Fix usage of whole-row variables in WCO and RLS policy expressions.

Since WITH CHECK OPTION was introduced, ExecInitModifyTable has
initialized WCO expressions with the wrong plan node as parent -- that is,
it passed its input subplan not the ModifyTable node itself.  Up to now
we thought this was harmless, but bug #16006 from Vinay Banakar shows it's
not: if the input node is a SubqueryScan then ExecInitWholeRowVar can get
confused into doing the wrong thing.  (The fact that ExecInitWholeRowVar
contains such logic is certainly a horrid kluge that doesn't deserve to
live, but figuring out another way to do that is a task for some other day.)

Andres had already noticed the wrong-parent mistake and fixed it in commit
148e632, but not being aware of any user-visible consequences, he quite
reasonably didn't back-patch.  This patch is simply a back-patch of
148e632, plus addition of a test case based on bug #16006.  I also added
the test case to v12/HEAD, even though the bug is already fixed there.

Back-patch to all supported branches.  9.4 lacks RLS policies so the
new test case doesn't work there, but I'm pretty sure a test could be
devised based on using a whole-row Var in a plain WITH CHECK OPTION
condition.  (I lack the cycles to do so myself, though.)

Andres Freund and Tom Lane

Discussion: https://postgr.es/m/16006-99290d2e4642cbd5@postgresql.org
Discussion: https://postgr.es/m/20181205225213.hiwa3kgoxeybqcqv@alap3.anarazel.de

Author: tglsfdc

src/test/regress/expected/rowsecurity.out

@@ -3958,6 +3958,40 @@ DROP OPERATOR <<< (int, int);
 DROP FUNCTION op_leak(int, int);
 RESET SESSION AUTHORIZATION;
 DROP TABLE rls_tbl;
+-- Bug #16006: whole-row Vars in a policy don't play nice with sub-selects
+SET SESSION AUTHORIZATION regress_rls_alice;
+CREATE TABLE rls_tbl (a int, b int, c int);
+CREATE POLICY p1 ON rls_tbl USING (rls_tbl >= ROW(1,1,1));
+ALTER TABLE rls_tbl ENABLE ROW LEVEL SECURITY;
+ALTER TABLE rls_tbl FORCE ROW LEVEL SECURITY;
+INSERT INTO rls_tbl SELECT 10, 20, 30;
+EXPLAIN (VERBOSE, COSTS OFF)
+INSERT INTO rls_tbl
+  SELECT * FROM (SELECT b, c FROM rls_tbl ORDER BY a) ss;
+                             QUERY PLAN                             
+--------------------------------------------------------------------
+ Insert on regress_rls_schema.rls_tbl
+   ->  Subquery Scan on ss
+         Output: ss.b, ss.c, NULL::integer
+         ->  Sort
+               Output: rls_tbl_1.b, rls_tbl_1.c, rls_tbl_1.a
+               Sort Key: rls_tbl_1.a
+               ->  Seq Scan on regress_rls_schema.rls_tbl rls_tbl_1
+                     Output: rls_tbl_1.b, rls_tbl_1.c, rls_tbl_1.a
+                     Filter: (rls_tbl_1.* >= '(1,1,1)'::record)
+(9 rows)
+
+INSERT INTO rls_tbl
+  SELECT * FROM (SELECT b, c FROM rls_tbl ORDER BY a) ss;
+SELECT * FROM rls_tbl;
+ a  | b  | c  
+----+----+----
+ 10 | 20 | 30
+ 20 | 30 |   
+(2 rows)
+
+DROP TABLE rls_tbl;
+RESET SESSION AUTHORIZATION;
 --
 -- Clean up objects
 --

src/test/regress/sql/rowsecurity.sql

@@ -1810,6 +1810,25 @@ DROP FUNCTION op_leak(int, int);
 RESET SESSION AUTHORIZATION;
 DROP TABLE rls_tbl;
 
+-- Bug #16006: whole-row Vars in a policy don't play nice with sub-selects
+SET SESSION AUTHORIZATION regress_rls_alice;
+CREATE TABLE rls_tbl (a int, b int, c int);
+CREATE POLICY p1 ON rls_tbl USING (rls_tbl >= ROW(1,1,1));
+
+ALTER TABLE rls_tbl ENABLE ROW LEVEL SECURITY;
+ALTER TABLE rls_tbl FORCE ROW LEVEL SECURITY;
+
+INSERT INTO rls_tbl SELECT 10, 20, 30;
+EXPLAIN (VERBOSE, COSTS OFF)
+INSERT INTO rls_tbl
+  SELECT * FROM (SELECT b, c FROM rls_tbl ORDER BY a) ss;
+INSERT INTO rls_tbl
+  SELECT * FROM (SELECT b, c FROM rls_tbl ORDER BY a) ss;
+SELECT * FROM rls_tbl;
+
+DROP TABLE rls_tbl;
+RESET SESSION AUTHORIZATION;
+
 --
 -- Clean up objects
 --