MDEV-5345 - Deadlock between mysql_change_user(), SHOW VARIABLES and

            INSTALL PLUGIN

There was mixed lock order between LOCK_plugin, LOCK_global_system_variables
and LOCK_system_variables_hash. This patch ensures that write-lock on
LOCK_system_variables_hash doesn't intersect with LOCK_plugin.

Fixed by moving initialization/deinitialization of plugin options from
plugin_add()/plugin_del() to plugin_initialize()/plugin_deinitalize().
So that plugin options are handled without protection of LOCK_plugin.

mysql-test/r/plugin_vars.result

+#
+# MDEV-5345 - Deadlock between mysql_change_user(), SHOW VARIABLES and
+#             INSTALL PLUGIN
+#
+CREATE PROCEDURE p_install(x INT)
+BEGIN
+DECLARE CONTINUE HANDLER FOR 1126 BEGIN END;
+WHILE x DO
+SET x= x - 1;
+INSTALL PLUGIN no_such_plugin SONAME 'no_such_object';
+END WHILE;
+END|
+CREATE PROCEDURE p_show_vars(x INT)
+WHILE x DO
+SET x= x - 1;
+SHOW VARIABLES;
+END WHILE|
+CALL p_install(100);
+CALL p_show_vars(100);
+USE test;
+DROP PROCEDURE p_install;
+DROP PROCEDURE p_show_vars;

mysql-test/t/plugin_vars.test

+--echo #
+--echo # MDEV-5345 - Deadlock between mysql_change_user(), SHOW VARIABLES and
+--echo #             INSTALL PLUGIN
+--echo #
+
+# Prepare test
+delimiter |;
+CREATE PROCEDURE p_install(x INT)
+BEGIN
+  DECLARE CONTINUE HANDLER FOR 1126 BEGIN END;
+  WHILE x DO
+    SET x= x - 1;
+    INSTALL PLUGIN no_such_plugin SONAME 'no_such_object';
+  END WHILE;
+END|
+
+CREATE PROCEDURE p_show_vars(x INT)
+WHILE x DO
+  SET x= x - 1;
+  SHOW VARIABLES;
+END WHILE|
+delimiter ;|
+
+connect(con1, localhost, root,,);
+connect(con2, localhost, root,,);
+
+# Start test
+connection con1;
+--send CALL p_install(100)
+
+connection con2;
+--send CALL p_show_vars(100)
+
+connection default;
+
+disable_result_log;
+let $i= 100;
+while ($i)
+{
+  change_user;
+  dec $i;
+}
+
+# Cleanup
+connection con1;
+reap;
+connection con2;
+reap;
+connection default;
+enable_result_log;
+
+disconnect con1;
+disconnect con2;
+USE test;
+DROP PROCEDURE p_install;
+DROP PROCEDURE p_show_vars;

sql/set_var.cc

@@ -441,10 +441,10 @@ int mysql_del_sys_var_chain(sys_var *first)
 {
   int result= 0;
 
-  /* A write lock should be held on LOCK_system_variables_hash */
-
+  mysql_rwlock_wrlock(&LOCK_system_variables_hash);
   for (sys_var *var= first; var; var= var->next)
     result|= my_hash_delete(&system_variable_hash, (uchar*) var);
+  mysql_rwlock_unlock(&LOCK_system_variables_hash);
 
   return result;
 }