MDEV-35050 Found wrong usage of mutex upon setting plugin session variables

mysql-test/suite/plugins/r/fulltext_plugin.result

+#
+# BUG#39746 - Debug flag breaks struct definition (server crash)
+#
 INSTALL PLUGIN simple_parser SONAME 'mypluglib.so';
 CREATE TABLE t1(a TEXT, b TEXT, FULLTEXT(a) WITH PARSER simple_parser);
 ALTER TABLE t1 ADD FULLTEXT(b) WITH PARSER simple_parser;
@@ -5,3 +8,11 @@ DROP TABLE t1;
 UNINSTALL PLUGIN simple_parser;
 show status like 'a%status';
 Variable_name	Value
+#
+# MDEV-35050 Found wrong usage of mutex upon setting plugin session variables
+#
+install soname 'mypluglib';
+set session_track_system_variables="*";
+set session simple_parser_simple_thdvar_one = 10;
+uninstall soname 'mypluglib';
+# End of 10.5 tests

mysql-test/suite/plugins/t/fulltext_plugin.test

 --source include/have_simple_parser.inc
 
-#
-# BUG#39746 - Debug flag breaks struct definition (server crash)
-#
+--echo #
+--echo # BUG#39746 - Debug flag breaks struct definition (server crash)
+--echo #
 --replace_result .dll .so
 eval INSTALL PLUGIN simple_parser SONAME '$MYPLUGLIB_SO';
 CREATE TABLE t1(a TEXT, b TEXT, FULLTEXT(a) WITH PARSER simple_parser);
@@ -15,3 +15,12 @@ UNINSTALL PLUGIN simple_parser;
 #
 show status like 'a%status';
 
+--echo #
+--echo # MDEV-35050 Found wrong usage of mutex upon setting plugin session variables
+--echo #
+install soname 'mypluglib';
+set session_track_system_variables="*";
+set session simple_parser_simple_thdvar_one = 10;
+uninstall soname 'mypluglib';
+
+--echo # End of 10.5 tests

sql/session_tracker.cc

@@ -428,6 +428,7 @@ bool Session_sysvars_tracker::vars_list::store(THD *thd, String *buf)
     SHOW_VAR show;
     CHARSET_INFO *charset;
     size_t val_length, length;
+    mysql_mutex_lock(&LOCK_global_system_variables);
     mysql_mutex_lock(&LOCK_plugin);
     if (!*node->test_load)
     {
@@ -444,13 +445,12 @@ bool Session_sysvars_tracker::vars_list::store(THD *thd, String *buf)
     show.name= svar->name.str;
     show.value= (char *) svar;
 
-    mysql_mutex_lock(&LOCK_global_system_variables);
     const char *value= get_one_variable(thd, &show, OPT_SESSION, SHOW_SYS, NULL,
                                         &charset, val_buf, &val_length);
-    mysql_mutex_unlock(&LOCK_global_system_variables);
 
     if (is_plugin)
       mysql_mutex_unlock(&LOCK_plugin);
+    mysql_mutex_unlock(&LOCK_global_system_variables);
 
     length= net_length_size(svar->name.length) +
       svar->name.length +