Always expect Pending when polling the Notified

Signed-off-by: Zahari Dichev <zaharidichev@gmail.com>

tokio/src/sync/mpsc/chan.rs

@@ -8,7 +8,7 @@ use crate::sync::Notify;
 
 use std::fmt;
 use std::process;
-use std::sync::atomic::Ordering::{AcqRel, Relaxed, SeqCst};
+use std::sync::atomic::Ordering::{AcqRel, Relaxed};
 use std::task::Poll::{Pending, Ready};
 use std::task::{Context, Poll};
 
@@ -223,19 +223,14 @@ where
         let notified = self.inner.notify_rx_closed.notified();
         pin!(notified);
 
-        // Polling this for first time will register the waiter and
-        // return `Pending` or return `Ready` right away. If `Ready`
-        // is returned we are done
-        let aquired_lost_notification =
-            crate::future::poll_fn(|cx| match Pin::new(&mut notified).poll(cx) {
-                Poll::Ready(()) => Poll::Ready(true),
-                Poll::Pending => Poll::Ready(false),
-            })
-            .await;
-
-        if aquired_lost_notification {
-            return;
-        }
+        // Polling the future once is guaranteed to return `Pending` as `watch`
+        // only notifies using `notify_waiters`.
+        crate::future::poll_fn(|cx| {
+            let res = Pin::new(&mut notified).poll(cx);
+            assert!(!res.is_ready());
+            Poll::Ready(())
+        })
+        .await;
 
         if self.inner.semaphore.closed() {
             return;