MDEV-26242: Assertion `i >= 0' failed on setting default_tmp_storage_engine to 'DEFAULT' in 10.7

Locking / unlocking plugins is already handled by Sys_var_plugin::check
method. No need to do that in the specialized checking code, use
var->save_value instead to get the plugin handle.

mysql-test/suite/sys_vars/r/default_tmp_storage_engine_rocksdb.result

@@ -49,3 +49,54 @@ DROP TABLE t2;
 CREATE TEMPORARY TABLE t2 LIKE t1;
 ERROR HY000: Table storage engine 'ROCKSDB' does not support the create option 'TEMPORARY'
 DROP TABLE t1;
+#
+# MDEV-26242: Assertion `i >= 0' failed on setting 
+#             default_tmp_storage_engine to 'DEFAULT' in 10.7
+#
+SHOW VARIABLES like '%storage_engine';
+Variable_name	Value
+default_storage_engine	ROCKSDB
+default_tmp_storage_engine	Aria
+enforce_storage_engine	
+storage_engine	ROCKSDB
+SELECT @@GLOBAL.default_tmp_storage_engine, @@GLOBAL.default_storage_engine, @@GLOBAL.storage_engine, @@SESSION.default_tmp_storage_engine, @@SESSION.default_storage_engine, @@SESSION.storage_engine;
+@@GLOBAL.default_tmp_storage_engine	@@GLOBAL.default_storage_engine	@@GLOBAL.storage_engine	@@SESSION.default_tmp_storage_engine	@@SESSION.default_storage_engine	@@SESSION.storage_engine
+NULL	MyISAM	MyISAM	Aria	ROCKSDB	ROCKSDB
+SELECT @@GLOBAL.table_type;
+ERROR HY000: Unknown system variable 'table_type'
+SET default_storage_engine=DEFAULT;
+SET default_tmp_storage_engine=innodb;
+SELECT @@GLOBAL.default_tmp_storage_engine, @@GLOBAL.default_storage_engine, @@GLOBAL.storage_engine, @@SESSION.default_tmp_storage_engine, @@SESSION.default_storage_engine, @@SESSION.storage_engine;
+@@GLOBAL.default_tmp_storage_engine	@@GLOBAL.default_storage_engine	@@GLOBAL.storage_engine	@@SESSION.default_tmp_storage_engine	@@SESSION.default_storage_engine	@@SESSION.storage_engine
+NULL	MyISAM	MyISAM	InnoDB	MyISAM	MyISAM
+CREATE TABLE t (t int);
+CREATE TEMPORARY TABLE t1 (t int);
+SHOW CREATE TABLE t;
+Table	Create Table
+t	CREATE TABLE `t` (
+  `t` int(11) DEFAULT NULL
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+SHOW CREATE TABLE t1;
+Table	Create Table
+t1	CREATE TEMPORARY TABLE `t1` (
+  `t` int(11) DEFAULT NULL
+) ENGINE=InnoDB DEFAULT CHARSET=latin1
+DROP TABLE t1, t;
+SET GLOBAL default_tmp_storage_engine='default';
+SELECT @@GLOBAL.default_tmp_storage_engine, @@GLOBAL.default_storage_engine, @@GLOBAL.storage_engine, @@SESSION.default_tmp_storage_engine, @@SESSION.default_storage_engine, @@SESSION.storage_engine;
+@@GLOBAL.default_tmp_storage_engine	@@GLOBAL.default_storage_engine	@@GLOBAL.storage_engine	@@SESSION.default_tmp_storage_engine	@@SESSION.default_storage_engine	@@SESSION.storage_engine
+MyISAM	MyISAM	MyISAM	InnoDB	MyISAM	MyISAM
+CREATE TABLE t (t int);
+CREATE TEMPORARY TABLE t1 (t int);
+SHOW CREATE TABLE t;
+Table	Create Table
+t	CREATE TABLE `t` (
+  `t` int(11) DEFAULT NULL
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+SHOW CREATE TABLE t1;
+Table	Create Table
+t1	CREATE TEMPORARY TABLE `t1` (
+  `t` int(11) DEFAULT NULL
+) ENGINE=InnoDB DEFAULT CHARSET=latin1
+DROP TABLE t1, t;
+SET GLOBAL default_tmp_storage_engine=NULL;

mysql-test/suite/sys_vars/t/default_tmp_storage_engine_rocksdb.test

@@ -4,6 +4,8 @@
 
 --source include/have_rocksdb.inc
 --source include/not_embedded.inc
+--source include/have_innodb.inc
+
 --error ER_UNKNOWN_STORAGE_ENGINE
 SET default_tmp_storage_engine = engine_doesnt_exist;
 --error ER_ILLEGAL_HA_CREATE_OPTION
@@ -50,3 +52,35 @@ DROP TABLE t2;
 CREATE TEMPORARY TABLE t2 LIKE t1;
 
 DROP TABLE t1;
+
+--echo #
+--echo # MDEV-26242: Assertion `i >= 0' failed on setting 
+--echo #             default_tmp_storage_engine to 'DEFAULT' in 10.7
+--echo #
+
+
+SHOW VARIABLES like '%storage_engine';
+SELECT @@GLOBAL.default_tmp_storage_engine, @@GLOBAL.default_storage_engine, @@GLOBAL.storage_engine, @@SESSION.default_tmp_storage_engine, @@SESSION.default_storage_engine, @@SESSION.storage_engine;
+--error ER_UNKNOWN_SYSTEM_VARIABLE
+SELECT @@GLOBAL.table_type;
+
+SET default_storage_engine=DEFAULT;
+SET default_tmp_storage_engine=innodb;
+SELECT @@GLOBAL.default_tmp_storage_engine, @@GLOBAL.default_storage_engine, @@GLOBAL.storage_engine, @@SESSION.default_tmp_storage_engine, @@SESSION.default_storage_engine, @@SESSION.storage_engine;
+
+CREATE TABLE t (t int);
+CREATE TEMPORARY TABLE t1 (t int);
+SHOW CREATE TABLE t;
+SHOW CREATE TABLE t1;
+DROP TABLE t1, t;
+
+SET GLOBAL default_tmp_storage_engine='default';
+SELECT @@GLOBAL.default_tmp_storage_engine, @@GLOBAL.default_storage_engine, @@GLOBAL.storage_engine, @@SESSION.default_tmp_storage_engine, @@SESSION.default_storage_engine, @@SESSION.storage_engine;
+
+CREATE TABLE t (t int);
+CREATE TEMPORARY TABLE t1 (t int);
+SHOW CREATE TABLE t;
+SHOW CREATE TABLE t1;
+DROP TABLE t1, t;
+
+SET GLOBAL default_tmp_storage_engine=NULL;

sql/sys_vars.cc

@@ -742,13 +742,9 @@ static Sys_var_charptr_fscs Sys_character_sets_dir(
 
 static bool check_engine_supports_temporary(sys_var *self, THD *thd, set_var *var)
 {
-  String str, *res;
+  plugin_ref plugin= var->save_result.plugin;
-  LEX_CSTRING name;
+  if (!plugin)
-  if (!var->value || var->value->is_null())
     return false;
-  res= var->value->val_str(&str);
-  res->get_value(&name);
-  plugin_ref plugin= ha_resolve_by_name(thd, &name, true);
   DBUG_ASSERT(plugin);
   handlerton *hton= plugin_hton(plugin);
   DBUG_ASSERT(hton);
@@ -756,10 +752,8 @@ static bool check_engine_supports_temporary(sys_var *self, THD *thd, set_var *va
   {
     my_error(ER_ILLEGAL_HA_CREATE_OPTION, MYF(0), hton_name(hton)->str,
              "TEMPORARY");
-    plugin_unlock(thd, plugin);
     return true;
   }
-  plugin_unlock(thd, plugin);
   return false;
 }