Fix the signal story on linux

- 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 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 +117,26 @@ _dispatch_muxnote_dispose(dispatch_muxnote_t dmn)
 	free(dmn);
 }
 
+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();
+	raise(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 +146,16 @@ _dispatch_muxnote_create(dispatch_unote_t du, uint32_t events)
 
 	switch (filter) {
 	case EVFILT_SIGNAL:
+		if (!sigismember(&signals_with_unotes, du._du->du_ident)) {
+			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");