Fix race condition in kqueue event system implementation (#2193)

This fixes a race condition in the kqueue back-end. Previously, it was possible for an unsubscribed (i.e. ASIO_DISPOSABLE) to be sent as an event in between being disposed and being sent as an ASIO_DISPOSABLE event. This could cause an event that had been unsubscribed to receive a read or a write event immediately before a dispose event.

src/libponyrt/asio/kqueue.c

@@ -161,28 +161,43 @@ DECLARE_THREAD_FN(ponyint_asio_backend_dispatch)
         switch(ep->filter)
         {
           case EVFILT_READ:
-            ev->readable = true;
-            pony_asio_event_send(ev, ASIO_READ, 0);
+            if(ev->flags & ASIO_READ)
+            {
+              ev->readable = true;
+              pony_asio_event_send(ev, ASIO_READ, 0);
+            }
             break;
 
           case EVFILT_WRITE:
             if(ep->flags & EV_EOF)
             {
-              ev->readable = true;
-              ev->writeable = true;
-              pony_asio_event_send(ev, ASIO_READ | ASIO_WRITE, 0);
+              if(ev->flags & (ASIO_READ | ASIO_WRITE))
+              {
+                ev->readable = true;
+                ev->writeable = true;
+                pony_asio_event_send(ev, ASIO_READ | ASIO_WRITE, 0);
+              }
             } else {
-              ev->writeable = true;
-              pony_asio_event_send(ev, ASIO_WRITE, 0);
+              if(ev->flags & ASIO_WRITE)
+              {
+                ev->writeable = true;
+                pony_asio_event_send(ev, ASIO_WRITE, 0);
+              }
             }
             break;
 
           case EVFILT_TIMER:
-            pony_asio_event_send(ev, ASIO_TIMER, 0);
+            if(ev->flags & ASIO_TIMER)
+            {
+              pony_asio_event_send(ev, ASIO_TIMER, 0);
+            }
             break;
 
           case EVFILT_SIGNAL:
-            pony_asio_event_send(ev, ASIO_SIGNAL, (uint32_t)ep->data);
+            if(ev->flags & ASIO_SIGNAL)
+            {
+              pony_asio_event_send(ev, ASIO_SIGNAL, (uint32_t)ep->data);
+            }
             break;
 
           default: {}