Bug#25571453: ASSERTION `MDL_CHECKER::IS_READ_LOCKED(M_THD, *OBJECT)' FAILED

The assertion was triggered when executing TRUNCATE TABLE if
the truncated table was used by a view in a different schema
and the view was locked with LOCK TABLES. In that case the
schema containing the truncated table was not locked. This
caused the assert to be triggered by the data dictionary code
as a schema lock is required in order to check if the table
has been locked properly. The problem had no ill effects on
release builds.

This patch fixes the problem by making sure that the schema
containing the truncated table is locked.

Author: jhauglid

mysql-test/r/truncate.result

@@ -159,3 +159,16 @@ HANDLER t1 READ FIRST;
 ERROR 42S02: Unknown table 't1' in HANDLER
 DROP TABLE t1;
 # End of 6.0 tests
+#
+# Bug#25571453: ASSERTION `MDL_CHECKER::IS_READ_LOCKED(M_THD, *OBJECT)'
+#               FAILED
+#
+CREATE TABLE t1(a INT);
+CREATE SCHEMA s1;
+CREATE VIEW s1.v1 AS SELECT * FROM t1;
+LOCK TABLE s1.v1 WRITE;
+TRUNCATE TABLE t1;
+UNLOCK TABLES;
+DROP VIEW s1.v1;
+DROP TABLE t1;
+DROP SCHEMA s1;

mysql-test/t/truncate.test

@@ -175,6 +175,21 @@ DROP TABLE t1;
 
 --echo # End of 6.0 tests
 
+--echo #
+--echo # Bug#25571453: ASSERTION `MDL_CHECKER::IS_READ_LOCKED(M_THD, *OBJECT)'
+--echo #               FAILED
+--echo #
+
+CREATE TABLE t1(a INT);
+CREATE SCHEMA s1;
+CREATE VIEW s1.v1 AS SELECT * FROM t1;
+LOCK TABLE s1.v1 WRITE;
+# This used to cause an assert to be triggered.
+TRUNCATE TABLE t1;
+UNLOCK TABLES;
+DROP VIEW s1.v1;
+DROP TABLE t1;
+DROP SCHEMA s1;
 
 ########################################
 ##### WL#7554 set schema `test` back to what it was

sql/sql_truncate.cc

@@ -489,6 +489,7 @@ bool Sql_cmd_truncate_table::truncate_table(THD *thd, TABLE_LIST *table_ref)
   DBUG_ASSERT((!table_ref->table) ||
               (table_ref->table && table_ref->table->s));
 
+  dd::Schema_MDL_locker mdl_locker(thd);
   dd::cache::Dictionary_client::Auto_releaser releaser(thd->dd_client());
 
   /* Initialize, or reinitialize in case of reexecution (SP). */
@@ -542,6 +543,9 @@ bool Sql_cmd_truncate_table::truncate_table(THD *thd, TABLE_LIST *table_ref)
   }
   else /* It's not a temporary table. */
   {
+    if (mdl_locker.ensure_locked(table_ref->db))
+      DBUG_RETURN(true);
+
     if (lock_table(thd, table_ref, &hton))
       DBUG_RETURN(TRUE);