Handle interrupts while waiting on Append's async subplans

We did not wake up on interrupts while waiting on async events on an
async-capable append node. For example, if you tried to cancel the
query, nothing would happen until one of the async subplans becomes
readable. To fix, add WL_LATCH_SET to the WaitEventSet.

Backpatch down to v14 where async Append execution was introduced.

Discussion: https://www.postgresql.org/message-id/37a40570-f558-40d3-b5ea-5c2079b3b30b@iki.fi

src/backend/executor/nodeAppend.c

@@ -1016,7 +1016,7 @@ ExecAppendAsyncRequest(AppendState *node, TupleTableSlot **result)
 static void
 ExecAppendAsyncEventWait(AppendState *node)
 {
-	int			nevents = node->as_nasyncplans + 1;
+	int			nevents = node->as_nasyncplans + 2;
 	long		timeout = node->as_syncdone ? -1 : 0;
 	WaitEvent	occurred_event[EVENT_BUFFER_SIZE];
 	int			noccurred;
@@ -1043,13 +1043,28 @@ ExecAppendAsyncEventWait(AppendState *node)
 		}
 
 		/*
-		 * If there are no configured events other than the postmaster death
-		 * event, we don't need to wait or poll.
+		 * No need for further processing if none of the subplans configured
+		 * any events.
 		 */
 		if (GetNumRegisteredWaitEvents(node->as_eventset) == 1)
 			noccurred = 0;
 		else
 		{
+			/*
+			 * Add the process latch to the set, so that we wake up to process
+			 * the standard interrupts with CHECK_FOR_INTERRUPTS().
+			 *
+			 * NOTE: For historical reasons, it's important that this is added
+			 * to the WaitEventSet after the ExecAsyncConfigureWait() calls.
+			 * Namely, postgres_fdw calls "GetNumRegisteredWaitEvents(set) ==
+			 * 1" to check if any other events are in the set.  That's a poor
+			 * design, it's questionable for postgres_fdw to be doing that in
+			 * the first place, but we cannot change it now.  The pattern has
+			 * possibly been copied to other extensions too.
+			 */
+			AddWaitEventToSet(node->as_eventset, WL_LATCH_SET, PGINVALID_SOCKET,
+							  MyLatch, NULL);
+
 			/* Return at most EVENT_BUFFER_SIZE events in one call. */
 			if (nevents > EVENT_BUFFER_SIZE)
 				nevents = EVENT_BUFFER_SIZE;
@@ -1098,6 +1113,13 @@ ExecAppendAsyncEventWait(AppendState *node)
 				ExecAsyncNotify(areq);
 			}
 		}
+
+		/* Handle standard interrupts */
+		if ((w->events & WL_LATCH_SET) != 0)
+		{
+			ResetLatch(MyLatch);
+			CHECK_FOR_INTERRUPTS();
+		}
 	}
 }