MDEV-7177 - Server hangs on shutdown after InnoDB error (main.plugin_loaderr

            fails in buildbot)

There was a race condition in timer functionality of query timeouts.

Race was as following:
main thread:          init_thr_timers()
timer handler thread: my_thread_init()
main thread:          end_thr_timer()/timer_thread_state= ABORTING
timer handler thread: timer_thread_state= RUNNING, continue normal op
main thread:          waits indefinitely for timer handler thread to go down

The original idea of the fix is to set RUNNING state in main thread, before
starting timer handler thread. But it didn't survive further cleanups:
- removed "timer_thread_state" and used "thr_timer_inited" for this purpose
- removed unused "timer_thread_running"
- removed code responisble for "timer handler thread" shutdown synchronization,
  use pthread_join() instead.

mysys/thr_timer.c

@@ -27,14 +27,6 @@
 #include <sys/syscall.h>
 #endif
 
-enum thread_state
-{
-  NOT_RUNNING= -1, RUNNING= 0, ABORTING=1
-};
-
-static enum thread_state timer_thread_state= NOT_RUNNING;
-
-volatile my_bool timer_thread_running= 0;
 struct timespec next_timer_expire_time;
 
 static my_bool thr_timer_inited= 0;
@@ -75,7 +67,7 @@ static thr_timer_t max_timer_data;
 my_bool init_thr_timer(uint alloc_timers)
 {
   pthread_attr_t thr_attr;
-  my_bool res;
+  my_bool res= 0;
   DBUG_ENTER("init_thr_timer");
 
   init_queue(&timer_queue, alloc_timers+2, offsetof(thr_timer_t,expire_time),
@@ -93,45 +85,39 @@ my_bool init_thr_timer(uint alloc_timers)
   /* Create a thread to handle timers */
   pthread_attr_init(&thr_attr);
   pthread_attr_setscope(&thr_attr,PTHREAD_SCOPE_PROCESS);
-  pthread_attr_setdetachstate(&thr_attr,PTHREAD_CREATE_DETACHED);
   pthread_attr_setstacksize(&thr_attr,8196);
-  res= mysql_thread_create(key_thread_timer,
-                           &timer_thread, &thr_attr, timer_handler, NULL) != 0;
+  thr_timer_inited= 1;
+  if (mysql_thread_create(key_thread_timer, &timer_thread, &thr_attr,
+                          timer_handler, NULL))
+  {
+    thr_timer_inited= 0;
+    res= 1;
+    mysql_mutex_destroy(&LOCK_timer);
+    mysql_cond_destroy(&COND_timer);
+    delete_queue(&timer_queue);
+  }
   pthread_attr_destroy(&thr_attr);
 
-  thr_timer_inited= 1;
   DBUG_RETURN(res);
 }
 
 
 void end_thr_timer(void)
 {
-  struct timespec abstime;
   DBUG_ENTER("end_thr_timer");
-  
+
   if (!thr_timer_inited)
     DBUG_VOID_RETURN;
 
   mysql_mutex_lock(&LOCK_timer);
-  timer_thread_state= ABORTING;                 /* Signal abort */
+  thr_timer_inited= 0;                          /* Signal abort */
   mysql_cond_signal(&COND_timer);
-
-  /* Wait until timer thread dies */
-  set_timespec(abstime, 10*1000);          /* Wait up to 10 seconds */
-  while (timer_thread_state == ABORTING)
-  {
-    int error= mysql_cond_timedwait(&COND_timer, &LOCK_timer, &abstime);
-    if (error == ETIME || error == ETIMEDOUT)
-      break;				/* Don't wait forever */
-  }
   mysql_mutex_unlock(&LOCK_timer);
-  if (timer_thread_state == NOT_RUNNING)
-  {
-    mysql_mutex_destroy(&LOCK_timer);
-    mysql_cond_destroy(&COND_timer);
-    delete_queue(&timer_queue);
-    thr_timer_inited= 0;
-  }
+  pthread_join(timer_thread, NULL);
+
+  mysql_mutex_destroy(&LOCK_timer);
+  mysql_cond_destroy(&COND_timer);
+  delete_queue(&timer_queue);
   DBUG_VOID_RETURN;
 }
 
@@ -277,18 +263,15 @@ static sig_handler process_timers(struct timespec *now)
 
 /*
   set up a timer thread to handle timeouts
-  This will be killed when timer_thread_state is set to ABORTING.
-  At end timer_aborted will be set to NOT_RUNNING
+  This will be killed when thr_timer_inited is set to false.
 */
 
 static void *timer_handler(void *arg __attribute__((unused)))
 {
   my_thread_init();
 
-  timer_thread_state= RUNNING;
-
   mysql_mutex_lock(&LOCK_timer);
-  while (likely(timer_thread_state == RUNNING))
+  while (likely(thr_timer_inited))
   {
     int error;
     struct timespec *top_time;
@@ -315,8 +298,6 @@ static void *timer_handler(void *arg __attribute__((unused)))
 #endif
     }
   }
-  timer_thread_state= NOT_RUNNING;		/* Mark thread ended */
-  mysql_cond_signal(&COND_timer);         	/* signal end_thr_timer() */
   mysql_mutex_unlock(&LOCK_timer);
   my_thread_end();
   pthread_exit(0);