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 adding
a pthread_key destructor that calls dispatch_sig_thread
(effectively stalling pthread exit until the program really exits).

This patch relies on the TSD destructors being called in order of
creation, which is currently the case in glibc.

Author: dgrove-oss

src/queue.c

@@ -48,6 +48,7 @@
 #define pthread_workqueue_t void*
 #endif
 
+static void _dispatch_sig_thread(void *ctxt);
 static void _dispatch_cache_cleanup(void *value);
 static void _dispatch_sync_f(dispatch_queue_t dq, void *ctxt,
 		dispatch_function_t func, pthread_priority_t pp);
@@ -5693,6 +5694,11 @@ 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__
+		pthread_key_t dispatch_main_key;
+		pthread_key_create(&dispatch_main_key, _dispatch_sig_thread);
+		pthread_setspecific(dispatch_main_key, &dispatch_main_key); // anything non NULL really
+#endif
 		pthread_exit(NULL);
 		DISPATCH_INTERNAL_CRASH(errno, "pthread_exit() returned");
 #if HAVE_PTHREAD_MAIN_NP
@@ -5776,11 +5782,13 @@ _dispatch_queue_cleanup2(void)
 	// overload the "probably" variable to mean that dispatch_main() or
 	// similar non-POSIX API was called
 	// this has to run before the DISPATCH_COCOA_COMPAT below
+#ifndef __linux__
 	if (_dispatch_program_is_probably_callback_driven) {
 		_dispatch_barrier_async_detached_f(_dispatch_get_root_queue(
 				_DISPATCH_QOS_CLASS_DEFAULT, true), NULL, _dispatch_sig_thread);
 		sleep(1); // workaround 6778970
 	}
+#endif
 
 #if DISPATCH_COCOA_COMPAT
 	dispatch_once_f(&_dispatch_main_q_port_pred, dq,