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

@@ -10424,6 +10424,32 @@ DROP TABLE local_tbl;
 DROP INDEX base_tbl1_idx;
 DROP INDEX base_tbl2_idx;
 DROP INDEX async_p3_idx;
+-- 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)
+
 -- Test that pending requests are processed properly
 SET enable_mergejoin TO false;
 SET enable_hashjoin TO false;

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -3317,6 +3317,11 @@ DROP INDEX base_tbl1_idx;
 DROP INDEX base_tbl2_idx;
 DROP INDEX async_p3_idx;
 
+-- 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;
+
 -- Test that pending requests are processed properly
 SET enable_mergejoin TO false;
 SET enable_hashjoin TO false;

src/backend/optimizer/plan/createplan.c

@@ -82,7 +82,7 @@ static List *get_gating_quals(PlannerInfo *root, List *quals);
 static Plan *create_gating_plan(PlannerInfo *root, Path *path, Plan *plan,
 								List *gating_quals);
 static Plan *create_join_plan(PlannerInfo *root, JoinPath *best_path);
-static bool is_async_capable_path(Path *path);
+static bool is_async_capable_plan(Plan *plan, Path *path);
 static Plan *create_append_plan(PlannerInfo *root, AppendPath *best_path,
 								int flags);
 static Plan *create_merge_append_plan(PlannerInfo *root, MergeAppendPath *best_path,
@@ -1109,11 +1109,11 @@ create_join_plan(PlannerInfo *root, JoinPath *best_path)
 }
 
 /*
- * is_async_capable_path
+ * is_async_capable_plan
- *		Check whether a given Path node is async-capable.
+ *		Check whether the Plan node created from a Path node is async-capable.
  */
 static bool
-is_async_capable_path(Path *path)
+is_async_capable_plan(Plan *plan, Path *path)
 {
 	switch (nodeTag(path))
 	{
@@ -1121,6 +1121,13 @@ is_async_capable_path(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))
@@ -1295,8 +1302,8 @@ create_append_plan(PlannerInfo *root, AppendPath *best_path, int flags)
 
 		subplans = lappend(subplans, subplan);
 
-		/* Check to see if subplan can be executed asynchronously */
+		/* If needed, check to see if subplan can be executed asynchronously */
-		if (consider_async && is_async_capable_path(subpath))
+		if (consider_async && is_async_capable_plan(subplan, subpath))
 		{
 			subplan->async_capable = true;
 			++nasyncplans;