MDEV-10043 - main.events_restart fails sporadically in buildbot (crashes upon

             shutdown)

There was race condition between shutdown thread and event worker threads.

Shutdown thread waits for thread_count to become 0 in close_connections(). It
may happen so that event worker thread was started but didn't increment
thread_count by this time. In this case shutdown thread may miss wait for this
working thread and continue deinitialization. Worker thread in turn may continue
execution and crash on deinitialized data.

Fixed by incrementing thread_count before thread is actually created like it is
done for connection threads.

Also let event scheduler not to inc/dec running threads counter for symmetry
with other "service" threads.

sql/event_scheduler.cc

@@ -131,12 +131,6 @@ post_init_event_thread(THD *thd)
     thd->cleanup();
     return TRUE;
   }
-
-  mysql_mutex_lock(&LOCK_thread_count);
-  threads.append(thd);
-  thread_count++;
-  inc_thread_running();
-  mysql_mutex_unlock(&LOCK_thread_count);
   return FALSE;
 }
 
@@ -158,7 +152,6 @@ deinit_event_thread(THD *thd)
   DBUG_PRINT("exit", ("Event thread finishing"));
   mysql_mutex_lock(&LOCK_thread_count);
   thread_count--;
-  dec_thread_running();
   delete thd;
   mysql_cond_broadcast(&COND_thread_count);
   mysql_mutex_unlock(&LOCK_thread_count);
@@ -195,6 +188,8 @@ pre_init_event_thread(THD* thd)
   thd->client_capabilities|= CLIENT_MULTI_RESULTS;
   mysql_mutex_lock(&LOCK_thread_count);
   thd->thread_id= thd->variables.pseudo_thread_id= thread_id++;
+  threads.append(thd);
+  thread_count++;
   mysql_mutex_unlock(&LOCK_thread_count);
 
   /*
@@ -241,13 +236,8 @@ event_scheduler_thread(void *arg)
   my_free(arg);
   if (!res)
     scheduler->run(thd);
-  else
-  {
-    thd->proc_info= "Clearing";
-    net_end(&thd->net);
-    delete thd;
-  }
 
+  deinit_event_thread(thd);
   DBUG_LEAVE;                               // Against gcc warnings
   my_thread_end();
   return 0;
@@ -308,6 +298,7 @@ Event_worker_thread::run(THD *thd, Event_queue_element_for_exec *event)
   DBUG_ENTER("Event_worker_thread::run");
   DBUG_PRINT("info", ("Time is %ld, THD: 0x%lx", (long) my_time(0), (long) thd));
 
+  inc_thread_running();
   if (res)
     goto end;
 
@@ -334,6 +325,7 @@ end:
              event->name.str));
 
   delete event;
+  dec_thread_running();
   deinit_event_thread(thd);
 
   DBUG_VOID_RETURN;
@@ -432,13 +424,9 @@ Event_scheduler::start(int *err_no)
                     " Can not create thread for event scheduler (errno=%d)",
                     *err_no);
 
-    new_thd->proc_info= "Clearing";
-    DBUG_ASSERT(new_thd->net.buff != 0);
-    net_end(&new_thd->net);
-
     state= INITIALIZED;
     scheduler_thd= NULL;
-    delete new_thd;
+    deinit_event_thread(new_thd);
 
     delete scheduler_param_value;
     ret= true;
@@ -505,7 +493,6 @@ Event_scheduler::run(THD *thd)
   }
 
   LOCK_DATA();
-  deinit_event_thread(thd);
   scheduler_thd= NULL;
   state= INITIALIZED;
   DBUG_PRINT("info", ("Broadcasting COND_state back to the stoppers"));
@@ -564,10 +551,7 @@ Event_scheduler::execute_top(Event_queue_element_for_exec *event_name)
     sql_print_error("Event_scheduler::execute_top: Can not create event worker"
                     " thread (errno=%d). Stopping event scheduler", res);
 
-    new_thd->proc_info= "Clearing";
-    DBUG_ASSERT(new_thd->net.buff != 0);
-    net_end(&new_thd->net);
-
+    deinit_event_thread(new_thd);
     goto error;
   }
 
@@ -579,9 +563,6 @@ Event_scheduler::execute_top(Event_queue_element_for_exec *event_name)
 
 error:
   DBUG_PRINT("error", ("Event_scheduler::execute_top() res: %d", res));
-  if (new_thd)
-    delete new_thd;
-
   delete event_name;
   DBUG_RETURN(TRUE);
 }