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Fix race condition between hot standby and restoring a full-page image.

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There was a window in RestoreBackupBlock where a page would be zeroed out,
but not yet locked. If a backend pinned and locked the page in that window,
it saw the zeroed page instead of the old page or new page contents, which
could lead to missing rows in a result set, or errors.

To fix, replace RBM_ZERO with RBM_ZERO_AND_LOCK, which atomically pins,
zeroes, and locks the page, if it's not in the buffer cache already.

In stable branches, the old RBM_ZERO constant is renamed to RBM_DO_NOT_USE,
to avoid breaking any 3rd party extensions that might use RBM_ZERO. More
importantly, this avoids renumbering the other enum values, which would
cause even bigger confusion in extensions that use ReadBufferExtended, but
haven't been recompiled.

Backpatch to all supported versions; this has been racy since hot standby
was introduced.
This commit is contained in:
Heikki Linnakangas
2014-11-13 19:47:44 +02:00
parent 35fed51626
commit 81c4508196
5 changed files with 78 additions and 32 deletions
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13
src/backend/access/hash/hashpage.c
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@@ -155,9 +155,8 @@ _hash_getinitbuf(Relation rel, BlockNumber blkno)
if (blkno == P_NEW)
elog(ERROR, "hash AM does not use P_NEW");
buf = ReadBufferExtended(rel, MAIN_FORKNUM, blkno, RBM_ZERO, NULL);
LockBuffer(buf, HASH_WRITE);
buf = ReadBufferExtended(rel, MAIN_FORKNUM, blkno, RBM_ZERO_AND_LOCK,
NULL);
/* ref count and lock type are correct */
@@ -198,11 +197,13 @@ _hash_getnewbuf(Relation rel, BlockNumber blkno, ForkNumber forkNum)
if (BufferGetBlockNumber(buf) != blkno)
elog(ERROR, "unexpected hash relation size: %u, should be %u",
BufferGetBlockNumber(buf), blkno);
LockBuffer(buf, HASH_WRITE);
}
else
buf = ReadBufferExtended(rel, forkNum, blkno, RBM_ZERO, NULL);
LockBuffer(buf, HASH_WRITE);
{
buf = ReadBufferExtended(rel, forkNum, blkno, RBM_ZERO_AND_LOCK,
NULL);
}
/* ref count and lock type are correct */