heapam: Use exclusive lock on old page in CLUSTER

To be able to guarantee that we can set the hint bit, acquire an exclusive
lock on the old buffer. We need the hint bits to be set as otherwise
reform_and_rewrite_tuple() -> rewrite_heap_tuple() -> heap_freeze_tuple() will
get confused.

It'd be better if we somehow coulda void setting hint bits on the old
page. One reason to use VACUUM FULL are very bloated tables - rewriting most
of the old table before during VACUUM FULL doesn't.

Author:
Reviewed-by:
Discussion: https://postgr.es/m/
Backpatch:

Author: anarazel

src/backend/access/heap/heapam_handler.c

@@ -837,7 +837,18 @@ heapam_relation_copy_for_cluster(Relation OldHeap, Relation NewHeap,
 		tuple = ExecFetchSlotHeapTuple(slot, false, NULL);
 		buf = hslot->buffer;
 
-		LockBuffer(buf, BUFFER_LOCK_SHARE);
+		/*
+		 * To be able to guarantee that we can set the hint bit, acquire an
+		 * exclusive lock on the old buffer. We need the hint bits to be set
+		 * as otherwise reform_and_rewrite_tuple() -> rewrite_heap_tuple() ->
+		 * heap_freeze_tuple() will get confused.
+		 *
+		 * It'd be better if we somehow could avoid setting hint bits on the
+		 * old page. One reason to use VACUUM FULL are very bloated tables -
+		 * rewriting most of the old table before during VACUUM FULL doesn't
+		 * exactly help...
+		 */
+		LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
 
 		switch (HeapTupleSatisfiesVacuum(tuple, OldestXmin, buf))
 		{

src/backend/access/heap/heapam_visibility.c

@@ -141,6 +141,13 @@ void
 HeapTupleSetHintBits(HeapTupleHeader tuple, Buffer buffer,
 					 uint16 infomask, TransactionId xid)
 {
+	/*
+	 * The uses from heapam.c rely on being able to perform the hint bit
+	 * updates, which can only be guaranteed if we are holding an exclusive
+	 * lock on the buffer - which all callers are doing.
+	 */
+	Assert(BufferIsLocal(buffer) || BufferLockHeldByMe(buffer, BUFFER_LOCK_EXCLUSIVE));
+
 	SetHintBits(tuple, buffer, infomask, xid);
 }
 

src/backend/storage/buffer/bufmgr.c

@@ -5908,7 +5908,7 @@ BufferLockHeldByMe(Buffer buffer, int mode)
 	{
 		Assert(false);
 		pg_unreachable();
-		lwmode = LW_EXCLUSIVE;  /* assuage compiler */
+		lwmode = LW_EXCLUSIVE;	/* assuage compiler */
 	}
 
 	return LWLockHeldByMeInMode(BufferDescriptorGetContentLock(buf), lwmode);