sync: add `mpsc::Sender::closed` future (#2840)

Adding closed future, makes it possible to select over closed and some other
work, so that the task is woken when the channel is closed and can proactively
cancel itself.

Added a mpsc::Sender::closed future that will become ready when the receiver
is closed.

1 files changed, 28 insertions, 0 deletions

diff --git a/tokio/src/sync/tests/loom_mpsc.rs b/tokio/src/sync/tests/loom_mpsc.rs
index e8db2dea..330e798b 100644
--- a/tokio/src/sync/tests/loom_mpsc.rs
+++ b/tokio/src/sync/tests/loom_mpsc.rs
@@ -41,6 +41,34 @@ fn closing_unbounded_tx() {
 }
 
 #[test]
+fn closing_bounded_rx() {
+    loom::model(|| {
+        let (mut tx1, rx) = mpsc::channel::<()>(16);
+        let mut tx2 = tx1.clone();
+        thread::spawn(move || {
+            drop(rx);
+        });
+
+        block_on(tx1.closed());
+        block_on(tx2.closed());
+    });
+}
+
+#[test]
+fn closing_unbounded_rx() {
+    loom::model(|| {
+        let (mut tx1, rx) = mpsc::unbounded_channel::<()>();
+        let mut tx2 = tx1.clone();
+        thread::spawn(move || {
+            drop(rx);
+        });
+
+        block_on(tx1.closed());
+        block_on(tx2.closed());
+    });
+}
+
+#[test]
 fn dropping_tx() {
     loom::model(|| {
         let (tx, mut rx) = mpsc::channel::<()>(16);