Set scan direction appropriately for SubPlans (bug #15336)

RhodiumToad · RhodiumToad · commit 807c1c555064 · 2018-08-17T16:25:52.000+01:00
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 c7ff766, 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
diff --git a/src/backend/executor/nodeSubplan.c b/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;
 }
 
 /*
