Fix confusion about number of subplans in partitioned INSERT setup.

ExecInitModifyTable() thought there was a plan per partition, but no,
there's only one.  The problem had escaped detection so far because there
would only be visible misbehavior if there were a SubPlan (not an InitPlan)
in the quals being duplicated for each partition.  However, valgrind
detected a bogus memory access in test cases added by commit 4f7a95be2,
and investigation of that led to discovery of the bug.  The additional
test case added here crashes without the patch.

Patch by Amit Langote, test case by me.

Discussion: https://postgr.es/m/10974.1497227727@sss.pgh.pa.us

src/backend/executor/nodeModifyTable.c

@@ -1841,10 +1841,21 @@ ExecInitModifyTable(ModifyTable *node, EState *estate, int eflags)
 	if (node->withCheckOptionLists != NIL && mtstate->mt_num_partitions > 0)
 	{
 		List	   *wcoList;
+		PlanState  *plan;
 
-		Assert(operation == CMD_INSERT);
-		resultRelInfo = mtstate->mt_partitions;
+		/*
+		 * In case of INSERT on partitioned tables, there is only one plan.
+		 * Likewise, there is only one WITH CHECK OPTIONS list, not one per
+		 * partition.  We make a copy of the WCO qual for each partition; note
+		 * that, if there are SubPlans in there, they all end up attached to
+		 * the one parent Plan node.
+		 */
+		Assert(operation == CMD_INSERT &&
+			   list_length(node->withCheckOptionLists) == 1 &&
+			   mtstate->mt_nplans == 1);
 		wcoList = linitial(node->withCheckOptionLists);
+		plan = mtstate->mt_plans[0];
+		resultRelInfo = mtstate->mt_partitions;
 		for (i = 0; i < mtstate->mt_num_partitions; i++)
 		{
 			Relation	partrel = resultRelInfo->ri_RelationDesc;
@@ -1858,9 +1869,9 @@ ExecInitModifyTable(ModifyTable *node, EState *estate, int eflags)
 													 partrel, rel);
 			foreach(ll, mapped_wcoList)
 			{
-				WithCheckOption *wco = (WithCheckOption *) lfirst(ll);
-				ExprState  *wcoExpr = ExecInitQual((List *) wco->qual,
-												   mtstate->mt_plans[i]);
+				WithCheckOption *wco = castNode(WithCheckOption, lfirst(ll));
+				ExprState  *wcoExpr = ExecInitQual(castNode(List, wco->qual),
+												   plan);
 
 				wcoExprs = lappend(wcoExprs, wcoExpr);
 			}

src/test/regress/expected/rowsecurity.out

@@ -1327,6 +1327,14 @@ SELECT * FROM part_document ORDER by did;
 ERROR:  query would be affected by row-level security policy for table "part_document"
 SELECT * FROM part_document_satire ORDER by did;
 ERROR:  query would be affected by row-level security policy for table "part_document_satire"
+-- Check behavior with a policy that uses a SubPlan not an InitPlan.
+SET SESSION AUTHORIZATION regress_rls_alice;
+SET row_security TO ON;
+CREATE POLICY pp3 ON part_document AS RESTRICTIVE
+    USING ((SELECT dlevel <= seclv FROM uaccount WHERE pguser = current_user));
+SET SESSION AUTHORIZATION regress_rls_carol;
+INSERT INTO part_document VALUES (100, 11, 5, 'regress_rls_carol', 'testing pp3'); -- fail
+ERROR:  new row violates row-level security policy "pp3" for table "part_document"
 ----- Dependencies -----
 SET SESSION AUTHORIZATION regress_rls_alice;
 SET row_security TO ON;

src/test/regress/sql/rowsecurity.sql

@@ -450,6 +450,15 @@ SET row_security TO OFF;
 SELECT * FROM part_document ORDER by did;
 SELECT * FROM part_document_satire ORDER by did;
 
+-- Check behavior with a policy that uses a SubPlan not an InitPlan.
+SET SESSION AUTHORIZATION regress_rls_alice;
+SET row_security TO ON;
+CREATE POLICY pp3 ON part_document AS RESTRICTIVE
+    USING ((SELECT dlevel <= seclv FROM uaccount WHERE pguser = current_user));
+
+SET SESSION AUTHORIZATION regress_rls_carol;
+INSERT INTO part_document VALUES (100, 11, 5, 'regress_rls_carol', 'testing pp3'); -- fail
+
 ----- Dependencies -----
 SET SESSION AUTHORIZATION regress_rls_alice;
 SET row_security TO ON;