MDEV-29874: FederatedX error 10000 on multi-table UPDATE/DELETE

Multi-table UPDATE and DELETE statements employ mysql_select() calls
during their processing, so the server may try to instantiate
`select_handler` in an attempt to push down the statement to
a foreign engine.

However, the current implementation of `select_handler` for FederatedX
pushes down the whole query and not only its select part
(`thd->query()`, see `int ha_federatedx_select_handler::init_scan()`).
FederatedX engine does not support execution of DML statements
on the remote side, that is why the error occured.

Solution:
- Add an extra check to only allow SELECT statements pushdown
to FederatedX

Author: mariadb-OlegSmirnov

mysql-test/suite/federated/federatedx_create_handlers.result

@@ -606,6 +606,41 @@ id	name
 4	xxx
 5	yyy
 DEALLOCATE PREPARE stmt;
+#
+# FederatedX error 10000 on multi-table UPDATE/DELETE
+#
+connection slave;
+DROP TABLE IF EXISTS federated.t1, federated.t2;
+CREATE TABLE federated.t1 (a int, b int);
+INSERT INTO federated.t1 VALUES (1,1), (2,2), (3,3);
+CREATE TABLE federated.t2 (a int, b int);
+INSERT INTO federated.t2 VALUES (1,1), (2,2), (4,4);
+connection master;
+DROP TABLE IF EXISTS federated.t1, federated.t2;
+CREATE TABLE federated.t1 (a int, b int)
+ENGINE="FEDERATED" DEFAULT CHARSET=latin1
+CONNECTION='mysql://root@127.0.0.1:SLAVE_PORT/federated/t1';
+CREATE TABLE federated.t2 (a int, b int)
+ENGINE="FEDERATED" DEFAULT CHARSET=latin1
+CONNECTION='mysql://root@127.0.0.1:SLAVE_PORT/federated/t2';
+use federated;
+# Multi-table UPDATE
+UPDATE t1, t2 SET t1.a = 2 WHERE t1.a=t2.a;
+# Check the result
+SELECT * FROM t1;
+a	b
+2	1
+2	2
+3	3
+# Multi-table DELETE
+DELETE FROM t1 USING t1 JOIN t2 ON t1.a = t2.a WHERE t2.b > 1;
+SELECT * FROM t1;
+a	b
+3	3
+# Another form of multi-table DELETE
+DELETE FROM a1 USING t1 AS a1;
+SELECT * FROM t1;
+a	b
 set global federated_pushdown=0;
 connection master;
 DROP TABLE IF EXISTS federated.t1;

mysql-test/suite/federated/federatedx_create_handlers.test

@@ -431,6 +431,50 @@ EXECUTE stmt;
 EXECUTE stmt;
 DEALLOCATE PREPARE stmt;
 
+--echo #
+--echo # FederatedX error 10000 on multi-table UPDATE/DELETE
+--echo #
+
+connection slave;
+DROP TABLE IF EXISTS federated.t1, federated.t2;
+
+CREATE TABLE federated.t1 (a int, b int);
+INSERT INTO federated.t1 VALUES (1,1), (2,2), (3,3);
+
+CREATE TABLE federated.t2 (a int, b int);
+INSERT INTO federated.t2 VALUES (1,1), (2,2), (4,4);
+
+connection master;
+DROP TABLE IF EXISTS federated.t1, federated.t2;
+--replace_result $SLAVE_MYPORT SLAVE_PORT
+eval
+CREATE TABLE federated.t1 (a int, b int)
+ENGINE="FEDERATED" DEFAULT CHARSET=latin1
+CONNECTION='mysql://root@127.0.0.1:$SLAVE_MYPORT/federated/t1';
+
+--replace_result $SLAVE_MYPORT SLAVE_PORT
+eval
+CREATE TABLE federated.t2 (a int, b int)
+ENGINE="FEDERATED" DEFAULT CHARSET=latin1
+CONNECTION='mysql://root@127.0.0.1:$SLAVE_MYPORT/federated/t2';
+
+use federated;
+
+--echo # Multi-table UPDATE
+UPDATE t1, t2 SET t1.a = 2 WHERE t1.a=t2.a;
+
+--echo # Check the result
+SELECT * FROM t1;
+
+--echo # Multi-table DELETE
+DELETE FROM t1 USING t1 JOIN t2 ON t1.a = t2.a WHERE t2.b > 1;
+
+SELECT * FROM t1;
+
+--echo # Another form of multi-table DELETE
+DELETE FROM a1 USING t1 AS a1;
+
+SELECT * FROM t1;
 
 set global federated_pushdown=0;
 

storage/federatedx/federatedx_pushdown.cc

@@ -227,10 +227,16 @@ void ha_federatedx_derived_handler::print_error(int, unsigned long)
 }
 
 
+static bool is_supported_by_select_handler(enum_sql_command sql_command)
+{
+  return sql_command == SQLCOM_SELECT || sql_command == SQLCOM_INSERT_SELECT;
+}
+
+
 static select_handler*
 create_federatedx_select_handler(THD* thd, SELECT_LEX *sel)
 {
-  if (!use_pushdown)
+  if (!use_pushdown || !is_supported_by_select_handler(thd->lex->sql_command))
     return 0;
 
   ha_federatedx_select_handler* handler = NULL;