MDEV-8219: enforce_storage_engine cannot be set globally

Change session only variable enforce_storage_engine to be
session variable and make sure that also global value
is used if session variable is not set.

mysql-test/r/enforce_storage_engine.result

@@ -98,5 +98,24 @@ t1	CREATE TABLE `t1` (
   PRIMARY KEY (`c1`)
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 DROP TABLE t1;
+SET GLOBAL enforce_storage_engine=Memory;
+SET SESSION sql_mode='';
+select @@session.enforce_storage_engine;
+@@session.enforce_storage_engine
+MEMORY
+select @@global.enforce_storage_engine;
+@@global.enforce_storage_engine
+MEMORY
+CREATE TABLE t1 (c1 INT PRIMARY KEY AUTO_INCREMENT, c2 VARCHAR(10));
+Warnings:
+Note	1266	Using storage engine MEMORY for table 't1'
+SHOW CREATE TABLE t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `c1` int(11) NOT NULL AUTO_INCREMENT,
+  `c2` varchar(10) DEFAULT NULL,
+  PRIMARY KEY (`c1`)
+) ENGINE=MEMORY DEFAULT CHARSET=latin1
+DROP TABLE t1;
+SET SESSION enforce_storage_engine=NULL;
 SET GLOBAL enforce_storage_engine=NULL;
-ERROR HY000: Variable 'enforce_storage_engine' is a SESSION variable and can't be used with SET GLOBAL

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

@@ -725,10 +725,10 @@ READ_ONLY	YES
 COMMAND_LINE_ARGUMENT	OPTIONAL
 VARIABLE_NAME	ENFORCE_STORAGE_ENGINE
 SESSION_VALUE	
-GLOBAL_VALUE	NULL
+GLOBAL_VALUE	
 GLOBAL_VALUE_ORIGIN	COMPILE-TIME
 DEFAULT_VALUE	NULL
-VARIABLE_SCOPE	SESSION ONLY
+VARIABLE_SCOPE	SESSION
 VARIABLE_TYPE	VARCHAR
 VARIABLE_COMMENT	Force the use of a storage engine for new tables
 NUMERIC_MIN_VALUE	NULL

mysql-test/t/enforce_storage_engine.test

@@ -69,5 +69,18 @@ CREATE TABLE t1 (c1 INT PRIMARY KEY AUTO_INCREMENT, c2 VARCHAR(10));
 SHOW CREATE TABLE t1;
 DROP TABLE t1;
 
---error 1228
+SET GLOBAL enforce_storage_engine=Memory;
+SET SESSION sql_mode='';
+
+connect (con1,localhost,root,,);
+connection con1;
+select @@session.enforce_storage_engine;
+select @@global.enforce_storage_engine;
+CREATE TABLE t1 (c1 INT PRIMARY KEY AUTO_INCREMENT, c2 VARCHAR(10));
+SHOW CREATE TABLE t1;
+DROP TABLE t1;
+connection default;
+disconnect con1;
+
+SET SESSION enforce_storage_engine=NULL;
 SET GLOBAL enforce_storage_engine=NULL;

sql/sql_table.cc

@@ -9772,7 +9772,8 @@ static bool check_engine(THD *thd, const char *db_name,
   DBUG_ENTER("check_engine");
   handlerton **new_engine= &create_info->db_type;
   handlerton *req_engine= *new_engine;
-  handlerton *enf_engine= thd->variables.enforced_table_plugin ?  plugin_hton(thd->variables.enforced_table_plugin) : NULL;
+  handlerton *enf_engine= thd->variables.enforced_table_plugin ?
+    plugin_hton(thd->variables.enforced_table_plugin) : NULL;
   bool no_substitution= thd->variables.sql_mode & MODE_NO_ENGINE_SUBSTITUTION;
   *new_engine= ha_checktype(thd, req_engine, no_substitution);
   DBUG_ASSERT(*new_engine);

sql/sys_vars.cc

@@ -3426,7 +3426,7 @@ static Sys_var_plugin Sys_default_tmp_storage_engine(
 
 static Sys_var_plugin Sys_enforce_storage_engine(
        "enforce_storage_engine", "Force the use of a storage engine for new tables",
-       SESSION_ONLY(enforced_table_plugin),
+       SESSION_VAR(enforced_table_plugin),
        NO_CMD_LINE, MYSQL_STORAGE_ENGINE_PLUGIN,
        DEFAULT(&enforced_storage_engine), NO_MUTEX_GUARD, NOT_IN_BINLOG, ON_CHECK(check_has_super));