MDEV-11176: FTWRL confusing state about "worker thread pool"

The FLUSH TABLE WITH READ LOCK briefly set the state (in PROCESSLIST) to
"Waiting while replication worker thread pool is busy", even if there was
nothing to wait for. This is somewhat confusing on a server that might not
even have any replication configured, let alone replication workers.

Signed-off-by: Kristian Nielsen <knielsen@knielsen-hq.org>

Author: knielsen

mysql-test/suite/perfschema/r/stage_mdl_global.result

@@ -10,7 +10,6 @@ username	event_name	nesting_event_type
 username	event_name	nesting_event_type
 user1	stage/sql/starting	STATEMENT
 user1	stage/sql/starting	STATEMENT
-user1	stage/sql/starting	STATEMENT
 user1	stage/sql/Query end	STATEMENT
 user1	stage/sql/closing tables	STATEMENT
 user1	stage/sql/Query end	STATEMENT

sql/rpl_parallel.cc

@@ -552,6 +552,7 @@ pool_mark_busy(rpl_parallel_thread_pool *pool, THD *thd)
 {
   PSI_stage_info old_stage;
   int res= 0;
+  bool did_enter_cond= false;
 
   /*
     Wait here while the queue is busy. This is done to make FLUSH TABLES WITH
@@ -568,24 +569,28 @@ pool_mark_busy(rpl_parallel_thread_pool *pool, THD *thd)
   */
   DBUG_EXECUTE_IF("mark_busy_mdev_22370",my_sleep(1000000););
   mysql_mutex_lock(&pool->LOCK_rpl_thread_pool);
-  if (thd)
-  {
-    thd->set_time_for_next_stage();
-    thd->ENTER_COND(&pool->COND_rpl_thread_pool, &pool->LOCK_rpl_thread_pool,
-                    &stage_waiting_for_rpl_thread_pool, &old_stage);
-  }
-  while (pool->busy)
+  if (pool->busy)
   {
-    if (thd && unlikely(thd->check_killed()))
+    if (thd)
     {
-      res= 1;
-      break;
+      thd->set_time_for_next_stage();
+      thd->ENTER_COND(&pool->COND_rpl_thread_pool, &pool->LOCK_rpl_thread_pool,
+                      &stage_waiting_for_rpl_thread_pool, &old_stage);
+      did_enter_cond= true;
     }
-    mysql_cond_wait(&pool->COND_rpl_thread_pool, &pool->LOCK_rpl_thread_pool);
+    do
+    {
+      if (thd && unlikely(thd->check_killed()))
+      {
+        res= 1;
+        break;
+      }
+      mysql_cond_wait(&pool->COND_rpl_thread_pool, &pool->LOCK_rpl_thread_pool);
+    } while (pool->busy);
   }
   if (!res)
     pool->busy= true;
-  if (thd)
+  if (did_enter_cond)
     thd->EXIT_COND(&old_stage);
   else
     mysql_mutex_unlock(&pool->LOCK_rpl_thread_pool);