Get heap page max offset with buffer lock held.

petergeoghegan · petergeoghegan · commit 55e5352266b1 · 2020-12-30T17:21:41.000-08:00
On further reflection it seems better to call PageGetMaxOffsetNumber()
after acquiring a buffer lock on the page.  This shouldn't really
matter, but doing it this way is cleaner.

Follow-up to commit 42288174.

Backpatch: 12-, just like commit 42288174
diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
@@ -7042,10 +7042,10 @@ heap_compute_xid_horizon_for_tuples(Relation rel,
 			xid_horizon_prefetch_buffer(rel, &prefetch_state, 1);
 #endif
 
+			LockBuffer(buf, BUFFER_LOCK_SHARE);
+
 			page = BufferGetPage(buf);
 			maxoff = PageGetMaxOffsetNumber(page);
-
-			LockBuffer(buf, BUFFER_LOCK_SHARE);
 		}
 
 		/*
