Fix oversight in commit 1ec7fca859.

I failed to account for the possibility that when
ExecAppendAsyncEventWait() notifies multiple async-capable nodes using
postgres_fdw, a preceding node might invoke process_pending_request() to
process a pending asynchronous request made by a succeeding node.  In
that case the succeeding node should produce a tuple to return to the
parent Append node from tuples fetched by process_pending_request() when
notified.  Repair.

Per buildfarm via Michael Paquier.  Back-patch to v14, like the previous
commit.

Thanks to Tom Lane for testing.

Discussion: https://postgr.es/m/YQP0UPT8KmPiHTMs%40paquier.xyz

contrib/postgres_fdw/postgres_fdw.c

@@ -6896,12 +6896,26 @@ postgresForeignAsyncNotify(AsyncRequest *areq)
 	ForeignScanState *node = (ForeignScanState *) areq->requestee;
 	PgFdwScanState *fsstate = (PgFdwScanState *) node->fdw_state;
 
-	/* The request should be currently in-process */
-	Assert(fsstate->conn_state->pendingAreq == areq);
-
 	/* The core code would have initialized the callback_pending flag */
 	Assert(!areq->callback_pending);
 
+	/*
+	 * If process_pending_request() has been invoked on the given request
+	 * before we get here, we might have some tuples already; in which case
+	 * produce the next tuple
+	 */
+	if (fsstate->next_tuple < fsstate->num_tuples)
+	{
+		produce_tuple_asynchronously(areq, true);
+		return;
+	}
+
+	/* We must have run out of tuples */
+	Assert(fsstate->next_tuple >= fsstate->num_tuples);
+
+	/* The request should be currently in-process */
+	Assert(fsstate->conn_state->pendingAreq == areq);
+
 	/* On error, report the original query, not the FETCH. */
 	if (!PQconsumeInput(fsstate->conn))
 		pgfdw_report_error(ERROR, NULL, fsstate->conn, false, fsstate->query);
@@ -7027,7 +7041,7 @@ process_pending_request(AsyncRequest *areq)
 	/*
 	 * If we didn't get any tuples, must be end of data; complete the request
 	 * now.  Otherwise, we postpone completing the request until we are called
-	 * from postgresForeignAsyncConfigureWait().
+	 * from postgresForeignAsyncConfigureWait()/postgresForeignAsyncNotify().
 	 */
 	if (fsstate->next_tuple >= fsstate->num_tuples)
 	{

src/backend/executor/nodeAppend.c

@@ -1082,16 +1082,18 @@ ExecAppendAsyncEventWait(AppendState *node)
 		{
 			AsyncRequest *areq = (AsyncRequest *) w->user_data;
 
-			/*
-			 * Mark it as no longer needing a callback.  We must do this
-			 * before dispatching the callback in case the callback resets the
-			 * flag.
-			 */
-			Assert(areq->callback_pending);
-			areq->callback_pending = false;
-
-			/* Do the actual work. */
-			ExecAsyncNotify(areq);
+			if (areq->callback_pending)
+			{
+				/*
+				 * Mark it as no longer needing a callback.  We must do this
+				 * before dispatching the callback in case the callback resets
+				 * the flag.
+				 */
+				areq->callback_pending = false;
+
+				/* Do the actual work. */
+				ExecAsyncNotify(areq);
+			}
 		}
 	}
 }