BUG/MINOR: signals/poller: ensure wakeup from signals

Add self-wake in signal_handler() to fix a race condition with a signal
coming in between checking signal_queue_len and entering polling sleep.

The changes in commit 43c891dda ("BUG/MINOR: signals/poller: set the
poller timeout to 0 when there are signals") were insufficient.

Move the signal_queue_len check from the poll implementations to
run_poll_loop() to keep that logic in one place.

The poll loops are terminated either by the parameter wake being set or
wake up due to a write to their poller_wr_pipe by wake_thread() in
signal_handler().

This fixes issue #1841.

Must be backported in every stable version.

src/ev_epoll.c

@@ -222,11 +222,10 @@ static void _do_poll(struct poller *p, int exp, int wake)
 	thread_idle_now();
 	thread_harmless_now();
 
-	/* Now let's wait for polled events.
-	 * Check if the signal queue is not empty in case we received a signal
-	 * before entering the loop, so we don't wait MAX_DELAY_MS for nothing */
-	wait_time = (wake | signal_queue_len) ? 0 : compute_poll_timeout(exp);
+	/* Now let's wait for polled events. */
+	wait_time = wake ? 0 : compute_poll_timeout(exp);
 	clock_entering_poll();
+
 	do {
 		int timeout = (global.tune.options & GTUNE_BUSY_POLLING) ? 0 : wait_time;
 
@@ -239,7 +238,7 @@ static void _do_poll(struct poller *p, int exp, int wake)
 		}
 		if (timeout || !wait_time)
 			break;
-		if (signal_queue_len || wake)
+		if (wake)
 			break;
 		if (tick_isset(exp) && tick_is_expired(exp, now_ms))
 			break;

src/ev_evports.c

@@ -178,10 +178,8 @@ static void _do_poll(struct poller *p, int exp, int wake)
 	thread_idle_now();
 	thread_harmless_now();
 
-	/* Now let's wait for polled events.
-	 * Check if the signal queue is not empty in case we received a signal
-	 * before entering the loop, so we don't wait MAX_DELAY_MS for nothing */
-	wait_time = (wake | signal_queue_len) ? 0 : compute_poll_timeout(exp);
+	/* Now let's wait for polled events. */
+	wait_time = wake ? 0 : compute_poll_timeout(exp);
 	clock_entering_poll();
 
 	do {
@@ -219,7 +217,7 @@ static void _do_poll(struct poller *p, int exp, int wake)
 			break;
 		if (timeout || !wait_time)
 			break;
-		if (signal_queue_len || wake)
+		if (wake)
 			break;
 		if (tick_isset(exp) && tick_is_expired(exp, now_ms))
 			break;

src/ev_kqueue.c

@@ -166,10 +166,8 @@ static void _do_poll(struct poller *p, int exp, int wake)
 	}
 	fd_nbupdt = 0;
 
-	/* Now let's wait for polled events.
-	 * Check if the signal queue is not empty in case we received a signal
-	 * before entering the loop, so we don't wait MAX_DELAY_MS for nothing */
-	wait_time = (wake | signal_queue_len) ? 0 : compute_poll_timeout(exp);
+	/* Now let's wait for polled events. */
+	wait_time = wake ? 0 : compute_poll_timeout(exp);
 	fd = global.tune.maxpollevents;
 	clock_entering_poll();
 
@@ -193,7 +191,7 @@ static void _do_poll(struct poller *p, int exp, int wake)
 		}
 		if (timeout || !wait_time)
 			break;
-		if (signal_queue_len || wake)
+		if (wake)
 			break;
 		if (tick_isset(exp) && tick_is_expired(exp, now_ms))
 			break;

src/ev_poll.c

@@ -204,10 +204,8 @@ static void _do_poll(struct poller *p, int exp, int wake)
 		}
 	}
 
-	/* Now let's wait for polled events.
-	 * Check if the signal queue is not empty in case we received a signal
-	 * before entering the loop, so we don't wait MAX_DELAY_MS for nothing */
-	wait_time = (wake | signal_queue_len) ? 0 : compute_poll_timeout(exp);
+	/* Now let's wait for polled events. */
+	wait_time = wake ? 0 : compute_poll_timeout(exp);
 	clock_entering_poll();
 	status = poll(poll_events, nbfd, wait_time);
 	clock_update_date(wait_time, status);

src/haproxy.c

@@ -2810,7 +2810,7 @@ void run_poll_loop()
 		if (killed > 1)
 			break;
 
-		/* expire immediately if events are pending */
+		/* expire immediately if events or signals are pending */
 		wake = 1;
 		if (thread_has_tasks())
 			activity[tid].wake_tasks++;
@@ -2821,6 +2821,10 @@ void run_poll_loop()
 			if (thread_has_tasks()) {
 				activity[tid].wake_tasks++;
 				_HA_ATOMIC_AND(&th_ctx->flags, ~TH_FL_SLEEPING);
+			} else if (signal_queue_len) {
+				/* this check is required after setting TH_FL_SLEEPING to avoid
+				 * a race with wakeup on signals using wake_threads() */
+				_HA_ATOMIC_AND(&th_ctx->flags, ~TH_FL_SLEEPING);
 			} else
 				wake = 0;
 		}

src/signal.c

@@ -56,6 +56,9 @@ void signal_handler(int sig)
 	signal_state[sig].count++;
 	if (sig)
 		signal(sig, signal_handler); /* re-arm signal */
+
+	/* If the thread is TH_FL_SLEEPING we need to wake it */
+	wake_thread(tid);
 }
 
 /* Call handlers of all pending signals and clear counts and queue length. The