Further fix for EvalPlanQual with mix of local and foreign partitions.

We assume that direct-modify ForeignScan nodes cannot be re-evaluated
during EvalPlanQual processing, but the rework for inherited
UPDATE/DELETE in commit 86dc90056 changed things, without considering
that, so that such ForeignScan nodes get called as part of the
EvalPlanQual subtree during EvalPlanQual processing in the case of an
inherited UPDATE/DELETE where the inheritance set contains foreign
target relations.  To avoid re-evaluating such ForeignScan nodes during
EvalPlanQual processing, commit c3928b467 modified nodeForeignscan.c,
but the assumption made there that ExecForeignScan() should never be
called for such ForeignScan nodes during EvalPlanQual processing turned
out to be wrong in some cases, leading to a segmentation fault or a
"cannot re-evaluate a Foreign Update or Delete during EvalPlanQual"
error.

Fix by modifying nodeForeignscan.c further to avoid re-evaluating such
ForeignScan nodes even in ExecForeignScan()/ExecReScanForeignScan()
during EvalPlanQual processing.  Since this makes non-reachable the
test-and-elog added to ForeignNext() by commit c3928b467 that produced
the aforesaid error, convert the test-and-elog to an Assert.

Per bug #17355 from Alexander Lakhin.  Back-patch to v14 where both
commits came in.

Patch by me, reviewed and tested by Alexander Lakhin and Amit Langote.

Discussion: https://postgr.es/m/17355-de8e362eb7001a96@postgresql.org

src/backend/executor/nodeForeignscan.c

@@ -55,8 +55,7 @@ ForeignNext(ForeignScanState *node)
 		 * direct modifications cannot be re-evaluated, so shouldn't get here
 		 * during EvalPlanQual processing
 		 */
-		if (estate->es_epq_active != NULL)
-			elog(ERROR, "cannot re-evaluate a Foreign Update or Delete during EvalPlanQual");
+		Assert(estate->es_epq_active == NULL);
 
 		slot = node->fdwroutine->IterateDirectModify(node);
 	}
@@ -121,6 +120,15 @@ static TupleTableSlot *
 ExecForeignScan(PlanState *pstate)
 {
 	ForeignScanState *node = castNode(ForeignScanState, pstate);
+	ForeignScan *plan = (ForeignScan *) node->ss.ps.plan;
+	EState	   *estate = node->ss.ps.state;
+
+	/*
+	 * Ignore direct modifications when EvalPlanQual is active --- they are
+	 * irrelevant for EvalPlanQual rechecking
+	 */
+	if (estate->es_epq_active != NULL && plan->operation != CMD_SELECT)
+		return NULL;
 
 	return ExecScan(&node->ss,
 					(ExecScanAccessMtd) ForeignNext,
@@ -265,9 +273,12 @@ ExecInitForeignScan(ForeignScan *node, EState *estate, int eflags)
 	{
 		/*
 		 * Direct modifications cannot be re-evaluated by EvalPlanQual, so
-		 * don't bother preparing the FDW.  There can be ForeignScan nodes in
-		 * the EvalPlanQual subtree, but ExecForeignScan should never be
-		 * called on them when EvalPlanQual is active.
+		 * don't bother preparing the FDW.
+		 *
+		 * In case of an inherited UPDATE/DELETE with foreign targets there
+		 * can be direct-modify ForeignScan nodes in the EvalPlanQual subtree,
+		 * so we need to ignore such ForeignScan nodes during EvalPlanQual
+		 * processing.  See also ExecForeignScan/ExecReScanForeignScan.
 		 */
 		if (estate->es_epq_active == NULL)
 			fdwroutine->BeginDirectModify(scanstate, eflags);
@@ -321,8 +332,17 @@ ExecEndForeignScan(ForeignScanState *node)
 void
 ExecReScanForeignScan(ForeignScanState *node)
 {
+	ForeignScan *plan = (ForeignScan *) node->ss.ps.plan;
+	EState	   *estate = node->ss.ps.state;
 	PlanState  *outerPlan = outerPlanState(node);
 
+	/*
+	 * Ignore direct modifications when EvalPlanQual is active --- they are
+	 * irrelevant for EvalPlanQual rechecking
+	 */
+	if (estate->es_epq_active != NULL && plan->operation != CMD_SELECT)
+		return;
+
 	node->fdwroutine->ReScanForeignScan(node);
 
 	/*