MDEV-30364 Assertion MDL_EXCLUSIVE on DISCARD TABLESPACE in LOCK TABLE mode

In locked_tables_mode when table is opened without
MYSQL_OPEN_GET_NEW_TABLE flag it is taken from pre-opened and locked
tables. In that case we upgrade its MDL ticket to MDL_EXCLUSIVE before
the operation and downgrade after operation.

mysql-test/main/debug_sync.result

@@ -320,3 +320,30 @@ SHOW VARIABLES LIKE 'DEBUG_SYNC';
 Variable_name	Value
 debug_sync	ON - current signals: 's2,s7,s1,s5'
 SET DEBUG_SYNC= 'RESET';
+#
+# MDEV-30364 Assertion MDL_EXCLUSIVE on DISCARD TABLESPACE in LOCK TABLE mode
+#
+create table t (c int) engine=innodb;
+connect con1,localhost,root;
+set debug_sync='get_schema_column WAIT_FOR go';
+select column_name from information_schema.columns
+where table_schema='test' and table_name='t';
+connection default;
+lock table t write;
+alter table t discard tablespace;
+connect con2,localhost,root;
+disconnect con2;
+connection default;
+ERROR 70100: Query execution was interrupted
+set debug_sync='now SIGNAL go';
+connection con1;
+column_name
+c
+disconnect con1;
+connection default;
+unlock tables;
+drop table t;
+set debug_sync= 'reset';
+#
+# End of 10.6 tests
+#

mysql-test/main/debug_sync.test

@@ -18,6 +18,7 @@
 # We need the Debug Sync Facility.
 #
 --source include/have_debug_sync.inc
+--source include/have_innodb.inc
 
 #
 # We are checking privileges, which the embedded server cannot do.
@@ -448,3 +449,42 @@ SHOW VARIABLES LIKE 'DEBUG_SYNC';
 #
 SET DEBUG_SYNC= 'RESET';
 
+--echo #
+--echo # MDEV-30364 Assertion MDL_EXCLUSIVE on DISCARD TABLESPACE in LOCK TABLE mode
+--echo #
+create table t (c int) engine=innodb;
+--connect con1,localhost,root
+set debug_sync='get_schema_column WAIT_FOR go';
+send select column_name from information_schema.columns
+where table_schema='test' and table_name='t';
+
+--connection default
+let $wait_condition=select 1 from information_schema.processlist where state like 'debug sync point%';
+--source include/wait_condition.inc
+let $connid=`select connection_id()`;
+lock table t write;
+send alter table t discard tablespace;
+
+--connect con2,localhost,root
+--disable_query_log
+--eval kill query $connid
+--enable_query_log
+--disconnect con2
+
+--connection default
+--error ER_QUERY_INTERRUPTED
+reap;
+set debug_sync='now SIGNAL go';
+
+--connection con1
+reap;
+--disconnect con1
+
+--connection default
+unlock tables;
+drop table t;
+set debug_sync= 'reset';
+
+--echo #
+--echo # End of 10.6 tests
+--echo #

sql/sql_table.cc

@@ -5946,6 +5946,8 @@ int mysql_discard_or_import_tablespace(THD *thd,
 {
   Alter_table_prelocking_strategy alter_prelocking_strategy;
   int error;
+  TABLE *table;
+  enum_mdl_type mdl_downgrade= MDL_NOT_INITIALIZED;
   DBUG_ENTER("mysql_discard_or_import_tablespace");
 
   mysql_audit_alter_table(thd, table_list);
@@ -5978,7 +5980,21 @@ int mysql_discard_or_import_tablespace(THD *thd,
     DBUG_RETURN(-1);
   }
 
-  error= table_list->table->file->ha_discard_or_import_tablespace(discard);
+  table= table_list->table;
+  DBUG_ASSERT(table->mdl_ticket || table->s->tmp_table);
+  if (table->mdl_ticket && table->mdl_ticket->get_type() < MDL_EXCLUSIVE)
+  {
+    DBUG_ASSERT(thd->locked_tables_mode);
+    mdl_downgrade= table->mdl_ticket->get_type();
+    if (thd->mdl_context.upgrade_shared_lock(table->mdl_ticket, MDL_EXCLUSIVE,
+                                             thd->variables.lock_wait_timeout))
+    {
+      error= 1;
+      goto err;
+    }
+  }
+
+  error= table->file->ha_discard_or_import_tablespace(discard);
 
   THD_STAGE_INFO(thd, stage_end);
 
@@ -6004,6 +6020,9 @@ int mysql_discard_or_import_tablespace(THD *thd,
 err:
   thd->tablespace_op=FALSE;
 
+  if (mdl_downgrade > MDL_NOT_INITIALIZED)
+    table->mdl_ticket->downgrade_lock(mdl_downgrade);
+
   if (likely(error == 0))
   {
     my_ok(thd);