io: avoid unnecessary wake in registration (#2221)

See discussion in #2222. This wake/notify call has been there in one
form or another since the very early days of tokio. Currently though, it
is not clear that it is needed; the contract for polling is that you
must keep polling until you get `Pending`, so doing a wakeup when we are
about to return `Ready` is premature.

tokio/tests/rt_basic.rs

@@ -2,7 +2,7 @@
 #![cfg(feature = "full")]
 
 use tokio::runtime::Runtime;
-use tokio::sync::oneshot;
+use tokio::sync::{mpsc, oneshot};
 use tokio_test::{assert_err, assert_ok};
 
 use std::thread;
@@ -27,6 +27,69 @@ fn spawned_task_does_not_progress_without_block_on() {
     assert_eq!(out, "hello");
 }
 
+#[test]
+fn no_extra_poll() {
+    use std::pin::Pin;
+    use std::sync::{
+        atomic::{AtomicUsize, Ordering::SeqCst},
+        Arc,
+    };
+    use std::task::{Context, Poll};
+    use tokio::stream::{Stream, StreamExt};
+
+    struct TrackPolls<S> {
+        npolls: Arc<AtomicUsize>,
+        s: S,
+    }
+
+    impl<S> Stream for TrackPolls<S>
+    where
+        S: Stream,
+    {
+        type Item = S::Item;
+        fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+            // safety: we do not move s
+            let this = unsafe { self.get_unchecked_mut() };
+            this.npolls.fetch_add(1, SeqCst);
+            // safety: we are pinned, and so is s
+            unsafe { Pin::new_unchecked(&mut this.s) }.poll_next(cx)
+        }
+    }
+
+    let (tx, rx) = mpsc::unbounded_channel();
+    let mut rx = TrackPolls {
+        npolls: Arc::new(AtomicUsize::new(0)),
+        s: rx,
+    };
+    let npolls = Arc::clone(&rx.npolls);
+
+    let mut rt = rt();
+
+    rt.spawn(async move { while let Some(_) = rx.next().await {} });
+    rt.block_on(async {
+        tokio::task::yield_now().await;
+    });
+
+    // should have been polled exactly once: the initial poll
+    assert_eq!(npolls.load(SeqCst), 1);
+
+    tx.send(()).unwrap();
+    rt.block_on(async {
+        tokio::task::yield_now().await;
+    });
+
+    // should have been polled twice more: once to yield Some(), then once to yield Pending
+    assert_eq!(npolls.load(SeqCst), 1 + 2);
+
+    drop(tx);
+    rt.block_on(async {
+        tokio::task::yield_now().await;
+    });
+
+    // should have been polled once more: to yield None
+    assert_eq!(npolls.load(SeqCst), 1 + 2 + 1);
+}
+
 #[test]
 fn acquire_mutex_in_drop() {
     use futures::future::pending;