From 6302fe6b28e24e87d78e457e85362dc2fa9ca119 Mon Sep 17 00:00:00 2001
From: Andrew Gierth <rhodiumtoad@postgresql.org>
Date: Fri, 17 Aug 2018 15:04:26 +0100
Subject: [PATCH] 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      | 26 +++++++++++++++++++++++--
 src/test/regress/expected/subselect.out | 17 ++++++++++++++++
 src/test/regress/sql/subselect.sql      | 16 +++++++++++++++
 3 files changed, 57 insertions(+), 2 deletions(-)

diff --git a/src/backend/executor/nodeSubplan.c b/src/backend/executor/nodeSubplan.c
index 2cf169f9566..b4883c4f8df 100644
--- a/src/backend/executor/nodeSubplan.c
+++ b/src/backend/executor/nodeSubplan.c
@@ -73,6 +73,9 @@ 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;
@@ -85,11 +88,19 @@ ExecSubPlan(SubPlanState *node,
 	if (subplan->setParam != NIL && subplan->subLinkType != MULTIEXPR_SUBLINK)
 		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;
 }
 
 /*
@@ -944,6 +955,8 @@ 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   *pvar;
@@ -957,6 +970,12 @@ 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;
+
 	/* Initialize ArrayBuildStateAny in caller's context, if needed */
 	if (subLinkType == ARRAY_SUBLINK)
 		astate = initArrayResultAny(subplan->firstColType,
@@ -1110,6 +1129,9 @@ ExecSetParamPlan(SubPlanState *node, ExprContext *econtext)
 	}
 
 	MemoryContextSwitchTo(oldcontext);
+
+	/* restore scan direction */
+	estate->es_direction = dir;
 }
 
 /*
diff --git a/src/test/regress/expected/subselect.out b/src/test/regress/expected/subselect.out
index bf076f3df38..22d8e65c41e 100644
--- a/src/test/regress/expected/subselect.out
+++ b/src/test/regress/expected/subselect.out
@@ -933,3 +933,20 @@ 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;
diff --git a/src/test/regress/sql/subselect.sql b/src/test/regress/sql/subselect.sql
index 74c4ef745d4..5fb0f2b84f7 100644
--- a/src/test/regress/sql/subselect.sql
+++ b/src/test/regress/sql/subselect.sql
@@ -496,3 +496,19 @@ 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;