Bug#42850 race condition in my_thr_init.c

 - Create the "dummy" thread joinable and wait for it to
   exit before continuing in 'my_thread_global_init'
 - This way we know that the pthread library is initialized
   by one thread only

mysys/my_thr_init.c

@@ -107,10 +107,11 @@ my_bool my_thread_global_init(void)
     pthread_attr_t  dummy_thread_attr;
 
     pthread_attr_init(&dummy_thread_attr);
-    pthread_attr_setdetachstate(&dummy_thread_attr, PTHREAD_CREATE_DETACHED);
+    pthread_attr_setdetachstate(&dummy_thread_attr, PTHREAD_CREATE_JOINABLE);
 
-    pthread_create(&dummy_thread,&dummy_thread_attr,
-                   nptl_pthread_exit_hack_handler, NULL);
+    if (pthread_create(&dummy_thread,&dummy_thread_attr,
+                       nptl_pthread_exit_hack_handler, NULL) == 0)
+      (void)pthread_join(dummy_thread, NULL);
   }
 #endif /* TARGET_OS_LINUX */