pgno); aHash[iKey]; iKey=walNextHash(iKey)){
      u32 iFrame = aHash[iKey] + iZero;
      if( iFrame<=iLast && aPgno[iFrame]==pgno && iFrame>iRead ){

        iRead = iFrame;
      }
    }
  }
  assert( iRead==0 || pWal->pWiData[walIndexEntry(iRead)]==pgno );

#ifdef SQLITE_ENABLE_EXPENSIVE_ASSERT

  assert( pInfo==walCkptInfo(pWal) );
  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));
    if( rc==SQLITE_OK ){
      if( memcmp(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.
        **
        ** However, if frames have been appended to the log (or if the log 
        ** is wrapped and written for that matter) before the READ_LOCK(0)
        ** is obtained, that is not necessarily true. A checkpointer may
        ** have started to backfill the appended frames but crashed before
        ** it finished. Leaving a corrupt image in the database file.
        */
        walUnlockShared(pWal, WAL_READ_LOCK(0));
        return WAL_RETRY;
      }
      pWal->readLock = 0;
      return SQLITE_OK;
    }else if( rc!=SQLITE_BUSY ){
      return rc;
................................................................................
  **
  **   (aPgno[iFrame]==pgno): 
  **     This condition filters out normal hash-table collisions.
  **
  **   (iFrame<=iLast): 
  **     This condition filters out entries that were added to the hash
  **     table after the current read-transaction had started.























  */
  for(iHash=iLast; iHash>0 && iRead==0; iHash-=HASHTABLE_NPAGE){
    volatile HASHTABLE_DATATYPE *aHash;  /* Pointer to hash table */
    volatile u32 *aPgno;                 /* Pointer to array of page numbers */
    u32 iZero;                    /* Frame number corresponding to aPgno[0] */
    int iKey;                     /* Hash slot index */
    int mxHash;                   /* upper bound on aHash[] values */

    walHashFind(pWal, iHash, &aHash, &aPgno, &iZero);
    mxHash = iLast - iZero;
    if( mxHash > HASHTABLE_NPAGE )  mxHash = HASHTABLE_NPAGE;
    for(iKey=walHash(pgno); aHash[iKey]; iKey=walNextHash(iKey)){
      u32 iFrame = aHash[iKey] + iZero;
      if( iFrame<=iLast && aPgno[iFrame]==pgno ){
        assert( iFrame>iRead );
        iRead = iFrame;
      }
    }
  }
  assert( iRead==0 || pWal->pWiData[walIndexEntry(iRead)]==pgno );

#ifdef SQLITE_ENABLE_EXPENSIVE_ASSERT

  list $::testrc $::testmsg
} {1 {disk I/O error}}

db close
T delete

#-------------------------------------------------------------------------
# When opening a read-transaction on a database 



#













































































































































finish_test

  list $::testrc $::testmsg
} {1 {disk I/O error}}

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 test case 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.
#
#   + The attempt to obtain the lock on aReadMark[0] fails with SQLITE_BUSY.
#     This can happen if a checkpoint is ongoing. In this case also simply
#     obtain a different read-lock.
#
catch {db close}
testvfs T -default 1
do_test wal3-6.1.1 {
  file delete -force test.db test.db-journal test.db wal
  sqlite3 db test.db
  execsql { PRAGMA journal_mode = WAL }
  execsql {
    CREATE TABLE t1(a, b);
    INSERT INTO t1 VALUES('o', 't');
    INSERT INTO t1 VALUES('t', 'f');
  }
} {}
do_test wal3-6.1.2 {
  sqlite3 db2 test.db
  sqlite3 db3 test.db
  execsql { BEGIN ; SELECT * FROM t1 } db3
} {o t t f}
do_test wal3-6.1.3 {
  execsql { PRAGMA wal_checkpoint } db2
} {}

# At this point the log file has been fully checkpointed. However, 
# connection [db3] holds a lock that prevents the log from being wrapped.
# Test case 3.6.1.4 has [db] attempt a read-lock on aReadMark[0]. But
# as it is obtaining the lock, [db2] appends to the log file.
#
T filter xShmLock
T script lock_callback
proc lock_callback {method file handle spec} {
  if {$spec == "3 1 lock shared"} {
    # This is the callback for [db] to obtain the read lock on aReadMark[0].
    # Disable future callbacks using [T filter {}] and write to the log
    # file using [db2]. [db3] is preventing [db2] from wrapping the log
    # here, so this is an append.
    T filter {}
    db2 eval { INSERT INTO t1 VALUES('f', 's') }
  }
  return SQLITE_OK
}
do_test wal3-6.1.4 {
  execsql {
    BEGIN;
    SELECT * FROM t1;
  }
} {o t t f f s}

# [db] should be left holding a read-lock on some slot other than 
# aReadMark[0]. Test this by demonstrating that the read-lock is preventing
# the log from being wrapped.
#
do_test wal3-6.1.5 {
  db3 eval COMMIT
  db2 eval { PRAGMA wal_checkpoint }
  set sz1 [file size test.db-wal]
  db2 eval { INSERT INTO t1 VALUES('s', 'e') }
  set sz2 [file size test.db-wal]
  expr {$sz2>$sz1}
} {1}

# Test that if [db2] had not interfered when [db] was trying to grab
# aReadMark[0], it would have been possible to wrap the log in 3.6.1.5.
#
do_test wal3-6.1.6 {
  execsql { COMMIT }
  execsql { PRAGMA wal_checkpoint } db2
  execsql {
    BEGIN;
    SELECT * FROM t1;
  }
} {o t t f f s s e}
do_test wal3-6.1.7 {
  db2 eval { PRAGMA wal_checkpoint }
  set sz1 [file size test.db-wal]
  db2 eval { INSERT INTO t1 VALUES('n', 't') }
  set sz2 [file size test.db-wal]
  expr {$sz2==$sz1}
} {1}

db3 close
db2 close
db close

do_test wal3-6.2.1 {
  file delete -force test.db test.db-journal test.db wal
  sqlite3 db test.db
  sqlite3 db2 test.db
  execsql { PRAGMA journal_mode = WAL }
  execsql {
    CREATE TABLE t1(a, b);
    INSERT INTO t1 VALUES('h', 'h');
    INSERT INTO t1 VALUES('l', 'b');
  }
} {}

T filter xShmLock
T script lock_callback
proc lock_callback {method file handle spec} {
  if {$spec == "3 1 unlock exclusive"} {
    T filter {}
    set ::R [db2 eval {
      BEGIN;
      SELECT * FROM t1;
    }]
  }
}
do_test wal3-6.2.2 {
  execsql { PRAGMA wal_checkpoint }
} {}
do_test wal3-6.2.3 {
  set ::R
} {h h l b}
do_test wal3-6.2.4 {
  set sz1 [file size test.db-wal]
  execsql { INSERT INTO t1 VALUES('b', 'c'); }
  set sz2 [file size test.db-wal]
  expr {$sz2 > $sz1}
} {1}
do_test wal3-6.2.5 {
  db2 eval { COMMIT }
  execsql { PRAGMA wal_checkpoint }
  set sz1 [file size test.db-wal]
  execsql { INSERT INTO t1 VALUES('n', 'o'); }
  set sz2 [file size test.db-wal]
  expr {$sz2 == $sz1}
} {1}
 
db2 close
db close
T delete


finish_test