Remove listener if one already exists

This commit makes it so EventListener::listen() does not overwrite
existing listeners if they already exist. If the listener is already
registered in an event listener list, it removes the listener from that
list before registering the new listener. This soundness bug was missed
during #94.

This is a patch-compatible fix for #100. We may also want an API-level
fix for #100 as well. This is up for further discussion.

Signed-off-by: John Nunley <dev@notgull.net>

src/lib.rs

@@ -810,13 +810,24 @@ impl<T> EventListener<T> {
     ///
     /// This method can only be called after the listener has been pinned, and must be called before
     /// the listener is polled.
+    ///
+    /// Notifications that exist when this function is called will be discarded.
     pub fn listen(mut self: Pin<&mut Self>, event: &Event<T>) {
         let inner = {
             let inner = event.inner();
             unsafe { Arc::clone(&ManuallyDrop::new(Arc::from_raw(inner))) }
         };
 
-        let ListenerProject { event, listener } = self.as_mut().project().listener.project();
+        let ListenerProject {
+            event,
+            mut listener,
+        } = self.as_mut().project().listener.project();
+
+        // If an event is already registered, make sure to remove it.
+        if let Some(current_event) = event.as_ref() {
+            current_event.remove(listener.as_mut(), false);
+        }
+
         let inner = event.insert(inner);
         inner.insert(listener);
 

tests/notify.rs

@@ -189,3 +189,16 @@ fn notify_all_fair() {
         .poll(&mut Context::from_waker(&waker3))
         .is_ready());
 }
+
+#[test]
+fn more_than_one_event() {
+    let event = Event::new();
+    let event2 = Event::new();
+
+    let mut listener = Box::pin(EventListener::<()>::new());
+    listener.as_mut().listen(&event);
+    listener.as_mut().listen(&event2);
+
+    drop(listener);
+    event.notify(1);
+}