Restore thread existence check

sql/rpl_parallel.cc

@@ -2673,6 +2673,8 @@ rpl_parallel_entry::choose_thread_internal(sched_bucket *cur_thr,
   Relay_log_info *rli= rgi->rli;
   rpl_parallel_thread *thr= cur_thr->thr;
 
+  if (thr)
+  {
     *did_enter_cond= false;
     mysql_mutex_lock(&thr->LOCK_rpl_thread);
     for (;;)
@@ -2709,17 +2711,16 @@ rpl_parallel_entry::choose_thread_internal(sched_bucket *cur_thr,
             and this can cause THD::awake to use the wrong mutex.
           */
 #ifdef ENABLED_DEBUG_SYNC
-        DBUG_EXECUTE_IF("rpl_parallel_wait_queue_max",
-                        {
-                          debug_sync_set_action(rli->sql_driver_thd,
+          DBUG_EXECUTE_IF("rpl_parallel_wait_queue_max", {
+            debug_sync_set_action(
+                rli->sql_driver_thd,
                 STRING_WITH_LEN("now SIGNAL wait_queue_ready"));
           };);
 #endif
           rli->sql_driver_thd->set_time_for_next_stage();
-        rli->sql_driver_thd->ENTER_COND(&thr->COND_rpl_thread_queue,
-                                        &thr->LOCK_rpl_thread,
-                                        &stage_waiting_for_room_in_worker_thread,
-                                        old_stage);
+          rli->sql_driver_thd->ENTER_COND(
+              &thr->COND_rpl_thread_queue, &thr->LOCK_rpl_thread,
+              &stage_waiting_for_room_in_worker_thread, old_stage);
           *did_enter_cond= true;
         }
 
@@ -2729,9 +2730,9 @@ rpl_parallel_entry::choose_thread_internal(sched_bucket *cur_thr,
                               did_enter_cond, old_stage);
           my_error(ER_CONNECTION_KILLED, MYF(0));
 #ifdef ENABLED_DEBUG_SYNC
-        DBUG_EXECUTE_IF("rpl_parallel_wait_queue_max",
-                        {
-                          debug_sync_set_action(rli->sql_driver_thd,
+          DBUG_EXECUTE_IF("rpl_parallel_wait_queue_max", {
+            debug_sync_set_action(
+                rli->sql_driver_thd,
                 STRING_WITH_LEN("now SIGNAL wait_queue_killed"));
           };);
 #endif
@@ -2742,6 +2743,7 @@ rpl_parallel_entry::choose_thread_internal(sched_bucket *cur_thr,
         mysql_cond_wait(&thr->COND_rpl_thread_queue, &thr->LOCK_rpl_thread);
       }
     }
+  }
 
   if (!thr)
     cur_thr->thr= thr=