In Envoy today, we sometimes have network socket FDs setup with event_assign to watch for EV_CLOSED | EV_ET | EV_PERSIST. The callback function is invoked on EV_READ or EV_WRITE however, as a result of the following call chain:

1. epoll_dispatch issuing read/write events with EV_ET set, see

   libevent/epoll.c

   Line 504 in 0d2f170

   evmap_io_active_(base, events[i].data.fd, ev | EV_ET);

   .

2. evmap_io_active masking out EV_READ and EV_WRITE (since we're only looking for EV_CLOSED) but preserving EV_ET in

   libevent/evmap.c

   Line 436 in 0d2f170

   event_active_nolock_(ev, ev->ev_events & events, 1);

   .

3. event_active_nolock_ delivering an event to the callback with res set to EV_ET, despite no registered event actually firing.

This can be fixed with the following patch:

diff --git a/evmap.c b/evmap.c
index ffc991f5..d824c062 100644
--- a/evmap.c
+++ b/evmap.c
@@ -432,7 +432,7 @@ evmap_io_active_(struct event_base *base, evutil_socket_t fd, short events)
        if (NULL == ctx)
                return;
        LIST_FOREACH(ev, &ctx->events, ev_io_next) {
-               if (ev->ev_events & events)
+               if (ev->ev_events & (events & (EV_READ | EV_WRITE | EV_CLOSED)))
                        event_active_nolock_(ev, ev->ev_events & events, 1);
        }
 }

I'm wondering if folks agree that this is a bug and whether the above patch is the correct fix?

CC @asraa @adisuissa