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.

Author: hlinnaka

src/backend/access/hash/hashpage.c

@@ -158,9 +158,8 @@ _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 */
 
@@ -201,11 +200,13 @@ _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 */
 

src/backend/access/heap/heapam.c

@@ -4367,9 +4367,8 @@ heap_xlog_newpage(XLogRecPtr lsn, XLogRecord *record)
 	 * not do anything that assumes we are touching a heap.
 	 */
 	buffer = XLogReadBufferExtended(xlrec->node, xlrec->forknum, xlrec->blkno,
-									RBM_ZERO);
+									RBM_ZERO_AND_LOCK);
 	Assert(BufferIsValid(buffer));
-	LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);
 	page = (Page) BufferGetPage(buffer);
 
 	Assert(record->xl_len == SizeOfHeapNewpage + BLCKSZ);

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);
 

src/backend/access/transam/xlogutils.c

@@ -234,16 +234,17 @@ XLogCheckInvalidPages(void)
  * The returned buffer is exclusively-locked.
  *
  * For historical reasons, instead of a ReadBufferMode argument, this only
- * supports RBM_ZERO (init == true) and RBM_NORMAL (init == false) modes.
+ * supports RBM_ZERO_AND_LOCK (init == true) and RBM_NORMAL (init == false)
+ * modes.
  */
 Buffer
 XLogReadBuffer(RelFileNode rnode, BlockNumber blkno, bool init)
 {
 	Buffer		buf;
 
 	buf = XLogReadBufferExtended(rnode, MAIN_FORKNUM, blkno,
-								 init ? RBM_ZERO : RBM_NORMAL);
-	if (BufferIsValid(buf))
+								 init ? RBM_ZERO_AND_LOCK : RBM_NORMAL);
+	if (BufferIsValid(buf) && !init)
 		LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
 
 	return buf;
@@ -262,8 +263,8 @@ XLogReadBuffer(RelFileNode rnode, BlockNumber blkno, bool init)
  * dropped or truncated. If we don't see evidence of that later in the WAL
  * sequence, we'll complain at the end of WAL replay.)
  *
- * In RBM_ZERO and RBM_ZERO_ON_ERROR modes, if the page doesn't exist, the
- * relation is extended with all-zeroes pages up to the given block number.
+ * In RBM_ZERO_* modes, if the page doesn't exist, the relation is extended
+ * with all-zeroes pages up to the given block number.
  *
  * In RBM_NORMAL_NO_LOG mode, we return InvalidBuffer if the page doesn't
  * exist, and we don't check for all-zeroes.  Thus, no log entry is made
@@ -317,14 +318,20 @@ XLogReadBufferExtended(RelFileNode rnode, ForkNumber forknum,
 		do
 		{
 			if (buffer != InvalidBuffer)
+			{
+				if (mode == RBM_ZERO_AND_LOCK || mode == RBM_ZERO_AND_CLEANUP_LOCK)
+					LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
 				ReleaseBuffer(buffer);
+			}
 			buffer = ReadBufferWithoutRelcache(rnode, forknum,
 											   P_NEW, mode, NULL);
 		}
 		while (BufferGetBlockNumber(buffer) < blkno);
 		/* Handle the corner case that P_NEW returns non-consecutive pages */
 		if (BufferGetBlockNumber(buffer) != blkno)
 		{
+			if (mode == RBM_ZERO_AND_LOCK || mode == RBM_ZERO_AND_CLEANUP_LOCK)
+				LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
 			ReleaseBuffer(buffer);
 			buffer = ReadBufferWithoutRelcache(rnode, forknum, blkno,
 											   mode, NULL);

src/backend/storage/buffer/bufmgr.c

@@ -208,14 +208,19 @@ ReadBuffer(Relation reln, BlockNumber blockNum)
  * valid, the page is zeroed instead of throwing an error. This is intended
  * for non-critical data, where the caller is prepared to repair errors.
  *
- * In RBM_ZERO mode, if the page isn't in buffer cache already, it's filled
- * with zeros instead of reading it from disk.  Useful when the caller is
- * going to fill the page from scratch, since this saves I/O and avoids
+ * In RBM_ZERO_AND_LOCK mode, if the page isn't in buffer cache already, it's
+ * filled with zeros instead of reading it from disk.  Useful when the caller
+ * is going to fill the page from scratch, since this saves I/O and avoids
  * unnecessary failure if the page-on-disk has corrupt page headers.
+ * The page is returned locked to ensure that the caller has a chance to
+ * initialize the page before it's made visible to others.
  * Caution: do not use this mode to read a page that is beyond the relation's
  * current physical EOF; that is likely to cause problems in md.c when
  * the page is modified and written out. P_NEW is OK, though.
  *
+ * RBM_ZERO_AND_CLEANUP_LOCK is the same as RBM_ZERO_AND_LOCK, but acquires
+ * a cleanup-strength lock on the page.
+ *
  * RBM_NORMAL_NO_LOG mode is treated the same as RBM_NORMAL here.
  *
  * If strategy is not NULL, a nondefault buffer access strategy is used.
@@ -356,6 +361,18 @@ ReadBuffer_common(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 											  isExtend,
 											  found);
 
+			/*
+			 * In RBM_ZERO_AND_LOCK mode, the caller expects the buffer to
+			 * be already locked on return.
+			 */
+			if (!isLocalBuf)
+			{
+				if (mode == RBM_ZERO_AND_LOCK)
+					LWLockAcquire(bufHdr->content_lock, LW_EXCLUSIVE);
+				else if (mode == RBM_ZERO_AND_CLEANUP_LOCK)
+					LockBufferForCleanup(BufferDescriptorGetBuffer(bufHdr));
+			}
+
 			return BufferDescriptorGetBuffer(bufHdr);
 		}
 
@@ -436,8 +453,11 @@ ReadBuffer_common(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 		 * Read in the page, unless the caller intends to overwrite it and
 		 * just wants us to allocate a buffer.
 		 */
-		if (mode == RBM_ZERO)
+		if (mode == RBM_ZERO_AND_LOCK || mode == RBM_ZERO_AND_CLEANUP_LOCK ||
+			mode == RBM_DO_NOT_USE)
+		{
 			MemSet((char *) bufBlock, 0, BLCKSZ);
+		}
 		else
 		{
 			smgrread(smgr, forkNum, blockNum, (char *) bufBlock);
@@ -464,6 +484,19 @@ ReadBuffer_common(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 		}
 	}
 
+	/*
+	 * In RBM_ZERO_AND_LOCK mode, grab the buffer content lock before marking
+	 * the page as valid, to make sure that no other backend sees the zeroed
+	 * page before the caller has had a chance to initialize it.
+	 *
+	 * Since no-one else can be looking at the page contents yet, there is no
+	 * difference between an exclusive lock and a cleanup-strength lock.
+	 * (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)
+		LWLockAcquire(bufHdr->content_lock, LW_EXCLUSIVE);
+
 	if (isLocalBuf)
 	{
 		/* Only need to adjust flags */

src/include/storage/bufmgr.h

@@ -36,11 +36,16 @@ typedef enum BufferAccessStrategyType
 typedef enum
 {
 	RBM_NORMAL,					/* Normal read */
-	RBM_ZERO,					/* Don't read from disk, caller will
-								 * initialize */
+	RBM_DO_NOT_USE,				/* This used to be RBM_ZERO. Only kept for
+								 * binary compatibility with 3rd party
+								 * extensions. */
 	RBM_ZERO_ON_ERROR,			/* Read, but return an all-zeros page on error */
-	RBM_NORMAL_NO_LOG			/* Don't log page as invalid during WAL
+	RBM_NORMAL_NO_LOG,			/* Don't log page as invalid during WAL
 								 * replay; otherwise same as RBM_NORMAL */
+	RBM_ZERO_AND_LOCK,			/* Don't read from disk, caller will
+								 * initialize. Also locks the page. */
+	RBM_ZERO_AND_CLEANUP_LOCK	/* Like RBM_ZERO_AND_LOCK, but locks the page
+								 * in "cleanup" mode */
 } ReadBufferMode;
 
 /* in globals.c ... this duplicates miscadmin.h */