MDEV-22696 Threadpool : make sure thd->event_scheduler.data does not change as long as THD is in server_threads.

Author: vaintroub

sql/threadpool_common.cc

@@ -211,12 +211,11 @@ void tp_callback(TP_connection *c)
 
 error:
  c->thd= 0;
- delete c;
-
  if (thd)
  {
  threadpool_remove_connection(thd);
  }
+ delete c;
  worker_context.restore();
 }
 
@@ -243,9 +242,11 @@ static THD* threadpool_add_connection(CONNECT *connect, void *scheduler_data)
  return NULL;
  }
  delete connect;
+
+ thd->event_scheduler.data = scheduler_data;
  server_threads.insert(thd);
  thd->set_mysys_var(mysys_var);
- thd->event_scheduler.data= scheduler_data;
+
 
  /* Login. */
  thread_attach(thd);
@@ -280,7 +281,6 @@ static THD* threadpool_add_connection(CONNECT *connect, void *scheduler_data)
 static void threadpool_remove_connection(THD *thd)
 {
  thread_attach(thd);
- thd->event_scheduler.data= 0;
  thd->net.reading_or_writing = 0;
  end_connection(thd);
  close_connection(thd, 0);