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.

If plausible signals are added to the sigsuspend mask,
then they are properly routed to the signalfd and the
DispatchSourceSignal event handler runs as expected.

This commit also changes event_epoll to use pthread_sigmask
to block the signal when creating a signalfd because
the Linux man page for sigprocmask states that sigprocmask's
behavior is undefined for multi-threaded programs and
pthread_sigmask should be used instead.

Author: dgrove-oss

src/event/event_epoll.c

@@ -135,7 +135,7 @@ _dispatch_muxnote_create(dispatch_unote_t du, uint32_t events)
 		if (fd < 0) {
 			return NULL;
 		}
-		sigprocmask(SIG_BLOCK, &sigmask, NULL);
+		pthread_sigmask(SIG_BLOCK, &sigmask, NULL);
 		break;
 
 	case EVFILT_WRITE:

src/queue.c

@@ -5762,7 +5762,25 @@ DISPATCH_NOINLINE DISPATCH_NORETURN
 static void
 _dispatch_sigsuspend(void)
 {
-	static const sigset_t mask;
+	sigset_t mask;
+	int r;
+
+	r = sigemptyset(&mask);
+	(void)dispatch_assume_zero(r);
+
+#if DISPATCH_EVENT_BACKEND_EPOLL
+	// If there are signals that the user may have reasonably
+	// created a DispatchSignalSource to monitor and might
+	// reasonably be sent to the main thread after it has called
+	// dispatch_main, we have to include them in the sigsuspend mask
+	// or they won't be properly delivered to the signalfd.
+	r = sigaddset(&mask, SIGUSR1);
+	(void)dispatch_assume_zero(r);
+	r = sigaddset(&mask, SIGUSR2);
+	(void)dispatch_assume_zero(r);
+	r = sigaddset(&mask, SIGCHLD);
+	(void)dispatch_assume_zero(r);
+#endif
 
 	for (;;) {
 		sigsuspend(&mask);