Fix RBM_ZERO_AND_LOCK mode to not acquire lock on local buffers.

hlinnaka · hlinnaka · commit f6c4a8690c93 · 2015-05-13T10:06:47.000+03:00
Commit 81c4508 introduced a new RBM_ZERO_AND_LOCK mode to ReadBuffer, which takes a lock on the buffer before zeroing it. However, you cannot take a lock on a local buffer, and you got a segfault instead. The version of that patch committed to master included a check for !isLocalBuf, and therefore didn't crash, but oddly I missed that in the back-patched versions. This patch adds that check to the back-branches too. RBM_ZERO_AND_LOCK mode is only used during WAL replay, and in hash indexes. WAL replay only deals with shared buffers, so the only way to trigger the bug is with a temporary hash index. Reported by Artem Ignatyev, analysis by Tom Lane.
diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
@@ -494,7 +494,8 @@ ReadBuffer_common(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 	 * (Note that we cannot use LockBuffer() of LockBufferForCleanup() here,
 	 * because they assert that the buffer is already valid.)
 	 */
-	if (mode == RBM_ZERO_AND_LOCK || mode == RBM_ZERO_AND_CLEANUP_LOCK)
+	if ((mode == RBM_ZERO_AND_LOCK || mode == RBM_ZERO_AND_CLEANUP_LOCK) &&
+		!isLocalBuf)
 		LWLockAcquire(bufHdr->content_lock, LW_EXCLUSIVE);
 
 	if (isLocalBuf)
