lib: avoid signal-handling race with event loop poll call

Manage the main pthread's signal mask to avoid a signal-handling
race. Before entering poll, check for pending signals that the
application needs to handle. Use ppoll() to re-enable those
signals during the poll call.

Signed-off-by: Mark Stapp <mjs@voltanet.io>

lib/thread.c

@@ -727,9 +727,13 @@ static void thread_free(struct thread_master *master, struct thread *thread)
 	XFREE(MTYPE_THREAD, thread);
 }
 
-static int fd_poll(struct thread_master *m, struct pollfd *pfds, nfds_t pfdsize,
-		   nfds_t count, const struct timeval *timer_wait)
+static int fd_poll(struct thread_master *m, const struct timeval *timer_wait,
+		   bool *eintr_p)
 {
+	sigset_t origsigs;
+	unsigned char trash[64];
+	nfds_t count = m->handler.copycount;
+
 	/*
 	 * If timer_wait is null here, that means poll() should block
 	 * indefinitely, unless the thread_master has overridden it by setting
@@ -760,15 +764,58 @@ static int fd_poll(struct thread_master *m, struct pollfd *pfds, nfds_t pfdsize,
 	rcu_assert_read_unlocked();
 
 	/* add poll pipe poker */
-	assert(count + 1 < pfdsize);
-	pfds[count].fd = m->io_pipe[0];
-	pfds[count].events = POLLIN;
-	pfds[count].revents = 0x00;
+	assert(count + 1 < m->handler.pfdsize);
+	m->handler.copy[count].fd = m->io_pipe[0];
+	m->handler.copy[count].events = POLLIN;
+	m->handler.copy[count].revents = 0x00;
+
+	/* We need to deal with a signal-handling race here: we
+	 * don't want to miss a crucial signal, such as SIGTERM or SIGINT,
+	 * that may arrive just before we enter poll(). We will block the
+	 * key signals, then check whether any have arrived - if so, we return
+	 * before calling poll(). If not, we'll re-enable the signals
+	 * in the ppoll() call.
+	 */
+
+	sigemptyset(&origsigs);
+	if (m->handle_signals) {
+		/* Main pthread that handles the app signals */
+		if (frr_sigevent_check(&origsigs)) {
+			/* Signal to process - restore signal mask and return */
+			pthread_sigmask(SIG_SETMASK, &origsigs, NULL);
+			num = -1;
+			*eintr_p = true;
+			goto done;
+		}
+	} else {
+		/* Don't make any changes for the non-main pthreads */
+		pthread_sigmask(SIG_SETMASK, NULL, &origsigs);
+	}
 
-	num = poll(pfds, count + 1, timeout);
+#if defined(HAVE_PPOLL)
+	struct timespec ts, *tsp;
 
-	unsigned char trash[64];
-	if (num > 0 && pfds[count].revents != 0 && num--)
+	if (timeout >= 0) {
+		ts.tv_sec = timeout / 1000;
+		ts.tv_nsec = (timeout % 1000) * 1000000;
+		tsp = &ts;
+	} else
+		tsp = NULL;
+
+	num = ppoll(m->handler.copy, count + 1, tsp, &origsigs);
+	pthread_sigmask(SIG_SETMASK, &origsigs, NULL);
+#else
+	/* Not ideal - there is a race after we restore the signal mask */
+	pthread_sigmask(SIG_SETMASK, &origsigs, NULL);
+	num = poll(m->handler.copy, count + 1, timeout);
+#endif
+
+done:
+
+	if (num < 0 && errno == EINTR)
+		*eintr_p = true;
+
+	if (num > 0 && m->handler.copy[count].revents != 0 && num--)
 		while (read(m->io_pipe[0], &trash, sizeof(trash)) > 0)
 			;
 
@@ -1395,7 +1442,7 @@ struct thread *thread_fetch(struct thread_master *m, struct thread *fetch)
 	struct timeval zerotime = {0, 0};
 	struct timeval tv;
 	struct timeval *tw = NULL;
-
+	bool eintr_p = false;
 	int num = 0;
 
 	do {
@@ -1467,14 +1514,14 @@ struct thread *thread_fetch(struct thread_master *m, struct thread *fetch)
 
 		pthread_mutex_unlock(&m->mtx);
 		{
-			num = fd_poll(m, m->handler.copy, m->handler.pfdsize,
-				      m->handler.copycount, tw);
+			eintr_p = false;
+			num = fd_poll(m, tw, &eintr_p);
 		}
 		pthread_mutex_lock(&m->mtx);
 
 		/* Handle any errors received in poll() */
 		if (num < 0) {
-			if (errno == EINTR) {
+			if (eintr_p) {
 				pthread_mutex_unlock(&m->mtx);
 				/* loop around to signal handler */
 				continue;