Don't wake oneshot sender after it has been dropped

Author: Nemo157

futures-channel/src/oneshot.rs

@@ -214,12 +214,19 @@ impl<T> Inner<T> {
         // and deadlock might be possible, as was observed in
         // https://github.com/rust-lang-nursery/futures-rs/pull/219.
         self.complete.store(true, SeqCst);
+
         if let Some(mut slot) = self.rx_task.try_lock() {
             if let Some(task) = slot.take() {
                 drop(slot);
                 task.wake();
             }
         }
+
+        // If we registered a task for cancel notification drop it to reduce
+        // spurious wakeups
+        if let Some(mut slot) = self.tx_task.try_lock() {
+            drop(slot.take());
+        }
     }
 
     fn close_rx(&self) {

futures-channel/tests/oneshot.rs

@@ -4,6 +4,7 @@ use futures::channel::oneshot::{self, Sender};
 use futures::executor::block_on;
 use futures::future::{Future, FutureExt, poll_fn};
 use futures::task::{Context, Poll};
+use futures_test::task::panic_waker_ref;
 use std::pin::Pin;
 use std::sync::mpsc;
 use std::thread;
@@ -69,6 +70,15 @@ fn cancel_lots() {
     t.join().unwrap();
 }
 
+#[test]
+fn cancel_after_sender_drop_doesnt_notify() {
+    let (mut tx, rx) = oneshot::channel::<u32>();
+    let mut cx = Context::from_waker(panic_waker_ref());
+    assert_eq!(tx.poll_cancel(&mut cx), Poll::Pending);
+    drop(tx);
+    drop(rx);
+}
+
 #[test]
 fn close() {
     let (mut tx, mut rx) = oneshot::channel::<u32>();