Linux: avoid zombie process when dispatch_main is called

If the main thread calls pthread_exit on Linux, the process
becomes a zombie. Modify dispatch_main to avoid this by manually
performing thread cleanup and pausing the thread instead of calling
pthread_exit.  This is an updated version of an old patch for this
issue (https://lists.macosforge.org/pipermail/libdispatch-dev/2011-May/000522.html).

Author: dgrove-oss

src/queue.c

@@ -5693,8 +5693,49 @@ dispatch_main(void)
 		_dispatch_object_debug(&_dispatch_main_q, "%s", __func__);
 		_dispatch_program_is_probably_callback_driven = true;
 		_dispatch_ktrace0(ARIADNE_ENTER_DISPATCH_MAIN_CODE);
+#ifdef __linux__
+		// On Linux, if the main thread calls pthread_exit the process becomes a zombie.
+		// Avoid zombification by manually doing the pthread cleanup
+		// and then pausing after masking all signals.
+		sigset_t mask;
+		void *p;
+#if DISPATCH_USE_THREAD_LOCAL_STORAGE
+		p = pthread_getspecific(__dispatch_tsd_key);
+		if (p != NULL)
+			_libdispatch_tsd_cleanup(p);
+#else
+		p = pthread_getspecific(dispatch_queue_key);
+		if (p != NULL)
+			_dispatch_queue_cleanup(p);
+		p = pthread_getspecific(dispatch_deferred_items_key);
+		if (p != NULL)
+			_dispatch_deferred_items_cleanup(p);
+		p = pthread_getspecific(dispatch_frame_key);
+		if (p != NULL)
+			_dispatch_frame_cleanup(p);
+		p = pthread_getspecific(dispatch_voucher_key);
+		if (p != NULL)
+			_voucher_thread_cleanup(p);
+		p = pthread_getspecific(dispatch_cache_key);
+		if (p != NULL)
+			_dispatch_cache_cleanup(p);
+		p = pthread_getspecific(dispatch_context_key);
+		if (p != NULL)
+			_dispatch_context_cleanup(p);
+#if DISPATCH_LOCK_USE_SEMAPHORE_FALLBACK
+		p = pthread_getspecific(dispatch_sema4_key);
+		if (p != NULL)
+			_dispatch_thread_semaphore_dispose(p);
+#endif
+#endif
+		sigfillset(&mask);
+		pthread_sigmask(SIG_SETMASK, &mask, NULL);
+		pause();
+		DISPATCH_INTERNAL_CRASH(errno, "pause() returned");
+#else
 		pthread_exit(NULL);
 		DISPATCH_INTERNAL_CRASH(errno, "pthread_exit() returned");
+#endif
 #if HAVE_PTHREAD_MAIN_NP
 	}
 	DISPATCH_CLIENT_CRASH(0, "dispatch_main() must be called on the main thread");