SR-4201: DispatchSourceSignal not working on Linux

The observed bug is that signals sent to the main thread
after it called dispatch_main were not triggering the
event handler of a DispatchSourceSignal that had been
previously created and activated to handle that signal.

This patch addresses the problem by:

- export _dispatch_sigmask() as the way to block signals in a process
  using dispatch
- call it when dispatch_main() is called
- each time dispatch registers a signal handler, register a sigaction that
  will catch signals for threads with wrong masks and will fix the thread
  configuration then raise the signal again by directing a pthread_kill
  to the manager thread so that it is delivered to the signalfd as expected.

Patch by Pierre Habouzit <phabouzit@apple.com> from PR-231
with minor compilation fixes by Dave Grove.

Author: dgrove-oss

src/event/event_epoll.c

@@ -117,9 +117,28 @@ _dispatch_muxnote_dispose(dispatch_muxnote_t dmn)
 	free(dmn);
 }
 
+static pthread_t manager_thread;
+
+static void
+_dispatch_muxnote_signal_block_and_raise(int signo)
+{
+	// On linux, for signals to be delivered to the signalfd, signals
+	// must be blocked, else any thread that hasn't them blocked may
+	// receive them.  Fix that by lazily noticing, blocking said signal,
+	// and raising the signal again when it happens
+	_dispatch_sigmask();
+	pthread_kill(manager_thread, signo);
+}
+
 static dispatch_muxnote_t
 _dispatch_muxnote_create(dispatch_unote_t du, uint32_t events)
 {
+	static sigset_t signals_with_unotes;
+	static struct sigaction sa = {
+		.sa_handler = _dispatch_muxnote_signal_block_and_raise,
+		.sa_flags = SA_RESTART,
+	};
+
 	dispatch_muxnote_t dmn;
 	struct stat sb;
 	int fd = du._du->du_ident;
@@ -129,13 +148,17 @@ _dispatch_muxnote_create(dispatch_unote_t du, uint32_t events)
 
 	switch (filter) {
 	case EVFILT_SIGNAL:
+		if (!sigismember(&signals_with_unotes, du._du->du_ident)) {
+			manager_thread = pthread_self();
+			sigaddset(&signals_with_unotes, du._du->du_ident);
+			sigaction(du._du->du_ident, &sa, NULL);
+		}
 		sigemptyset(&sigmask);
 		sigaddset(&sigmask, du._du->du_ident);
 		fd = signalfd(-1, &sigmask, SFD_NONBLOCK | SFD_CLOEXEC);
 		if (fd < 0) {
 			return NULL;
 		}
-		sigprocmask(SIG_BLOCK, &sigmask, NULL);
 		break;
 
 	case EVFILT_WRITE:

src/init.c

@@ -72,6 +72,29 @@ dispatch_atfork_child(void)
 	_dispatch_unsafe_fork = 0;
 }
 
+int
+_dispatch_sigmask(void)
+{
+	sigset_t mask;
+	int r = 0;
+
+	/* Workaround: 6269619 Not all signals can be delivered on any thread */
+	r |= sigfillset(&mask);
+	r |= sigdelset(&mask, SIGILL);
+	r |= sigdelset(&mask, SIGTRAP);
+#if HAVE_DECL_SIGEMT
+	r |= sigdelset(&mask, SIGEMT);
+#endif
+	r |= sigdelset(&mask, SIGFPE);
+	r |= sigdelset(&mask, SIGBUS);
+	r |= sigdelset(&mask, SIGSEGV);
+	r |= sigdelset(&mask, SIGSYS);
+	r |= sigdelset(&mask, SIGPIPE);
+	r |= sigdelset(&mask, SIGPROF);
+	r |= pthread_sigmask(SIG_BLOCK, &mask, NULL);
+	(void)dispatch_assume_zero(r);
+}
+
 #pragma mark -
 #pragma mark dispatch_globals
 

src/internal.h

@@ -990,6 +990,8 @@ extern int _dispatch_evfilt_machport_direct_enabled;
 #endif // DISPATCH_USE_EVFILT_MACHPORT_DIRECT
 
 
+int _dispatch_sigmask(void);
+
 /* #includes dependent on internal.h */
 #include "object_internal.h"
 #include "semaphore_internal.h"

src/queue.c

@@ -66,7 +66,6 @@ static void _dispatch_worker_thread2(int priority, int options, void *context);
 #endif
 #if DISPATCH_USE_PTHREAD_POOL
 static void *_dispatch_worker_thread(void *context);
-static int _dispatch_pthread_sigmask(int how, sigset_t *set, sigset_t *oset);
 #endif
 
 #if DISPATCH_COCOA_COMPAT
@@ -5466,13 +5465,8 @@ _dispatch_worker_thread(void *context)
 		pqc->dpq_thread_configure();
 	}
 
-	sigset_t mask;
-	int r;
 	// workaround tweaks the kernel workqueue does for us
-	r = sigfillset(&mask);
-	(void)dispatch_assume_zero(r);
-	r = _dispatch_pthread_sigmask(SIG_BLOCK, &mask, NULL);
-	(void)dispatch_assume_zero(r);
+	_dispatch_sigmask();
 	_dispatch_introspection_thread_add();
 
 	const int64_t timeout = 5ull * NSEC_PER_SEC;
@@ -5489,37 +5483,6 @@ _dispatch_worker_thread(void *context)
 
 	return NULL;
 }
-
-int
-_dispatch_pthread_sigmask(int how, sigset_t *set, sigset_t *oset)
-{
-	int r;
-
-	/* Workaround: 6269619 Not all signals can be delivered on any thread */
-
-	r = sigdelset(set, SIGILL);
-	(void)dispatch_assume_zero(r);
-	r = sigdelset(set, SIGTRAP);
-	(void)dispatch_assume_zero(r);
-#if HAVE_DECL_SIGEMT
-	r = sigdelset(set, SIGEMT);
-	(void)dispatch_assume_zero(r);
-#endif
-	r = sigdelset(set, SIGFPE);
-	(void)dispatch_assume_zero(r);
-	r = sigdelset(set, SIGBUS);
-	(void)dispatch_assume_zero(r);
-	r = sigdelset(set, SIGSEGV);
-	(void)dispatch_assume_zero(r);
-	r = sigdelset(set, SIGSYS);
-	(void)dispatch_assume_zero(r);
-	r = sigdelset(set, SIGPIPE);
-	(void)dispatch_assume_zero(r);
-	r = sigdelset(set, SIGPROF);
-	(void)dispatch_assume_zero(r);
-
-	return pthread_sigmask(how, set, oset);
-}
 #endif // DISPATCH_USE_PTHREAD_POOL
 
 #pragma mark -
@@ -5749,6 +5712,7 @@ dispatch_main(void)
 		pthread_key_t dispatch_main_key;
 		pthread_key_create(&dispatch_main_key, _dispatch_sig_thread);
 		pthread_setspecific(dispatch_main_key, &dispatch_main_key);
+		_dispatch_sigmask();
 #endif
 		pthread_exit(NULL);
 		DISPATCH_INTERNAL_CRASH(errno, "pthread_exit() returned");