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

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;

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -3410,6 +3410,19 @@ UNION ALL
 SELECT * FROM result_tbl ORDER BY a;
 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;
+
+EXPLAIN (VERBOSE, COSTS OFF)
+(SELECT * FROM async_p1 WHERE CURRENT_USER = SESSION_USER)
+UNION ALL
+(SELECT * FROM async_p2 WHERE CURRENT_USER = SESSION_USER);
+
+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;
+
 -- Test that pending requests are processed properly
 SET enable_mergejoin TO false;
 SET enable_hashjoin TO false;

src/backend/optimizer/plan/createplan.c

@@ -1112,9 +1112,9 @@ create_join_plan(PlannerInfo *root, JoinPath *best_path)
 
 /*
  * mark_async_capable_plan
- *		Check whether a given Path node is async-capable, and if so, mark the
+ *		Check whether the Plan node created from a Path node is async-capable,
- *		Plan node created from it as such and return true, otherwise return
+ *		and if so, mark the Plan node as such and return true, otherwise
- *		false.
+ *		return false.
  */
 static bool
 mark_async_capable_plan(Plan *plan, Path *path)
@@ -1125,6 +1125,13 @@ mark_async_capable_plan(Plan *plan, Path *path)
 			{
 				SubqueryScan *scan_plan = (SubqueryScan *) plan;
 
+				/*
+				 * If the generated plan node includes a gating Result node,
+				 * we can't execute it asynchronously.
+				 */
+				if (IsA(plan, Result))
+					return false;
+
 				/*
 				 * If a SubqueryScan node atop of an async-capable plan node
 				 * is deletable, consider it as async-capable.
@@ -1139,6 +1146,13 @@ mark_async_capable_plan(Plan *plan, Path *path)
 			{
 				FdwRoutine *fdwroutine = path->parent->fdwroutine;
 
+				/*
+				 * If the generated plan node includes a gating Result node,
+				 * we can't execute it asynchronously.
+				 */
+				if (IsA(plan, Result))
+					return false;
+
 				Assert(fdwroutine != NULL);
 				if (fdwroutine->IsForeignPathAsyncCapable != NULL &&
 					fdwroutine->IsForeignPathAsyncCapable((ForeignPath *) path))
@@ -1148,11 +1162,17 @@ mark_async_capable_plan(Plan *plan, Path *path)
 		case T_ProjectionPath:
 
 			/*
-			 * If the generated plan node doesn't include a Result node,
+			 * If the generated plan node includes a Result node for
-			 * consider it as async-capable if the subpath is async-capable.
+			 * the projection, we can't execute it asynchronously.
 			 */
-			if (!IsA(plan, Result) &&
+			if (IsA(plan, Result))
-				mark_async_capable_plan(plan,
+				return false;
+
+			/*
+			 * create_projection_plan() would have pulled up the subplan, so
+			 * check the capability using the subpath.
+			 */
+			if (mark_async_capable_plan(plan,
 										((ProjectionPath *) path)->subpath))
 				return true;
 			return false;