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

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.

1 files changed, 1 insertions, 5 deletions

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index dafd7f67919..45028d28288 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -3620,12 +3620,8 @@ RestoreBackupBlock(XLogRecPtr lsn, XLogRecord *record, int block_index,
 		{
 			/* Found it, apply the update */
 			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);