fix(client): handle race condition when sending while connection is closing

src/client/dispatch.rs

@@ -156,38 +156,14 @@ impl<T, U> Stream for Receiver<T, U> {
     }
 }
 
-/*
-TODO: with futures 0.2, bring this Drop back and toss Envelope
-
-The problem is, there is a bug in futures 0.1 mpsc channel, where
-even though you may call `rx.close()`, `rx.poll()` may still think
-there are messages and so should park the current task. In futures
-0.2, we can use `try_next`, and not even risk such a bug.
-
-For now, use an `Envelope` that has this drop guard logic instead.
-
 impl<T, U> Drop for Receiver<T, U> {
     fn drop(&mut self) {
+        // Notify the giver about the closure first, before dropping
+        // the mpsc::Receiver.
         self.taker.cancel();
-        self.inner.close();
-
-        // This poll() is safe to call in `Drop`, because we've
-        // called, `close`, which promises that no new messages
-        // will arrive, and thus, once we reach the end, we won't
-        // see a `NotReady` (and try to park), but a Ready(None).
-        //
-        // All other variants:
-        // - Ready(None): the end. we want to stop looping
-        // - NotReady: unreachable
-        // - Err: unreachable
-        while let Ok(Async::Ready(Some((val, cb)))) = self.inner.poll() {
-            let _ = cb.send(Err((::Error::new_canceled(None::<::Error>), Some(val))));
     }
 }
 
-}
-*/
-
 struct Envelope<T, U>(Option<(T, Callback<T, U>)>);
 
 impl<T, U> Drop for Envelope<T, U> {

src/client/mod.rs

@@ -250,7 +250,32 @@ where C: Connect + Sync + 'static,
             if ver == Ver::Http1 {
                 set_relative_uri(req.uri_mut(), pooled.is_proxied);
             }
-            let fut = pooled.send_request_retryable(req)
+            let fut = pooled.send_request_retryable(req);
+
+            // As of futures@0.1.21, there is a race condition in the mpsc
+            // channel, such that sending when the receiver is closing can
+            // result in the message being stuck inside the queue. It won't
+            // ever notify until the Sender side is dropped.
+            //
+            // To counteract this, we must check if our senders 'want' channel
+            // has been closed after having tried to send. If so, error out...
+            if pooled.is_closed() {
+                drop(pooled);
+                let fut = fut
+                    .map_err(move |(err, orig_req)| {
+                        if let Some(req) = orig_req {
+                            ClientError::Canceled {
+                                connection_reused: conn_reused,
+                                reason: err,
+                                req,
+                            }
+                        } else {
+                            ClientError::Normal(err)
+                        }
+                    });
+                Either::A(fut)
+            } else {
+                let fut = fut
                     .map_err(move |(err, orig_req)| {
                         if let Some(req) = orig_req {
                             ClientError::Canceled {
@@ -292,8 +317,8 @@ where C: Connect + Sync + 'static,
                         }
                         Ok(res)
                     });
-
-            fut
+                Either::B(fut)
+            }
         });
 
         Box::new(resp)