fix(client): more reliably detect closed pooled connections (#1434)

src/client/dispatch.rs

@@ -0,0 +1,73 @@
+use futures::{Async, Future, Poll, Stream};
+use futures::sync::{mpsc, oneshot};
+
+use common::Never;
+use super::cancel::{Cancel, Canceled};
+
+pub type Callback<U> = oneshot::Sender<::Result<U>>;
+pub type Promise<U> = oneshot::Receiver<::Result<U>>;
+
+pub fn channel<T, U>() -> (Sender<T, U>, Receiver<T, U>) {
+    let (tx, rx) = mpsc::unbounded();
+    let (cancel, canceled) = Cancel::new();
+    let tx = Sender {
+        cancel: cancel,
+        inner: tx,
+    };
+    let rx = Receiver {
+        canceled: canceled,
+        inner: rx,
+    };
+    (tx, rx)
+}
+
+pub struct Sender<T, U> {
+    cancel: Cancel,
+    inner: mpsc::UnboundedSender<(T, Callback<U>)>,
+}
+
+impl<T, U> Sender<T, U> {
+    pub fn is_closed(&self) -> bool {
+        self.cancel.is_canceled()
+    }
+
+    pub fn cancel(&self) {
+        self.cancel.cancel();
+    }
+
+    pub fn send(&self, val: T) -> Result<Promise<U>, T> {
+        let (tx, rx) = oneshot::channel();
+        self.inner.unbounded_send((val, tx))
+            .map(move |_| rx)
+            .map_err(|e| e.into_inner().0)
+    }
+}
+
+impl<T, U> Clone for Sender<T, U> {
+    fn clone(&self) -> Sender<T, U> {
+        Sender {
+            cancel: self.cancel.clone(),
+            inner: self.inner.clone(),
+        }
+    }
+}
+
+pub struct Receiver<T, U> {
+    canceled: Canceled,
+    inner: mpsc::UnboundedReceiver<(T, Callback<U>)>,
+}
+
+impl<T, U> Stream for Receiver<T, U> {
+    type Item = (T, Callback<U>);
+    type Error = Never;
+
+    fn poll(&mut self) -> Poll<Option<Self::Item>, Self::Error> {
+        if let Async::Ready(()) = self.canceled.poll()? {
+            return Ok(Async::Ready(None));
+        }
+        self.inner.poll()
+            .map_err(|()| unreachable!("mpsc never errors"))
+    }
+}
+
+//TODO: Drop for Receiver should consume inner