notifier: recheck for readiness after registering waker

Make sure we can't lose any event inbetween that small window of time

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>

Author: Keruspe

src/notifier.rs

@@ -32,10 +32,15 @@ impl Future for Notifier {
     type Output = ();
 
     fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        if self.ready() {
+            return Poll::Ready(());
+        }
+        self.wakers.register(cx.waker());
+        // Re-check after registering to close the lost-wakeup window between the
+        // first check and the waker registration.
         if self.ready() {
             Poll::Ready(())
         } else {
-            self.wakers.register(cx.waker());
             Poll::Pending
         }
     }