Disable asynchronous execution if using gating Result nodes.

mark_async_capable_plan(), which is called from create_append_plan() to
determine whether subplans are async-capable, failed to take into
account that the given subplan created from a given subpath might
include a gating Result node if the subpath is a SubqueryScanPath or
ForeignPath, causing a segmentation fault there when the subplan created
from a SubqueryScanPath includes the Result node, or causing
ExecAsyncRequest() to throw an error about an unrecognized node type
when the subplan created from a ForeignPath includes the Result node,
because in the latter case the Result node was unintentionally
considered as async-capable, but we don't currently support executing
Result nodes asynchronously.  Fix by modifying mark_async_capable_plan()
to disable asynchronous execution in such cases.  Also, adjust code in
the ProjectionPath case in mark_async_capable_plan(), for consistency
with other cases, and adjust/improve comments there.

is_async_capable_path() added in commit 27e1f1456, which was rewritten
to mark_async_capable_plan() in a later commit, has the same issue,
causing the error at execution mentioned above, so back-patch to v14
where the aforesaid commit went in.

Per report from Justin Pryzby.

Etsuro Fujita, reviewed by Zhihong Yu and Justin Pryzby.

Discussion: https://postgr.es/m/20220408124338.GK24419%40telsasoft.com

1 files changed, 66 insertions, 0 deletions

diff --git a/contrib/postgres_fdw/expected/postgres_fdw.out b/contrib/postgres_fdw/expected/postgres_fdw.out
index 477de09a876..44457f930c2 100644
--- a/contrib/postgres_fdw/expected/postgres_fdw.out
+++ b/contrib/postgres_fdw/expected/postgres_fdw.out
@@ -10734,6 +10734,72 @@ SELECT * FROM result_tbl ORDER BY a;
 (12 rows)
 
 DELETE FROM result_tbl;
+-- Disable async execution if we use gating Result nodes for pseudoconstant
+-- quals
+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT * FROM async_pt WHERE CURRENT_USER = SESSION_USER;
+                           QUERY PLAN                           
+----------------------------------------------------------------
+ Append
+   ->  Result
+         Output: async_pt_1.a, async_pt_1.b, async_pt_1.c
+         One-Time Filter: (CURRENT_USER = SESSION_USER)
+         ->  Foreign Scan on public.async_p1 async_pt_1
+               Output: async_pt_1.a, async_pt_1.b, async_pt_1.c
+               Remote SQL: SELECT a, b, c FROM public.base_tbl1
+   ->  Result
+         Output: async_pt_2.a, async_pt_2.b, async_pt_2.c
+         One-Time Filter: (CURRENT_USER = SESSION_USER)
+         ->  Foreign Scan on public.async_p2 async_pt_2
+               Output: async_pt_2.a, async_pt_2.b, async_pt_2.c
+               Remote SQL: SELECT a, b, c FROM public.base_tbl2
+   ->  Result
+         Output: async_pt_3.a, async_pt_3.b, async_pt_3.c
+         One-Time Filter: (CURRENT_USER = SESSION_USER)
+         ->  Seq Scan on public.async_p3 async_pt_3
+               Output: async_pt_3.a, async_pt_3.b, async_pt_3.c
+(18 rows)
+
+EXPLAIN (VERBOSE, COSTS OFF)
+(SELECT * FROM async_p1 WHERE CURRENT_USER = SESSION_USER)
+UNION ALL
+(SELECT * FROM async_p2 WHERE CURRENT_USER = SESSION_USER);
+                           QUERY PLAN                           
+----------------------------------------------------------------
+ Append
+   ->  Result
+         Output: async_p1.a, async_p1.b, async_p1.c
+         One-Time Filter: (CURRENT_USER = SESSION_USER)
+         ->  Foreign Scan on public.async_p1
+               Output: async_p1.a, async_p1.b, async_p1.c
+               Remote SQL: SELECT a, b, c FROM public.base_tbl1
+   ->  Result
+         Output: async_p2.a, async_p2.b, async_p2.c
+         One-Time Filter: (CURRENT_USER = SESSION_USER)
+         ->  Foreign Scan on public.async_p2
+               Output: async_p2.a, async_p2.b, async_p2.c
+               Remote SQL: SELECT a, b, c FROM public.base_tbl2
+(13 rows)
+
+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT * FROM ((SELECT * FROM async_p1 WHERE b < 10) UNION ALL (SELECT * FROM async_p2 WHERE b < 10)) s WHERE CURRENT_USER = SESSION_USER;
+                                   QUERY PLAN                                    
+---------------------------------------------------------------------------------
+ Append
+   ->  Result
+         Output: async_p1.a, async_p1.b, async_p1.c
+         One-Time Filter: (CURRENT_USER = SESSION_USER)
+         ->  Foreign Scan on public.async_p1
+               Output: async_p1.a, async_p1.b, async_p1.c
+               Remote SQL: SELECT a, b, c FROM public.base_tbl1 WHERE ((b < 10))
+   ->  Result
+         Output: async_p2.a, async_p2.b, async_p2.c
+         One-Time Filter: (CURRENT_USER = SESSION_USER)
+         ->  Foreign Scan on public.async_p2
+               Output: async_p2.a, async_p2.b, async_p2.c
+               Remote SQL: SELECT a, b, c FROM public.base_tbl2 WHERE ((b < 10))
+(13 rows)
+
 -- Test that pending requests are processed properly
 SET enable_mergejoin TO false;
 SET enable_hashjoin TO false;