Prevent crashes on update with returning clauses (#6643)

If an update query on a reference table has a returns clause with a
subquery that accesses some other local table, we end-up with an crash.

This commit prevents the crash, but does not prevent other error
messages from happening due to Citus not being able to pushdown the
results of that subquery in a valid SQL command.

Related: #6634

src/backend/distributed/planner/combine_query_planner.c

@@ -338,6 +338,10 @@ FindCitusExtradataContainerRTE(Node *node, RangeTblEntry **result)
 		{
 			RangeTblFunction *rangeTblFunction = (RangeTblFunction *) linitial(
 				rangeTblEntry->functions);
+			if (!IsA(rangeTblFunction->funcexpr, FuncExpr))
+			{
+				return false;
+			}
 			FuncExpr *funcExpr = castNode(FuncExpr, rangeTblFunction->funcexpr);
 			if (funcExpr->funcid == CitusExtraDataContainerFuncId())
 			{

src/test/regress/expected/coordinator_evaluation.out

@@ -609,5 +609,18 @@ SELECT * FROM reference_table ORDER BY 1;
  (5)
 (5 rows)
 
+-- failing UPDATE on a reference table with a subquery in RETURNING clause that needs to be pushed-down.
+-- the error message is not great, but at least we no longer see crashes.
+CREATE TABLE ref (a int);
+SELECT create_reference_table('ref');
+ create_reference_table
+---------------------------------------------------------------------
+
+(1 row)
+
+UPDATE ref SET a = 1 RETURNING
+	(SELECT pg_catalog.max(latest_end_time) FROM pg_catalog.pg_stat_wal_receiver)
+        as c3;
+ERROR:  a column definition list is required for functions returning "record"
 SET client_min_messages TO ERROR;
 DROP SCHEMA coordinator_evaluation CASCADE;

src/test/regress/sql/coordinator_evaluation.sql

@@ -223,5 +223,13 @@ INSERT INTO reference_table VALUES ('(4)'), ('(5)');
 
 SELECT * FROM reference_table ORDER BY 1;
 
+-- failing UPDATE on a reference table with a subquery in RETURNING clause that needs to be pushed-down.
+-- the error message is not great, but at least we no longer see crashes.
+CREATE TABLE ref (a int);
+SELECT create_reference_table('ref');
+UPDATE ref SET a = 1 RETURNING
+	(SELECT pg_catalog.max(latest_end_time) FROM pg_catalog.pg_stat_wal_receiver)
+        as c3;
+
 SET client_min_messages TO ERROR;
 DROP SCHEMA coordinator_evaluation CASCADE;