Prevent asynchronous execution of direct foreign-table modifications.

Commits 27e1f145 and 86dc9005, which were independently discussed,
cause a crash when executing an inherited foreign UPDATE/DELETE query
with asynchronous execution enabled, where children of an Append node
that is the direct/indirect child of the ModifyTable node are rewritten
so as to modify foreign tables directly by postgresPlanDirectModify();
as in that case the direct modifications are executed asynchronously,
which is not currently supported by asynchronous execution.  Fix by
disabling asynchronous execution of the direct modifications in that
function.

Author: Etsuro Fujita
Reviewed-by: Amit Langote
Discussion: https://postgr.es/m/CAPmGK158e9sJOfuWxfn%2B0ynrspXQU3JhNjSCbaoeSzMvnga%2Bbw%40mail.gmail.com

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -10154,6 +10154,61 @@ DROP TABLE base_tbl3;
 DROP TABLE base_tbl4;
 RESET enable_mergejoin;
 RESET enable_hashjoin;
+-- Test that UPDATE/DELETE with inherited target works with async_capable enabled
+EXPLAIN (VERBOSE, COSTS OFF)
+UPDATE async_pt SET c = c || c WHERE b = 0 RETURNING *;
+                                                QUERY PLAN                                                
+----------------------------------------------------------------------------------------------------------
+ Update on public.async_pt
+   Output: async_pt_1.a, async_pt_1.b, async_pt_1.c
+   Foreign Update on public.async_p1 async_pt_1
+   Foreign Update on public.async_p2 async_pt_2
+   Update on public.async_p3 async_pt_3
+   ->  Append
+         ->  Foreign Update on public.async_p1 async_pt_1
+               Remote SQL: UPDATE public.base_tbl1 SET c = (c || c) WHERE ((b = 0)) RETURNING a, b, c
+         ->  Foreign Update on public.async_p2 async_pt_2
+               Remote SQL: UPDATE public.base_tbl2 SET c = (c || c) WHERE ((b = 0)) RETURNING a, b, c
+         ->  Seq Scan on public.async_p3 async_pt_3
+               Output: (async_pt_3.c || async_pt_3.c), async_pt_3.tableoid, async_pt_3.ctid, NULL::record
+               Filter: (async_pt_3.b = 0)
+(13 rows)
+
+UPDATE async_pt SET c = c || c WHERE b = 0 RETURNING *;
+  a   | b |    c     
+------+---+----------
+ 1000 | 0 | 00000000
+ 2000 | 0 | 00000000
+ 3000 | 0 | 00000000
+(3 rows)
+
+EXPLAIN (VERBOSE, COSTS OFF)
+DELETE FROM async_pt WHERE b = 0 RETURNING *;
+                                        QUERY PLAN                                        
+------------------------------------------------------------------------------------------
+ Delete on public.async_pt
+   Output: async_pt_1.a, async_pt_1.b, async_pt_1.c
+   Foreign Delete on public.async_p1 async_pt_1
+   Foreign Delete on public.async_p2 async_pt_2
+   Delete on public.async_p3 async_pt_3
+   ->  Append
+         ->  Foreign Delete on public.async_p1 async_pt_1
+               Remote SQL: DELETE FROM public.base_tbl1 WHERE ((b = 0)) RETURNING a, b, c
+         ->  Foreign Delete on public.async_p2 async_pt_2
+               Remote SQL: DELETE FROM public.base_tbl2 WHERE ((b = 0)) RETURNING a, b, c
+         ->  Seq Scan on public.async_p3 async_pt_3
+               Output: async_pt_3.tableoid, async_pt_3.ctid
+               Filter: (async_pt_3.b = 0)
+(13 rows)
+
+DELETE FROM async_pt WHERE b = 0 RETURNING *;
+  a   | b |    c     
+------+---+----------
+ 1000 | 0 | 00000000
+ 2000 | 0 | 00000000
+ 3000 | 0 | 00000000
+(3 rows)
+
 -- Check EXPLAIN ANALYZE for a query that scans empty partitions asynchronously
 DELETE FROM async_p1;
 DELETE FROM async_p2;

contrib/postgres_fdw/postgres_fdw.c

@@ -2530,6 +2530,13 @@ postgresPlanDirectModify(PlannerInfo *root,
 			rebuild_fdw_scan_tlist(fscan, returningList);
 	}
 
+	/*
+	 * Finally, unset the async-capable flag if it is set, as we currently
+	 * don't support asynchronous execution of direct modifications.
+	 */
+	if (fscan->scan.plan.async_capable)
+		fscan->scan.plan.async_capable = false;
+
 	table_close(rel, NoLock);
 	return true;
 }

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -3237,6 +3237,14 @@ DROP TABLE base_tbl4;
 RESET enable_mergejoin;
 RESET enable_hashjoin;
 
+-- Test that UPDATE/DELETE with inherited target works with async_capable enabled
+EXPLAIN (VERBOSE, COSTS OFF)
+UPDATE async_pt SET c = c || c WHERE b = 0 RETURNING *;
+UPDATE async_pt SET c = c || c WHERE b = 0 RETURNING *;
+EXPLAIN (VERBOSE, COSTS OFF)
+DELETE FROM async_pt WHERE b = 0 RETURNING *;
+DELETE FROM async_pt WHERE b = 0 RETURNING *;
+
 -- Check EXPLAIN ANALYZE for a query that scans empty partitions asynchronously
 DELETE FROM async_p1;
 DELETE FROM async_p2;