Backport patch for Bug#31222 (com_% global status counters behave randomly

with mysql_change_user) to 5.0.

mysql-test/r/not_embedded_server.result

@@ -3,3 +3,10 @@ execute stmt1;
 Id	User	Host	db	Command	Time	State	Info
 number	root	localhost	test	Query	time	NULL	show full processlist
 deallocate prepare stmt1;
+FLUSH STATUS;
+SHOW GLOBAL STATUS LIKE 'com_select';
+Variable_name	Value
+Com_select	101
+SHOW GLOBAL STATUS LIKE 'com_select';
+Variable_name	Value
+Com_select	101

mysql-test/t/not_embedded_server.test

@@ -16,3 +16,31 @@ execute stmt1;
 deallocate prepare stmt1;
 
 # End of 4.1 tests
+
+#
+# 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

@@ -391,6 +391,10 @@ 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();
   cleanup_done= 0;
   init();