From d2ecc27c374ad04b06fc62f365ff9445854153fd 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 9eb4d63e1ac..cecd8d9f469 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;
 }
 
 /*
@@ -948,6 +959,8 @@ 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;
@@ -961,6 +974,12 @@ 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,
@@ -1114,6 +1133,9 @@ 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 4cc40913d2d..7002049eb0d 100644
--- a/src/test/regress/expected/subselect.out
+++ b/src/test/regress/expected/subselect.out
@@ -913,3 +913,20 @@ 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 a1e726debc9..c12b3ecb6b3 100644
--- a/src/test/regress/sql/subselect.sql
+++ b/src/test/regress/sql/subselect.sql
@@ -491,3 +491,19 @@ 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;