Fix bogus coding in ExecAppendAsyncEventWait().

No configured-by-FDW events would result in "return" directly out of a
PG_TRY block, making the exception stack dangling.  Repair.

Oversight in commit 501cfd07d; back-patch to v14, like that commit, but
as we do not have this issue in HEAD (cf. commit 50c67c201), no need to
apply this patch to it.

In passing, improve a comment about the handling of in-process requests
in a postgres_fdw.c function called from this function.

Alexander Pyhalov, with comment adjustment/improvement by me.

Discussion: https://postgr.es/m/425fa29a429b21b0332737c42a4fdc70%40postgrespro.ru

contrib/postgres_fdw/postgres_fdw.c

@@ -7174,14 +7174,16 @@ postgresForeignAsyncConfigureWait(AsyncRequest *areq)
 	{
 		/*
 		 * This is the case when the in-process request was made by another
-		 * Append.  Note that it might be useless to process the request,
-		 * because the query might not need tuples from that Append anymore.
-		 * If there are any child subplans of the same parent that are ready
-		 * for new requests, skip the given request.  Likewise, if there are
-		 * any configured events other than the postmaster death event, skip
-		 * it.  Otherwise, process the in-process request, then begin a fetch
-		 * to configure the event below, because we might otherwise end up
-		 * with no configured events other than the postmaster death event.
+		 * Append.  Note that it might be useless to process the request made
+		 * by that Append, because the query might not need tuples from that
+		 * Append anymore; so we avoid processing it to begin a fetch for the
+		 * given request if possible.  If there are any child subplans of the
+		 * same parent that are ready for new requests, skip the given
+		 * request.  Likewise, if there are any configured events other than
+		 * the postmaster death event, skip it.  Otherwise, process the
+		 * in-process request, then begin a fetch to configure the event
+		 * below, because we might otherwise end up with no configured events
+		 * other than the postmaster death event.
 		 */
 		if (!bms_is_empty(requestor->as_needrequest))
 			return;

src/backend/executor/nodeAppend.c

@@ -1043,26 +1043,25 @@ ExecAppendAsyncEventWait(AppendState *node)
 		}
 
 		/*
-		 * No need for further processing if there are no configured events
-		 * other than the postmaster death event.
+		 * If there are no configured events other than the postmaster death
+		 * event, we don't need to wait or poll.
 		 */
 		if (GetNumRegisteredWaitEvents(node->as_eventset) == 1)
+			noccurred = 0;
+		else
 		{
-			FreeWaitEventSet(node->as_eventset);
-			node->as_eventset = NULL;
-			return;
-		}
+			/* Return at most EVENT_BUFFER_SIZE events in one call. */
+			if (nevents > EVENT_BUFFER_SIZE)
+				nevents = EVENT_BUFFER_SIZE;
 
-		/* Return at most EVENT_BUFFER_SIZE events in one call. */
-		if (nevents > EVENT_BUFFER_SIZE)
-			nevents = EVENT_BUFFER_SIZE;
-
-		/*
-		 * If the timeout is -1, wait until at least one event occurs.  If the
-		 * timeout is 0, poll for events, but do not wait at all.
-		 */
-		noccurred = WaitEventSetWait(node->as_eventset, timeout, occurred_event,
-									 nevents, WAIT_EVENT_APPEND_READY);
+			/*
+			 * If the timeout is -1, wait until at least one event occurs.  If
+			 * the timeout is 0, poll for events, but do not wait at all.
+			 */
+			noccurred = WaitEventSetWait(node->as_eventset, timeout,
+										 occurred_event, nevents,
+										 WAIT_EVENT_APPEND_READY);
+		}
 	}
 	PG_FINALLY();
 	{