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

Author: hlinnaka

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;
@@ -1041,8 +1041,8 @@ ExecAppendAsyncEventWait(AppendState *node)
 	}
 
 	/*
-	 * No need for further processing if there are no configured events other
-	 * than the postmaster death event.
+	 * No need for further processing if none of the subplans configured any
+	 * events.
 	 */
 	if (GetNumRegisteredWaitEvents(node->as_eventset) == 1)
 	{
@@ -1051,6 +1051,21 @@ ExecAppendAsyncEventWait(AppendState *node)
 		return;
 	}
 
+	/*
+	 * 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;
@@ -1092,6 +1107,13 @@ ExecAppendAsyncEventWait(AppendState *node)
 				ExecAsyncNotify(areq);
 			}
 		}
+
+		/* Handle standard interrupts */
+		if ((w->events & WL_LATCH_SET) != 0)
+		{
+			ResetLatch(MyLatch);
+			CHECK_FOR_INTERRUPTS();
+		}
 	}
 }