WL#2936

  Move copying of global variables into thd.variables into plugin_thdvar_init().
  plugin_thdvar_init() may be called multiple times when THD::change_user() is called.
  This fixes a plugin ref-count leak when running the test mysql_client_test


sql/sql_class.cc:
  wl2936
    Initialize thd.variables to zero in constructor.
    Move copying of global variables into thd.variables into plugin_thdvar_init()
sql/sql_plugin.cc:
  wl2936
    Move copying of global variables into thd.variables into plugin_thdvar_init().
    plugin_thdvar_init() may be called multiple times when THD::change_user() is called.
    This fixes a plugin ref-count leak when running the test mysql_client_test

sql/sql_class.cc

@@ -345,6 +345,7 @@ THD::THD()
   time_after_lock=(time_t) 0;
   current_linfo =  0;
   slave_thread = 0;
+  bzero(&variables, sizeof(variables));
   thread_id= variables.pseudo_thread_id= 0;
   one_shot_set= 0;
   file_id = 0;
@@ -455,8 +456,6 @@ void THD::pop_internal_handler()
 void THD::init(void)
 {
   pthread_mutex_lock(&LOCK_global_system_variables);
-  variables= global_system_variables;
-  variables.table_plugin= NULL;
   plugin_thdvar_init(this);
   variables.time_format= date_time_format_copy((THD*) 0,
 					       variables.time_format);

sql/sql_plugin.cc

@@ -2303,14 +2303,26 @@ static byte *mysql_sys_var_ptr(void* a_thd, int offset)
 
 void plugin_thdvar_init(THD *thd)
 {
+  plugin_ref old_table_plugin= thd->variables.table_plugin;
+  DBUG_ENTER("plugin_thdvar_init");
+  
+  thd->variables.table_plugin= NULL;
+  cleanup_variables(thd, &thd->variables);
+  
+  thd->variables= global_system_variables;
+  thd->variables.table_plugin= NULL;
+
   /* we are going to allocate these lazily */
   thd->variables.dynamic_variables_version= 0;
   thd->variables.dynamic_variables_size= 0;
   thd->variables.dynamic_variables_ptr= 0;
 
-  DBUG_ASSERT(!(thd->variables.table_plugin));
+  pthread_mutex_lock(&LOCK_plugin);  
   thd->variables.table_plugin=
-        my_plugin_lock(NULL, &global_system_variables.table_plugin);
+        my_intern_plugin_lock(NULL, global_system_variables.table_plugin);
+  intern_plugin_unlock(NULL, old_table_plugin);
+  pthread_mutex_unlock(&LOCK_plugin);
+  DBUG_VOID_RETURN;
 }