uloop: use a waker for notifying sigchld and loop cancel events

Fix a race condition when do_sigchld, uloop_cancelled were set just
before epoll_wait(timeout=-1), resulting the loop stuck in the syscall
without noticing the events just happened

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>

uloop-epoll.c

@@ -23,7 +23,7 @@
 #define EPOLLRDHUP 0x2000
 #endif
 
-int uloop_init(void)
+static int uloop_init_pollfd(void)
 {
 	if (poll_fd >= 0)
 		return 0;

uloop-kqueue.c

@@ -15,7 +15,7 @@
  * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
-int uloop_init(void)
+static int uloop_init_pollfd(void)
 {
 	struct timespec timeout = { 0, 0 };
 	struct kevent ev = {};

uloop.c

@@ -63,6 +63,8 @@ static bool do_sigchld = false;
 static struct uloop_fd_event cur_fds[ULOOP_MAX_EVENTS];
 static int cur_fd, cur_nfds;
 
+int uloop_fd_add(struct uloop_fd *sock, unsigned int flags);
+
 #ifdef USE_KQUEUE
 #include "uloop-kqueue.c"
 #endif
@@ -71,6 +73,59 @@ static int cur_fd, cur_nfds;
 #include "uloop-epoll.c"
 #endif
 
+static void waker_consume(struct uloop_fd *fd, unsigned int events)
+{
+	char buf[4];
+
+	while (read(fd->fd, buf, 4) > 0)
+		;
+}
+
+static int waker_pipe = -1;
+static struct uloop_fd waker_fd = {
+	.fd = -1,
+	.cb = waker_consume,
+};
+
+static void waker_init_fd(int fd)
+{
+	fcntl(fd, F_SETFD, fcntl(fd, F_GETFD) | FD_CLOEXEC);
+	fcntl(fd, F_SETFL, fcntl(fd, F_GETFL) | O_NONBLOCK);
+}
+
+static int waker_init(void)
+{
+	int fds[2];
+
+	if (waker_pipe >= 0)
+		return 0;
+
+	if (pipe(fds) < 0)
+		return -1;
+
+	waker_init_fd(fds[0]);
+	waker_init_fd(fds[1]);
+
+	waker_fd.fd = fds[0];
+	waker_fd.cb = waker_consume;
+	uloop_fd_add(&waker_fd, ULOOP_READ);
+
+	return 0;
+}
+
+int uloop_init(void)
+{
+	if (uloop_init_pollfd() < 0)
+		return -1;
+
+	if (waker_init() < 0) {
+		uloop_done();
+		return -1;
+	}
+
+	return 0;
+}
+
 static bool uloop_fd_stack_event(struct uloop_fd *fd, int events)
 {
 	struct uloop_fd_stack *cur;
@@ -328,14 +383,21 @@ static void uloop_handle_processes(void)
 
 }
 
+static void uloop_signal_wake(void)
+{
+	write(waker_pipe, "w", 1);
+}
+
 static void uloop_handle_sigint(int signo)
 {
 	uloop_cancelled = true;
+	uloop_signal_wake();
 }
 
 static void uloop_sigchld(int signo)
 {
 	do_sigchld = true;
+	uloop_signal_wake();
 }
 
 static void uloop_install_handler(int signum, void (*handler)(int), struct sigaction* old, bool add)
@@ -477,11 +539,17 @@ void uloop_run(void)
 
 void uloop_done(void)
 {
-	if (poll_fd < 0)
+	if (poll_fd >= 0) {
-		return;
-
 		close(poll_fd);
 		poll_fd = -1;
+	}
+
+	if (waker_pipe >= 0) {
+		uloop_fd_delete(&waker_fd);
+		close(waker_pipe);
+		close(waker_fd.fd);
+		waker_pipe = -1;
+	}
 
 	uloop_clear_timeouts();
 	uloop_clear_processes();