Make visibilitymap_set() return previous state of vmbits

It can be useful to know the state of a relation page's VM bits before
visibilitymap_set(). visibilitymap_set() has the old value on hand, so
returning it is simple. This commit does not use visibilitymap_set()'s
new return value.

Author: Melanie Plageman
Reviewed-by: Masahiko Sawada, Andres Freund, Nitin Jadhav, Bilal Yavuz
Discussion: https://postgr.es/m/flat/CAAKRu_ZQe26xdvAqo4weHLR%3DivQ8J4xrSfDDD8uXnh-O-6P6Lg%40mail.gmail.com#6d8d2b4219394f774889509bf3bdc13d,
https://postgr.es/m/ctdjzroezaxmiyah3gwbwm67defsrwj2b5fpfs4ku6msfpxeia%40mwjyqlhwr2wu

src/backend/access/heap/visibilitymap.c

@@ -239,8 +239,10 @@ visibilitymap_pin_ok(BlockNumber heapBlk, Buffer vmbuf)
  * You must pass a buffer containing the correct map page to this function.
  * Call visibilitymap_pin first to pin the right one. This function doesn't do
  * any I/O.
+ *
+ * Returns the state of the page's VM bits before setting flags.
  */
-void
+uint8
 visibilitymap_set(Relation rel, BlockNumber heapBlk, Buffer heapBuf,
 				  XLogRecPtr recptr, Buffer vmBuf, TransactionId cutoff_xid,
 				  uint8 flags)
@@ -250,6 +252,7 @@ visibilitymap_set(Relation rel, BlockNumber heapBlk, Buffer heapBuf,
 	uint8		mapOffset = HEAPBLK_TO_OFFSET(heapBlk);
 	Page		page;
 	uint8	   *map;
+	uint8		status;
 
 #ifdef TRACE_VISIBILITYMAP
 	elog(DEBUG1, "vm_set %s %d", RelationGetRelationName(rel), heapBlk);
@@ -274,7 +277,8 @@ visibilitymap_set(Relation rel, BlockNumber heapBlk, Buffer heapBuf,
 	map = (uint8 *) PageGetContents(page);
 	LockBuffer(vmBuf, BUFFER_LOCK_EXCLUSIVE);
 
-	if (flags != (map[mapByte] >> mapOffset & VISIBILITYMAP_VALID_BITS))
+	status = (map[mapByte] >> mapOffset) & VISIBILITYMAP_VALID_BITS;
+	if (flags != status)
 	{
 		START_CRIT_SECTION();
 
@@ -311,6 +315,7 @@ visibilitymap_set(Relation rel, BlockNumber heapBlk, Buffer heapBuf,
 	}
 
 	LockBuffer(vmBuf, BUFFER_LOCK_UNLOCK);
+	return status;
 }
 
 /*

src/include/access/visibilitymap.h

@@ -31,9 +31,12 @@ extern bool visibilitymap_clear(Relation rel, BlockNumber heapBlk,
 extern void visibilitymap_pin(Relation rel, BlockNumber heapBlk,
 							  Buffer *vmbuf);
 extern bool visibilitymap_pin_ok(BlockNumber heapBlk, Buffer vmbuf);
-extern void visibilitymap_set(Relation rel, BlockNumber heapBlk, Buffer heapBuf,
-							  XLogRecPtr recptr, Buffer vmBuf, TransactionId cutoff_xid,
-							  uint8 flags);
+extern uint8 visibilitymap_set(Relation rel,
+							   BlockNumber heapBlk, Buffer heapBuf,
+							   XLogRecPtr recptr,
+							   Buffer vmBuf,
+							   TransactionId cutoff_xid,
+							   uint8 flags);
 extern uint8 visibilitymap_get_status(Relation rel, BlockNumber heapBlk, Buffer *vmbuf);
 extern void visibilitymap_count(Relation rel, BlockNumber *all_visible, BlockNumber *all_frozen);
 extern BlockNumber visibilitymap_prepare_truncate(Relation rel,