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Overview
Comment:Fix for race condition in WAL locking code.
Downloads: Tarball | ZIP archive
Timelines: family | ancestors | mistake
Files: files | file ages | folders
SHA1: 7c102c7b5f90717f55d95e3d38f33684118784d6
User & Date: dan 2010-06-09 11:02:13.000
References
2010-06-09
11:28
Simpler fix for the race condition also fixed by [7c102c7b5f] (check-in: 3c2de82003 user: dan tags: trunk)
Context
2010-06-09
11:02
Fix for race condition in WAL locking code. (Closed-Leaf check-in: 7c102c7b5f user: dan tags: mistake)
2010-06-08
15:50
Close database opened by tester.tcl when it is sourced in all.test. Because test scripts are now run in slave interpreters, this connection was not being closed by the first script run as it was previously. (check-in: b072e9f69a user: dan tags: trunk)
Changes
Unified Diff Ignore Whitespace Patch
Changes to src/wal.c.
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  if( mxI==0 ){
    /* If we get here, it means that all of the aReadMark[] entries between
    ** 1 and WAL_NREADER-1 are zero.  Try to initialize aReadMark[1] to
    ** be mxFrame, then retry.
    */
    rc = walLockExclusive(pWal, WAL_READ_LOCK(1), 1);
    if( rc==SQLITE_OK ){


      pInfo->aReadMark[1] = pWal->hdr.mxFrame+1;

      walUnlockExclusive(pWal, WAL_READ_LOCK(1), 1);
      rc = WAL_RETRY;
    }else if( rc==SQLITE_BUSY ){
      rc = WAL_RETRY;
    }
    return rc;
  }else{
    if( mxReadMark < pWal->hdr.mxFrame ){
      for(i=1; i<WAL_NREADER; i++){
        rc = walLockExclusive(pWal, WAL_READ_LOCK(i), 1);
        if( rc==SQLITE_OK ){


          mxReadMark = pInfo->aReadMark[i] = pWal->hdr.mxFrame+1;
          mxI = i;

          walUnlockExclusive(pWal, WAL_READ_LOCK(i), 1);
          break;
        }else if( rc!=SQLITE_BUSY ){
          return rc;
        }
      }
    }








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  if( mxI==0 ){
    /* If we get here, it means that all of the aReadMark[] entries between
    ** 1 and WAL_NREADER-1 are zero.  Try to initialize aReadMark[1] to
    ** be mxFrame, then retry.
    */
    rc = walLockExclusive(pWal, WAL_READ_LOCK(1), 1);
    if( rc==SQLITE_OK ){
      sqlite3OsShmBarrier(pWal->pDbFd);
      if( 0==memcmp((void *)pHdr, &pWal->hdr, sizeof(WalIndexHdr)) ){
        pInfo->aReadMark[1] = pWal->hdr.mxFrame+1;
      }
      walUnlockExclusive(pWal, WAL_READ_LOCK(1), 1);
      rc = WAL_RETRY;
    }else if( rc==SQLITE_BUSY ){
      rc = WAL_RETRY;
    }
    return rc;
  }else{
    if( mxReadMark<=pWal->hdr.mxFrame ){
      for(i=1; i<WAL_NREADER; i++){
        rc = walLockExclusive(pWal, WAL_READ_LOCK(i), 1);
        if( rc==SQLITE_OK ){
          sqlite3OsShmBarrier(pWal->pDbFd);
          if( 0==memcmp((void *)pHdr, &pWal->hdr, sizeof(WalIndexHdr)) ){
            mxReadMark = pInfo->aReadMark[i] = pWal->hdr.mxFrame+1;
            mxI = i;
          }
          walUnlockExclusive(pWal, WAL_READ_LOCK(i), 1);
          break;
        }else if( rc!=SQLITE_BUSY ){
          return rc;
        }
      }
    }
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    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;
    }
  }
  return rc;
}

/*








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    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{
      assert( mxReadMark<=(pWal->hdr.mxFrame+1) );
      pWal->readLock = mxI;
    }
  }
  return rc;
}

/*
︙︙
Changes to test/wal2.test.
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  set ints [set_tvfs_hdr $file]
  set v [lindex $ints $idx]
  incr v $incrval
  lset ints $idx $v
  set_tvfs_hdr $file $ints
}














#-------------------------------------------------------------------------
# Test case wal2-1.*:
#
# Set up a small database containing a single table. The database is not
# checkpointed during the test - all content resides in the log file.
#








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  set ints [set_tvfs_hdr $file]
  set v [lindex $ints $idx]
  incr v $incrval
  lset ints $idx $v
  set_tvfs_hdr $file $ints
}

proc clear_tvfs_readmarks {file} {

  set blob [tvfs shm $file]
  binary scan $blob i21 ints
  lappend ints 0 0 0 0 0 

  binary scan $blob a104a* dummy tail
  set blob [binary format i26a* $ints $tail]
  tvfs shm $file $blob

}


#-------------------------------------------------------------------------
# Test case wal2-1.*:
#
# Set up a small database containing a single table. The database is not
# checkpointed during the test - all content resides in the log file.
#
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# After this, the header is corrupted again and the reader is allowed
# to run recovery. This time, it sees an up-to-date snapshot of the
# database file.
#
set WRITER [list 0 1 lock exclusive]
set LOCKS  [list \
  {0 1 lock exclusive} {0 1 unlock exclusive} \

  {4 1 lock shared}    {4 1 unlock shared}    \
]
do_test wal2-2.0 {

  testvfs tvfs
  tvfs script tvfs_cb
  proc tvfs_cb {method args} {








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# After this, the header is corrupted again and the reader is allowed
# to run recovery. This time, it sees an up-to-date snapshot of the
# database file.
#
set WRITER [list 0 1 lock exclusive]
set LOCKS  [list \
  {0 1 lock exclusive} {0 1 unlock exclusive} \
  {4 1 lock exclusive} {4 1 unlock exclusive} \
  {4 1 lock shared}    {4 1 unlock shared}    \
]
do_test wal2-2.0 {

  testvfs tvfs
  tvfs script tvfs_cb
  proc tvfs_cb {method args} {
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        }
      }
      if {$method == "xShmLock"} {
        set lock [lindex $args 2]
        lappend ::locks $lock
        if {$lock == $::WRITER} {
          set_tvfs_hdr $::shm_file $::oldhdr

        }
      }
      return SQLITE_OK
    }

    execsql { SELECT count(a), sum(a) FROM t1 } db2
  } $res0








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        }
      }
      if {$method == "xShmLock"} {
        set lock [lindex $args 2]
        lappend ::locks $lock
        if {$lock == $::WRITER} {
          set_tvfs_hdr $::shm_file $::oldhdr
          clear_tvfs_readmarks $::shm_file
        }
      }
      return SQLITE_OK
    }

    execsql { SELECT count(a), sum(a) FROM t1 } db2
  } $res0
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  5  $wih(2)                $wih(1)                {Barton Deakin Watson}   \
  6  $wih(2)                $wih(2)                {Barton Deakin Watson}   \
  7  $wih(1)                $wih(1)                {Barton Deakin}          \
  8  {0 0 0 0 0 0 0 0 0 0} {0 0 0 0 0 0 0 0 0 0}   {Barton Deakin Watson}   \
] {
  do_test wal2-9.$tn {
    set_tvfs_hdr $::filename $hdr1 $hdr2

    execsql { SELECT * FROM x } db2
  } $res
}

db2 close
db close

finish_test








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  5  $wih(2)                $wih(1)                {Barton Deakin Watson}   \
  6  $wih(2)                $wih(2)                {Barton Deakin Watson}   \
  7  $wih(1)                $wih(1)                {Barton Deakin}          \
  8  {0 0 0 0 0 0 0 0 0 0} {0 0 0 0 0 0 0 0 0 0}   {Barton Deakin Watson}   \
] {
  do_test wal2-9.$tn {
    set_tvfs_hdr $::filename $hdr1 $hdr2
    clear_tvfs_readmarks $::shm_file
    execsql { SELECT * FROM x } db2
  } $res
}

db2 close
db close

finish_test