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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 c2b4ed19f6
commit 861d3aa43d
6 changed files with 66 additions and 25 deletions
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6
src/backend/access/transam/xlog.c
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@ -3187,12 +3187,8 @@ RestoreBackupBlockContents(XLogRecPtr lsn, BkpBlock bkpb, char *blk,
Page page;
buffer = XLogReadBufferExtended(bkpb.node, bkpb.fork, bkpb.block,
RBM_ZERO);
get_cleanup_lock ? RBM_ZERO_AND_CLEANUP_LOCK : RBM_ZERO_AND_LOCK);
Assert(BufferIsValid(buffer));
if (get_cleanup_lock)
LockBufferForCleanup(buffer);
else
LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);
page = (Page) BufferGetPage(buffer);