Reject SELECT ... GROUP BY GROUPING SETS (()) FOR UPDATE.

This case should be disallowed, just as FOR UPDATE with a plain
GROUP BY is disallowed; FOR UPDATE only makes sense when each row
of the query result can be identified with a single table row.
However, we missed teaching CheckSelectLocking() to check
groupingSets as well as groupClause, so that it would allow
degenerate grouping sets.  That resulted in a bad plan and
a null-pointer dereference in the executor.

Looking around for other instances of the same bug, the only one
I found was in examine_simple_variable().  That'd just lead to
silly estimates, but it should be fixed too.

Per private report from Yaoguang Chen.
Back-patch to all supported branches.

src/backend/parser/analyze.c

@@ -2688,7 +2688,7 @@ CheckSelectLocking(Query *qry, LockClauseStrength strength)
 		  translator: %s is a SQL row locking clause such as FOR UPDATE */
 				 errmsg("%s is not allowed with DISTINCT clause",
 						LCS_asString(strength))));
-	if (qry->groupClause != NIL)
+	if (qry->groupClause != NIL || qry->groupingSets != NIL)
 		ereport(ERROR,
 				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
 		/*------

src/backend/utils/adt/selfuncs.c

@@ -5150,7 +5150,8 @@ examine_simple_variable(PlannerInfo *root, Var *var,
 		 * of learning something even with it.
 		 */
 		if (subquery->setOperations ||
-			subquery->groupClause)
+			subquery->groupClause ||
+			subquery->groupingSets)
 			return;
 
 		/*

src/test/regress/expected/errors.out

@@ -50,6 +50,11 @@ select distinct on (foobar) * from pg_database;
 ERROR:  column "foobar" does not exist
 LINE 1: select distinct on (foobar) * from pg_database;
                             ^
+-- grouping with FOR UPDATE
+select null from pg_database group by datname for update;
+ERROR:  FOR UPDATE is not allowed with GROUP BY clause
+select null from pg_database group by grouping sets (()) for update;
+ERROR:  FOR UPDATE is not allowed with GROUP BY clause
 --
 -- DELETE
 -- missing relation name (this had better not wildcard!)

src/test/regress/sql/errors.sql

@@ -37,6 +37,10 @@ select * from pg_database where pg_database.datname = nonesuch;
 -- bad attribute name in select distinct on
 select distinct on (foobar) * from pg_database;
 
+-- grouping with FOR UPDATE
+select null from pg_database group by datname for update;
+select null from pg_database group by grouping sets (()) for update;
+
 
 --
 -- DELETE