sd-event: re-check new epoll events when a child event is queued

yuwata · poettering · commit efd3be9de1dc · 2021-03-16T13:35:20.000+01:00
Previously, when a process outputs something and exit just after epoll_wait() but before process_child(), then the IO event is ignored even if the IO event has higher priority. See systemd#18190. This can be solved by checking epoll event again after process_child(). However, there exists a possibility that another process outputs and exits just after process_child() but before the second epoll_wait(). When the IO event has lower priority than the child event, still IO event is processed. So, this makes new epoll events and child events are checked in a loop until no new event is detected. To prevent an infinite loop, the number of maximum trial is set to 10. Fixes systemd#18190.
diff --git a/src/libsystemd/sd-event/sd-event.c b/src/libsystemd/sd-event/sd-event.c
@@ -1039,7 +1039,7 @@ static int source_set_pending(sd_event_source *s, bool b) {
                 }
         }
 
-        return 0;
+        return 1;
 }
 
 static sd_event_source *source_new(sd_event *e, bool floating, EventSourceType type) {
@@ -3116,11 +3116,19 @@ static int process_timer(
         return 0;
 }
 
-static int process_child(sd_event *e) {
+static int process_child(sd_event *e, int64_t threshold, int64_t *ret_min_priority) {
+        int64_t min_priority = threshold;
+        bool something_new = false;
         sd_event_source *s;
         int r;
 
         assert(e);
+        assert(ret_min_priority);
+
+        if (!e->need_process_child) {
+                *ret_min_priority = min_priority;
+                return 0;
+        }
 
         e->need_process_child = false;
 
@@ -3145,6 +3153,9 @@ static int process_child(sd_event *e) {
         HASHMAP_FOREACH(s, e->child_sources) {
                 assert(s->type == SOURCE_CHILD);
 
+                if (s->priority > threshold)
+                        continue;
+
                 if (s->pending)
                         continue;
 
@@ -3181,10 +3192,15 @@ static int process_child(sd_event *e) {
                         r = source_set_pending(s, true);
                         if (r < 0)
                                 return r;
+                        if (r > 0) {
+                                something_new = true;
+                                min_priority = MIN(min_priority, s->priority);
+                        }
                 }
         }
 
-        return 0;
+        *ret_min_priority = min_priority;
+        return something_new;
 }
 
 static int process_pidfd(sd_event *e, sd_event_source *s, uint32_t revents) {
@@ -3214,13 +3230,13 @@ static int process_pidfd(sd_event *e, sd_event_source *s, uint32_t revents) {
         return source_set_pending(s, true);
 }
 
-static int process_signal(sd_event *e, struct signal_data *d, uint32_t events) {
-        bool read_one = false;
+static int process_signal(sd_event *e, struct signal_data *d, uint32_t events, int64_t *min_priority) {
         int r;
 
         assert(e);
         assert(d);
         assert_return(events == EPOLLIN, -EIO);
+        assert(min_priority);
 
         /* If there's a signal queued on this priority and SIGCHLD is
            on this priority too, then make sure to recheck the
@@ -3246,7 +3262,7 @@ static int process_signal(sd_event *e, struct signal_data *d, uint32_t events) {
                 n = read(d->fd, &si, sizeof(si));
                 if (n < 0) {
                         if (IN_SET(errno, EAGAIN, EINTR))
-                                return read_one;
+                                return 0;
 
                         return -errno;
                 }
@@ -3256,8 +3272,6 @@ static int process_signal(sd_event *e, struct signal_data *d, uint32_t events) {
 
                 assert(SIGNAL_VALID(si.ssi_signo));
 
-                read_one = true;
-
                 if (e->signal_sources)
                         s = e->signal_sources[si.ssi_signo];
                 if (!s)
@@ -3271,12 +3285,16 @@ static int process_signal(sd_event *e, struct signal_data *d, uint32_t events) {
                 r = source_set_pending(s, true);
                 if (r < 0)
                         return r;
+                if (r > 0 && *min_priority >= s->priority) {
+                        *min_priority = s->priority;
+                        return 1; /* an event source with smaller priority is queued. */
+                }
 
-                return 1;
+                return 0;
         }
 }
 
-static int event_inotify_data_read(sd_event *e, struct inotify_data *d, uint32_t revents) {
+static int event_inotify_data_read(sd_event *e, struct inotify_data *d, uint32_t revents, int64_t threshold) {
         ssize_t n;
 
         assert(e);
@@ -3292,6 +3310,9 @@ static int event_inotify_data_read(sd_event *e, struct inotify_data *d, uint32_t
         if (d->buffer_filled > 0)
                 return 0;
 
+        if (d->priority > threshold)
+                return 0;
+
         n = read(d->fd, &d->buffer, sizeof(d->buffer));
         if (n < 0) {
                 if (IN_SET(errno, EAGAIN, EINTR))
@@ -3831,20 +3852,14 @@ static int epoll_wait_usec(
         return r;
 }
 
-_public_ int sd_event_wait(sd_event *e, uint64_t timeout) {
+static int process_epoll(sd_event *e, usec_t timeout, int64_t threshold, int64_t *ret_min_priority) {
+        int64_t min_priority = threshold;
+        bool something_new = false;
         size_t n_event_queue, m;
         int r;
 
-        assert_return(e, -EINVAL);
-        assert_return(e = event_resolve(e), -ENOPKG);
-        assert_return(!event_pid_changed(e), -ECHILD);
-        assert_return(e->state != SD_EVENT_FINISHED, -ESTALE);
-        assert_return(e->state == SD_EVENT_ARMED, -EBUSY);
-
-        if (e->exit_requested) {
-                e->state = SD_EVENT_PENDING;
-                return 1;
-        }
+        assert(e);
+        assert(ret_min_priority);
 
         n_event_queue = MAX(e->n_sources, 1u);
         if (!GREEDY_REALLOC(e->event_queue, e->event_queue_allocated, n_event_queue))
@@ -3856,12 +3871,8 @@ _public_ int sd_event_wait(sd_event *e, uint64_t timeout) {
 
         for (;;) {
                 r = epoll_wait_usec(e->epoll_fd, e->event_queue, e->event_queue_allocated, timeout);
-                if (r == -EINTR) {
-                        e->state = SD_EVENT_PENDING;
-                        return 1;
-                }
                 if (r < 0)
-                        goto finish;
+                        return r;
 
                 m = (size_t) r;
 
@@ -3877,7 +3888,9 @@ _public_ int sd_event_wait(sd_event *e, uint64_t timeout) {
                 timeout = 0;
         }
 
-        triple_timestamp_get(&e->timestamp);
+        /* Set timestamp only when this is called first time. */
+        if (threshold == INT64_MAX)
+                triple_timestamp_get(&e->timestamp);
 
         for (size_t i = 0; i < m; i++) {
 
@@ -3893,6 +3906,11 @@ _public_ int sd_event_wait(sd_event *e, uint64_t timeout) {
 
                                 assert(s);
 
+                                if (s->priority > threshold)
+                                        continue;
+
+                                min_priority = MIN(min_priority, s->priority);
+
                                 switch (s->type) {
 
                                 case SOURCE_IO:
@@ -3920,19 +3938,75 @@ _public_ int sd_event_wait(sd_event *e, uint64_t timeout) {
                         }
 
                         case WAKEUP_SIGNAL_DATA:
-                                r = process_signal(e, e->event_queue[i].data.ptr, e->event_queue[i].events);
+                                r = process_signal(e, e->event_queue[i].data.ptr, e->event_queue[i].events, &min_priority);
                                 break;
 
                         case WAKEUP_INOTIFY_DATA:
-                                r = event_inotify_data_read(e, e->event_queue[i].data.ptr, e->event_queue[i].events);
+                                r = event_inotify_data_read(e, e->event_queue[i].data.ptr, e->event_queue[i].events, threshold);
                                 break;
 
                         default:
                                 assert_not_reached("Invalid wake-up pointer");
                         }
                 }
+                if (r < 0)
+                        return r;
+                if (r > 0)
+                        something_new = true;
+        }
+
+        *ret_min_priority = min_priority;
+        return something_new;
+}
+
+_public_ int sd_event_wait(sd_event *e, uint64_t timeout) {
+        int r;
+
+        assert_return(e, -EINVAL);
+        assert_return(e = event_resolve(e), -ENOPKG);
+        assert_return(!event_pid_changed(e), -ECHILD);
+        assert_return(e->state != SD_EVENT_FINISHED, -ESTALE);
+        assert_return(e->state == SD_EVENT_ARMED, -EBUSY);
+
+        if (e->exit_requested) {
+                e->state = SD_EVENT_PENDING;
+                return 1;
+        }
+
+        for (int64_t threshold = INT64_MAX; ; threshold--) {
+                int64_t epoll_min_priority, child_min_priority;
+
+                /* There may be a possibility that new epoll (especially IO) and child events are
+                 * triggered just after process_epoll() call but before process_child(), and the new IO
+                 * events may have higher priority than the child events. To salvage these events,
+                 * let's call epoll_wait() again, but accepts only events with higher priority than the
+                 * previous. See issue https://github.com/systemd/systemd/issues/18190 and comments
+                 * https://github.com/systemd/systemd/pull/18750#issuecomment-785801085
+                 * https://github.com/systemd/systemd/pull/18922#issuecomment-792825226 */
+
+                r = process_epoll(e, timeout, threshold, &epoll_min_priority);
+                if (r == -EINTR) {
+                        e->state = SD_EVENT_PENDING;
+                        return 1;
+                }
+                if (r < 0)
+                        goto finish;
+                if (r == 0 && threshold < INT64_MAX)
+                        /* No new epoll event. */
+                        break;
+
+                r = process_child(e, threshold, &child_min_priority);
                 if (r < 0)
                         goto finish;
+                if (r == 0)
+                        /* No new child event. */
+                        break;
+
+                threshold = MIN(epoll_min_priority, child_min_priority);
+                if (threshold == INT64_MIN)
+                        break;
+
+                timeout = 0;
         }
 
         r = process_watchdog(e);
@@ -3959,19 +4033,12 @@ _public_ int sd_event_wait(sd_event *e, uint64_t timeout) {
         if (r < 0)
                 goto finish;
 
-        if (e->need_process_child) {
-                r = process_child(e);
-                if (r < 0)
-                        goto finish;
-        }
-
         r = process_inotify(e);
         if (r < 0)
                 goto finish;
 
         if (event_next_pending(e)) {
                 e->state = SD_EVENT_PENDING;
-
                 return 1;
         }
 
