Set scan direction appropriately for SubPlans (bug #15336)

When executing a SubPlan in an expression, the EState's direction
field was left alone, resulting in an attempt to execute the subplan
backwards if it was encountered during a backwards scan of a cursor.
Also, though much less likely, it was possible to reach the execution
of an InitPlan while in backwards-scan state.

Repair by saving/restoring estate->es_direction and forcing forward
scan mode in the relevant places.

Backpatch all the way, since this has been broken since 8.3 (prior to
commit c7ff7663e, SubPlans had their own EStates rather than sharing
the parent plan's, so there was no confusion over scan direction).

Per bug #15336 reported by Vladimir Baranoff; analysis and patch by
me, review by Tom Lane.

Discussion: https://postgr.es/m/153449812167.1304.1741624125628126322@wrigleys.postgresql.org

src/backend/executor/nodeSubplan.c

@@ -63,6 +63,9 @@ ExecSubPlan(SubPlanState *node,
 			ExprDoneCond *isDone)
 {
 	SubPlan    *subplan = (SubPlan *) node->xprstate.expr;
+	EState	   *estate = node->planstate->state;
+	ScanDirection dir = estate->es_direction;
+	Datum		retval;
 
 	/* Set default values for result flags: non-null, not a set result */
 	*isNull = false;
@@ -75,11 +78,19 @@ ExecSubPlan(SubPlanState *node,
 	if (subplan->setParam != NIL)
 		elog(ERROR, "cannot set parent params from subquery");
 
+	/* Force forward-scan mode for evaluation */
+	estate->es_direction = ForwardScanDirection;
+
 	/* Select appropriate evaluation strategy */
 	if (subplan->useHashTable)
-		return ExecHashSubPlan(node, econtext, isNull);
+		retval = ExecHashSubPlan(node, econtext, isNull);
 	else
-		return ExecScanSubPlan(node, econtext, isNull);
+		retval = ExecScanSubPlan(node, econtext, isNull);
+
+	/* restore scan direction */
+	estate->es_direction = dir;
+
+	return retval;
 }
 
 /*
@@ -906,6 +917,8 @@ ExecSetParamPlan(SubPlanState *node, ExprContext *econtext)
 	SubPlan    *subplan = (SubPlan *) node->xprstate.expr;
 	PlanState  *planstate = node->planstate;
 	SubLinkType subLinkType = subplan->subLinkType;
+	EState	   *estate = planstate->state;
+	ScanDirection dir = estate->es_direction;
 	MemoryContext oldcontext;
 	TupleTableSlot *slot;
 	ListCell   *l;
@@ -918,6 +931,12 @@ ExecSetParamPlan(SubPlanState *node, ExprContext *econtext)
 	if (subLinkType == CTE_SUBLINK)
 		elog(ERROR, "CTE subplans should not be executed via ExecSetParamPlan");
 
+	/*
+	 * Enforce forward scan direction regardless of caller. It's hard but not
+	 * impossible to get here in backward scan, so make it work anyway.
+	 */
+	estate->es_direction = ForwardScanDirection;
+
 	/*
 	 * Must switch to per-query memory context.
 	 */
@@ -1048,6 +1067,9 @@ ExecSetParamPlan(SubPlanState *node, ExprContext *econtext)
 	}
 
 	MemoryContextSwitchTo(oldcontext);
+
+	/* restore scan direction */
+	estate->es_direction = dir;
 }
 
 /*

src/test/regress/expected/subselect.out

@@ -851,3 +851,20 @@ select nextval('ts1');
       11
 (1 row)
 
+--
+-- Ensure that backward scan direction isn't propagated into
+-- expression subqueries (bug #15336)
+--
+begin;
+declare c1 scroll cursor for
+ select * from generate_series(1,4) i
+  where i <> all (values (2),(3));
+move forward all in c1;
+fetch backward all in c1;
+ i 
+---
+ 4
+ 1
+(2 rows)
+
+commit;

src/test/regress/sql/subselect.sql

@@ -464,3 +464,19 @@ select * from
   order by 1;
 
 select nextval('ts1');
+
+--
+-- Ensure that backward scan direction isn't propagated into
+-- expression subqueries (bug #15336)
+--
+
+begin;
+
+declare c1 scroll cursor for
+ select * from generate_series(1,4) i
+  where i <> all (values (2),(3));
+
+move forward all in c1;
+fetch backward all in c1;
+
+commit;