Ensure locks are acquired on RLS-added relations

During fireRIRrules(), get_row_security_policies can add to
securityQuals and withCheckOptions.  Make sure to lock any relations
added at that point and before firing RIR rules on those expressions.

Back-patch to 9.5 where RLS was added.

src/backend/rewrite/rewriteHandler.c

@@ -1787,6 +1787,8 @@ fireRIRrules(Query *parsetree, List *activeRIRs, bool forUpdatePushedDown)
 		{
 			if (hasSubLinks)
 			{
+				acquireLocksOnSubLinks_context context;
+
 				/*
 				 * Recursively process the new quals, checking for infinite
 				 * recursion.
@@ -1799,6 +1801,23 @@ fireRIRrules(Query *parsetree, List *activeRIRs, bool forUpdatePushedDown)
 
 				activeRIRs = lcons_oid(RelationGetRelid(rel), activeRIRs);
 
+				/*
+				 * get_row_security_policies just passed back securityQuals
+				 * and/or withCheckOptions, and there were SubLinks, make sure
+				 * we lock any relations which are referenced.
+				 *
+				 * These locks would normally be acquired by the parser, but
+				 * securityQuals and withCheckOptions are added post-parsing.
+				 */
+				context.for_execute = true;
+				(void) acquireLocksOnSubLinks((Node *) securityQuals, &context);
+				(void) acquireLocksOnSubLinks((Node *) withCheckOptions,
+											  &context);
+
+				/*
+				 * Now that we have the locks on anything added by
+				 * get_row_security_policies, fire any RIR rules for them.
+				 */
 				expression_tree_walker((Node *) securityQuals,
 									   fireRIRonSubLink, (void *) activeRIRs);