Unpin buffer before inplace update waits for an XID to end.

Commit a07e03fd8f changed inplace updates
to wait for heap_update() commands like GRANT TABLE and GRANT DATABASE.
By keeping the pin during that wait, a sequence of autovacuum workers
and an uncommitted GRANT starved one foreground LockBufferForCleanup()
for six minutes, on buildfarm member sarus.  Prevent, at the cost of a
bit of complexity.  Back-patch to v12, like the earlier commit.  That
commit and heap_inplace_lock() have not yet appeared in any release.

Discussion: https://postgr.es/m/20241026184936.ae.nmisch@google.com

src/backend/access/heap/heapam.c

@@ -6134,8 +6134,8 @@ heap_abort_speculative(Relation relation, ItemPointer tid)
  * transaction.  If compatible, return true with the buffer exclusive-locked,
  * and the caller must release that by calling
  * heap_inplace_update_and_unlock(), calling heap_inplace_unlock(), or raising
- * an error.  Otherwise, return false after blocking transactions, if any,
+ * an error.  Otherwise, call release_callback(arg), wait for blocking
- * have ended.
+ * transactions to end, and return false.
  *
  * Since this is intended for system catalogs and SERIALIZABLE doesn't cover
  * DDL, this doesn't guarantee any particular predicate locking.
@@ -6169,7 +6169,8 @@ heap_abort_speculative(Relation relation, ItemPointer tid)
  */
 bool
 heap_inplace_lock(Relation relation,
-				  HeapTuple oldtup_ptr, Buffer buffer)
+				  HeapTuple oldtup_ptr, Buffer buffer,
+				  void (*release_callback) (void *), void *arg)
 {
 	HeapTupleData oldtup = *oldtup_ptr; /* minimize diff vs. heap_update() */
 	TM_Result	result;
@@ -6234,6 +6235,7 @@ heap_inplace_lock(Relation relation,
 										lockmode, NULL))
 			{
 				LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
+				release_callback(arg);
 				ret = false;
 				MultiXactIdWait((MultiXactId) xwait, mxact_status, infomask,
 								relation, &oldtup.t_self, XLTW_Update,
@@ -6249,6 +6251,7 @@ heap_inplace_lock(Relation relation,
 		else
 		{
 			LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
+			release_callback(arg);
 			ret = false;
 			XactLockTableWait(xwait, relation, &oldtup.t_self,
 							  XLTW_Update);
@@ -6260,6 +6263,7 @@ heap_inplace_lock(Relation relation,
 		if (!ret)
 		{
 			LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
+			release_callback(arg);
 		}
 	}
 

src/backend/access/index/genam.c

@@ -805,6 +805,7 @@ systable_inplace_update_begin(Relation relation,
 	int			retries = 0;
 	SysScanDesc scan;
 	HeapTuple	oldtup;
+	BufferHeapTupleTableSlot *bslot;
 
 	/*
 	 * For now, we don't allow parallel updates.  Unlike a regular update,
@@ -826,10 +827,9 @@ systable_inplace_update_begin(Relation relation,
 	Assert(IsInplaceUpdateRelation(relation) || !IsSystemRelation(relation));
 
 	/* Loop for an exclusive-locked buffer of a non-updated tuple. */
-	for (;;)
+	do
 	{
 		TupleTableSlot *slot;
-		BufferHeapTupleTableSlot *bslot;
 
 		CHECK_FOR_INTERRUPTS();
 
@@ -855,11 +855,9 @@ systable_inplace_update_begin(Relation relation,
 		slot = scan->slot;
 		Assert(TTS_IS_BUFFERTUPLE(slot));
 		bslot = (BufferHeapTupleTableSlot *) slot;
-		if (heap_inplace_lock(scan->heap_rel,
+	} while (!heap_inplace_lock(scan->heap_rel,
-							  bslot->base.tuple, bslot->buffer))
+								bslot->base.tuple, bslot->buffer,
-			break;
+								(void (*) (void *)) systable_endscan, scan));
-		systable_endscan(scan);
-	};
 
 	*oldtupcopy = heap_copytuple(oldtup);
 	*state = scan;

src/include/access/heapam.h

@@ -164,7 +164,8 @@ extern TM_Result heap_lock_tuple(Relation relation, HeapTuple tuple,
 								 Buffer *buffer, struct TM_FailureData *tmfd);
 
 extern bool heap_inplace_lock(Relation relation,
-							  HeapTuple oldtup_ptr, Buffer buffer);
+							  HeapTuple oldtup_ptr, Buffer buffer,
+							  void (*release_callback) (void *), void *arg);
 extern void heap_inplace_update_and_unlock(Relation relation,
 										   HeapTuple oldtup, HeapTuple tuple,
 										   Buffer buffer);