Fix for Bug#31222: com_% global status counters

behave randomly with mysql_change_user.
  
The problem was that global status variables were not updated
in THD::check_user(), so thread statistics were lost after
COM_CHANGE_USER.
  
The fix is to update global status variables with the thread ones
before preparing the thread for new user.

mysql-test/r/change_user.result

@@ -44,3 +44,10 @@ IS_FREE_LOCK('bug31418')
 SELECT IS_USED_LOCK('bug31418');
 IS_USED_LOCK('bug31418')
 NULL
+FLUSH STATUS;
+SHOW GLOBAL STATUS LIKE 'com_select';
+Variable_name	Value
+Com_select	112
+SHOW GLOBAL STATUS LIKE 'com_select';
+Variable_name	Value
+Com_select	112

mysql-test/t/change_user.test

@@ -33,3 +33,31 @@ SELECT IS_USED_LOCK('bug31418') = CONNECTION_ID();
 --change_user
 SELECT IS_FREE_LOCK('bug31418');
 SELECT IS_USED_LOCK('bug31418');
+
+#
+# Bug#31222: com_% global status counters behave randomly with
+# mysql_change_user.
+#
+
+FLUSH STATUS;
+
+--disable_result_log
+--disable_query_log
+
+let $i = 100;
+
+while ($i)
+{
+  dec $i;
+
+  SELECT 1;
+}
+
+--enable_query_log
+--enable_result_log
+
+SHOW GLOBAL STATUS LIKE 'com_select';
+
+--change_user
+
+SHOW GLOBAL STATUS LIKE 'com_select';

sql/sql_class.cc

@@ -769,6 +769,10 @@ void THD::init_for_queries()
 
 void THD::change_user(void)
 {
+  pthread_mutex_lock(&LOCK_status);
+  add_to_status(&global_status_var, &status_var);
+  pthread_mutex_unlock(&LOCK_status);
+
   cleanup();
   killed= NOT_KILLED;
   cleanup_done= 0;