tarpc/rpc/src/client/channel.rs

// Copyright 2018 Google LLC
//
// Use of this source code is governed by an MIT-style
// license that can be found in the LICENSE file or at
// https://opensource.org/licenses/MIT.

use crate::{
    context,
    util::{Compact, TimeUntil},
    ClientMessage, PollIo, Request, Response, Transport,
};
use fnv::FnvHashMap;
use futures::{
    channel::{mpsc, oneshot},
    prelude::*,
    ready,
    stream::Fuse,
    task::Context,
    Poll,
};
use log::{debug, info, trace};
use pin_project::{pin_project, pinned_drop};
use std::{
    io,
    pin::Pin,
    sync::{
        atomic::{AtomicU64, Ordering},
        Arc,
    },
};
use tokio_timer::{timeout, Timeout};
use trace::SpanId;

use super::{Config, NewClient};

/// Handles communication from the client to request dispatch.
#[derive(Debug)]
pub struct Channel<Req, Resp> {
    to_dispatch: mpsc::Sender<DispatchRequest<Req, Resp>>,
    /// Channel to send a cancel message to the dispatcher.
    cancellation: RequestCancellation,
    /// The ID to use for the next request to stage.
    next_request_id: Arc<AtomicU64>,
}

impl<Req, Resp> Clone for Channel<Req, Resp> {
    fn clone(&self) -> Self {
        Self {
            to_dispatch: self.to_dispatch.clone(),
            cancellation: self.cancellation.clone(),
            next_request_id: self.next_request_id.clone(),
        }
    }
}

/// A future returned by [`Channel::send`] that resolves to a server response.
#[pin_project]
#[derive(Debug)]
#[must_use = "futures do nothing unless polled"]
struct Send<'a, Req, Resp> {
    #[pin]
    fut: MapOkDispatchResponse<SendMapErrConnectionReset<'a, Req, Resp>, Resp>,
}

type SendMapErrConnectionReset<'a, Req, Resp> = MapErrConnectionReset<
    futures::sink::Send<'a, mpsc::Sender<DispatchRequest<Req, Resp>>, DispatchRequest<Req, Resp>>,
>;

impl<'a, Req, Resp> Future for Send<'a, Req, Resp> {
    type Output = io::Result<DispatchResponse<Resp>>;

    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        self.as_mut().project().fut.poll(cx)
    }
}

/// A future returned by [`Channel::call`] that resolves to a server response.
#[pin_project]
#[derive(Debug)]
#[must_use = "futures do nothing unless polled"]
pub struct Call<'a, Req, Resp> {
    #[pin]
    fut: AndThenIdent<Send<'a, Req, Resp>, DispatchResponse<Resp>>,
}

impl<'a, Req, Resp> Future for Call<'a, Req, Resp> {
    type Output = io::Result<Resp>;

    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        self.as_mut().project().fut.poll(cx)
    }
}

impl<Req, Resp> Channel<Req, Resp> {
    /// Sends a request to the dispatch task to forward to the server, returning a [`Future`] that
    /// resolves when the request is sent (not when the response is received).
    fn send(&mut self, mut ctx: context::Context, request: Req) -> Send<Req, Resp> {
        // Convert the context to the call context.
        ctx.trace_context.parent_id = Some(ctx.trace_context.span_id);
        ctx.trace_context.span_id = SpanId::random(&mut rand::thread_rng());

        let timeout = ctx.deadline.time_until();
        trace!(
            "[{}] Queuing request with timeout {:?}.",
            ctx.trace_id(),
            timeout,
        );

        let (response_completion, response) = oneshot::channel();
        let cancellation = self.cancellation.clone();
        let request_id = self.next_request_id.fetch_add(1, Ordering::Relaxed);
        Send {
            fut: MapOkDispatchResponse::new(
                MapErrConnectionReset::new(self.to_dispatch.send(DispatchRequest {
                    ctx,
                    request_id,
                    request,
                    response_completion,
                })),
                DispatchResponse {
                    response: Timeout::new(response, timeout),
                    complete: false,
                    request_id,
                    cancellation,
                    ctx,
                },
            ),
        }
    }

    /// Sends a request to the dispatch task to forward to the server, returning a [`Future`] that
    /// resolves to the response.
    pub fn call(&mut self, context: context::Context, request: Req) -> Call<Req, Resp> {
        Call {
            fut: AndThenIdent::new(self.send(context, request)),
        }
    }
}

/// A server response that is completed by request dispatch when the corresponding response
/// arrives off the wire.
#[pin_project(PinnedDrop)]
#[derive(Debug)]
struct DispatchResponse<Resp> {
    response: Timeout<oneshot::Receiver<Response<Resp>>>,
    ctx: context::Context,
    complete: bool,
    cancellation: RequestCancellation,
    request_id: u64,
}

impl<Resp> Future for DispatchResponse<Resp> {
    type Output = io::Result<Resp>;

    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<Resp>> {
        let resp = ready!(self.response.poll_unpin(cx));

        Poll::Ready(match resp {
            Ok(resp) => {
                self.complete = true;
                match resp {
                    Ok(resp) => Ok(resp.message?),
                    Err(oneshot::Canceled) => {
                        // The oneshot is Canceled when the dispatch task ends. In that case,
                        // there's nothing listening on the other side, so there's no point in
                        // propagating cancellation.
                        Err(io::Error::from(io::ErrorKind::ConnectionReset))
                    }
                }
            }
            Err(timeout::Elapsed { .. }) => Err(io::Error::new(
                io::ErrorKind::TimedOut,
                "Client dropped expired request.".to_string(),
            )),
        })
    }
}

// Cancels the request when dropped, if not already complete.
#[pinned_drop]
impl<Resp> PinnedDrop for DispatchResponse<Resp> {
    fn drop(mut self: Pin<&mut Self>) {
        if !self.complete {
            // The receiver needs to be closed to handle the edge case that the request has not
            // yet been received by the dispatch task. It is possible for the cancel message to
            // arrive before the request itself, in which case the request could get stuck in the
            // dispatch map forever if the server never responds (e.g. if the server dies while
            // responding). Even if the server does respond, it will have unnecessarily done work
            // for a client no longer waiting for a response. To avoid this, the dispatch task
            // checks if the receiver is closed before inserting the request in the map. By
            // closing the receiver before sending the cancel message, it is guaranteed that if the
            // dispatch task misses an early-arriving cancellation message, then it will see the
            // receiver as closed.
            self.response.get_mut().close();
            let request_id = self.request_id;
            self.cancellation.cancel(request_id);
        }
    }
}

/// Returns a channel and dispatcher that manages the lifecycle of requests initiated by the
/// channel.
pub fn new<Req, Resp, C>(
    config: Config,
    transport: C,
) -> NewClient<Channel<Req, Resp>, RequestDispatch<Req, Resp, C>>
where
    C: Transport<ClientMessage<Req>, Response<Resp>>,
{
    let (to_dispatch, pending_requests) = mpsc::channel(config.pending_request_buffer);
    let (cancellation, canceled_requests) = cancellations();
    let canceled_requests = canceled_requests.fuse();

    NewClient {
        client: Channel {
            to_dispatch,
            cancellation,
            next_request_id: Arc::new(AtomicU64::new(0)),
        },
        dispatch: RequestDispatch {
            config,
            canceled_requests,
            transport: transport.fuse(),
            in_flight_requests: FnvHashMap::default(),
            pending_requests: pending_requests.fuse(),
        },
    }
}

/// Handles the lifecycle of requests, writing requests to the wire, managing cancellations,
/// and dispatching responses to the appropriate channel.
#[pin_project]
#[derive(Debug)]
pub struct RequestDispatch<Req, Resp, C> {
    /// Writes requests to the wire and reads responses off the wire.
    #[pin]
    transport: Fuse<C>,
    /// Requests waiting to be written to the wire.
    #[pin]
    pending_requests: Fuse<mpsc::Receiver<DispatchRequest<Req, Resp>>>,
    /// Requests that were dropped.
    #[pin]
    canceled_requests: Fuse<CanceledRequests>,
    /// Requests already written to the wire that haven't yet received responses.
    in_flight_requests: FnvHashMap<u64, InFlightData<Resp>>,
    /// Configures limits to prevent unlimited resource usage.
    config: Config,
}

impl<Req, Resp, C> RequestDispatch<Req, Resp, C>
where
    C: Transport<ClientMessage<Req>, Response<Resp>>,
{
    fn pump_read(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> PollIo<()> {
        Poll::Ready(
            match ready!(self.as_mut().project().transport.poll_next(cx)?) {
                Some(response) => {
                    self.complete(response);
                    Some(Ok(()))
                }
                None => None,
            },
        )
    }

    fn pump_write(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> PollIo<()> {
        enum ReceiverStatus {
            NotReady,
            Closed,
        }

        let pending_requests_status = match self.as_mut().poll_next_request(cx)? {
            Poll::Ready(Some(dispatch_request)) => {
                self.as_mut().write_request(dispatch_request)?;
                return Poll::Ready(Some(Ok(())));
            }
            Poll::Ready(None) => ReceiverStatus::Closed,
            Poll::Pending => ReceiverStatus::NotReady,
        };

        let canceled_requests_status = match self.as_mut().poll_next_cancellation(cx)? {
            Poll::Ready(Some((context, request_id))) => {
                self.as_mut().write_cancel(context, request_id)?;
                return Poll::Ready(Some(Ok(())));
            }
            Poll::Ready(None) => ReceiverStatus::Closed,
            Poll::Pending => ReceiverStatus::NotReady,
        };

        match (pending_requests_status, canceled_requests_status) {
            (ReceiverStatus::Closed, ReceiverStatus::Closed) => {
                ready!(self.as_mut().project().transport.poll_flush(cx)?);
                Poll::Ready(None)
            }
            (ReceiverStatus::NotReady, _) | (_, ReceiverStatus::NotReady) => {
                // No more messages to process, so flush any messages buffered in the transport.
                ready!(self.as_mut().project().transport.poll_flush(cx)?);

                // Even if we fully-flush, we return Pending, because we have no more requests
                // or cancellations right now.
                Poll::Pending
            }
        }
    }

    /// Yields the next pending request, if one is ready to be sent.
    fn poll_next_request(
        mut self: Pin<&mut Self>,
        cx: &mut Context<'_>,
    ) -> PollIo<DispatchRequest<Req, Resp>> {
        if self.as_mut().project().in_flight_requests.len() >= self.config.max_in_flight_requests {
            info!(
                "At in-flight request capacity ({}/{}).",
                self.as_mut().project().in_flight_requests.len(),
                self.config.max_in_flight_requests
            );

            // No need to schedule a wakeup, because timers and responses are responsible
            // for clearing out in-flight requests.
            return Poll::Pending;
        }

        while let Poll::Pending = self.as_mut().project().transport.poll_ready(cx)? {
            // We can't yield a request-to-be-sent before the transport is capable of buffering it.
            ready!(self.as_mut().project().transport.poll_flush(cx)?);
        }

        loop {
            match ready!(self.as_mut().project().pending_requests.poll_next_unpin(cx)) {
                Some(request) => {
                    if request.response_completion.is_canceled() {
                        trace!(
                            "[{}] Request canceled before being sent.",
                            request.ctx.trace_id()
                        );
                        continue;
                    }

                    return Poll::Ready(Some(Ok(request)));
                }
                None => return Poll::Ready(None),
            }
        }
    }

    /// Yields the next pending cancellation, and, if one is ready, cancels the associated request.
    fn poll_next_cancellation(
        mut self: Pin<&mut Self>,
        cx: &mut Context<'_>,
    ) -> PollIo<(context::Context, u64)> {
        while let Poll::Pending = self.as_mut().project().transport.poll_ready(cx)? {
            ready!(self.as_mut().project().transport.poll_flush(cx)?);
        }

        loop {
            let cancellation = self
                .as_mut()
                .project()
                .canceled_requests
                .poll_next_unpin(cx);
            match ready!(cancellation) {
                Some(request_id) => {
                    if let Some(in_flight_data) = self
                        .as_mut()
                        .project()
                        .in_flight_requests
                        .remove(&request_id)
                    {
                        self.as_mut().project().in_flight_requests.compact(0.1);
                        debug!("[{}] Removed request.", in_flight_data.ctx.trace_id());
                        return Poll::Ready(Some(Ok((in_flight_data.ctx, request_id))));
                    }
                }
                None => return Poll::Ready(None),
            }
        }
    }

    fn write_request(
        mut self: Pin<&mut Self>,
        dispatch_request: DispatchRequest<Req, Resp>,
    ) -> io::Result<()> {
        let request_id = dispatch_request.request_id;
        let request = ClientMessage::Request(Request {
            id: request_id,
            message: dispatch_request.request,
            context: context::Context {
                deadline: dispatch_request.ctx.deadline,
                trace_context: dispatch_request.ctx.trace_context,
                _non_exhaustive: (),
            },
            _non_exhaustive: (),
        });
        self.as_mut().project().transport.start_send(request)?;
        self.as_mut().project().in_flight_requests.insert(
            request_id,
            InFlightData {
                ctx: dispatch_request.ctx,
                response_completion: dispatch_request.response_completion,
            },
        );
        Ok(())
    }

    fn write_cancel(
        mut self: Pin<&mut Self>,
        context: context::Context,
        request_id: u64,
    ) -> io::Result<()> {
        let trace_id = *context.trace_id();
        let cancel = ClientMessage::Cancel {
            trace_context: context.trace_context,
            request_id,
        };
        self.as_mut().project().transport.start_send(cancel)?;
        trace!("[{}] Cancel message sent.", trace_id);
        Ok(())
    }

    /// Sends a server response to the client task that initiated the associated request.
    fn complete(mut self: Pin<&mut Self>, response: Response<Resp>) -> bool {
        if let Some(in_flight_data) = self
            .as_mut()
            .project()
            .in_flight_requests
            .remove(&response.request_id)
        {
            self.as_mut().project().in_flight_requests.compact(0.1);

            trace!("[{}] Received response.", in_flight_data.ctx.trace_id());
            let _ = in_flight_data.response_completion.send(response);
            return true;
        }

        debug!(
            "No in-flight request found for request_id = {}.",
            response.request_id
        );

        // If the response completion was absent, then the request was already canceled.
        false
    }
}

impl<Req, Resp, C> Future for RequestDispatch<Req, Resp, C>
where
    C: Transport<ClientMessage<Req>, Response<Resp>>,
{
    type Output = io::Result<()>;

    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
        loop {
            match (self.as_mut().pump_read(cx)?, self.as_mut().pump_write(cx)?) {
                (read, Poll::Ready(None)) => {
                    if self.as_mut().project().in_flight_requests.is_empty() {
                        info!("Shutdown: write half closed, and no requests in flight.");
                        return Poll::Ready(Ok(()));
                    }
                    info!(
                        "Shutdown: write half closed, and {} requests in flight.",
                        self.as_mut().project().in_flight_requests.len()
                    );
                    match read {
                        Poll::Ready(Some(())) => continue,
                        _ => return Poll::Pending,
                    }
                }
                (Poll::Ready(Some(())), _) | (_, Poll::Ready(Some(()))) => {}
                _ => return Poll::Pending,
            }
        }
    }
}

/// A server-bound request sent from a [`Channel`] to request dispatch, which will then manage
/// the lifecycle of the request.
#[derive(Debug)]
struct DispatchRequest<Req, Resp> {
    ctx: context::Context,
    request_id: u64,
    request: Req,
    response_completion: oneshot::Sender<Response<Resp>>,
}

#[derive(Debug)]
struct InFlightData<Resp> {
    ctx: context::Context,
    response_completion: oneshot::Sender<Response<Resp>>,
}

/// Sends request cancellation signals.
#[derive(Debug, Clone)]
struct RequestCancellation(mpsc::UnboundedSender<u64>);

/// A stream of IDs of requests that have been canceled.
#[derive(Debug)]
struct CanceledRequests(mpsc::UnboundedReceiver<u64>);

/// Returns a channel to send request cancellation messages.
fn cancellations() -> (RequestCancellation, CanceledRequests) {
    // Unbounded because messages are sent in the drop fn. This is fine, because it's still
    // bounded by the number of in-flight requests. Additionally, each request has a clone
    // of the sender, so the bounded channel would have the same behavior,
    // since it guarantees a slot.
    let (tx, rx) = mpsc::unbounded();
    (RequestCancellation(tx), CanceledRequests(rx))
}

impl RequestCancellation {
    /// Cancels the request with ID `request_id`.
    fn cancel(&mut self, request_id: u64) {
        let _ = self.0.unbounded_send(request_id);
    }
}

impl Stream for CanceledRequests {
    type Item = u64;

    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<u64>> {
        self.0.poll_next_unpin(cx)
    }
}

#[pin_project]
#[derive(Debug)]
#[must_use = "futures do nothing unless polled"]
struct MapErrConnectionReset<Fut> {
    #[pin]
    future: Fut,
    finished: Option<()>,
}

impl<Fut> MapErrConnectionReset<Fut> {
    fn new(future: Fut) -> MapErrConnectionReset<Fut> {
        MapErrConnectionReset {
            future,
            finished: Some(()),
        }
    }
}

impl<Fut> Future for MapErrConnectionReset<Fut>
where
    Fut: TryFuture,
{
    type Output = io::Result<Fut::Ok>;

    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        match self.as_mut().project().future.try_poll(cx) {
            Poll::Pending => Poll::Pending,
            Poll::Ready(result) => {
                self.project().finished.take().expect(
                    "MapErrConnectionReset must not be polled after it returned `Poll::Ready`",
                );
                Poll::Ready(result.map_err(|_| io::Error::from(io::ErrorKind::ConnectionReset)))
            }
        }
    }
}

#[pin_project]
#[derive(Debug)]
#[must_use = "futures do nothing unless polled"]
struct MapOkDispatchResponse<Fut, Resp> {
    #[pin]
    future: Fut,
    response: Option<DispatchResponse<Resp>>,
}

impl<Fut, Resp> MapOkDispatchResponse<Fut, Resp> {
    fn new(future: Fut, response: DispatchResponse<Resp>) -> MapOkDispatchResponse<Fut, Resp> {
        MapOkDispatchResponse {
            future,
            response: Some(response),
        }
    }
}

impl<Fut, Resp> Future for MapOkDispatchResponse<Fut, Resp>
where
    Fut: TryFuture,
{
    type Output = Result<DispatchResponse<Resp>, Fut::Error>;

    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        match self.as_mut().project().future.try_poll(cx) {
            Poll::Pending => Poll::Pending,
            Poll::Ready(result) => {
                let response = self
                    .as_mut()
                    .project()
                    .response
                    .take()
                    .expect("MapOk must not be polled after it returned `Poll::Ready`");
                Poll::Ready(result.map(|_| response))
            }
        }
    }
}

#[pin_project]
#[derive(Debug)]
#[must_use = "futures do nothing unless polled"]
struct AndThenIdent<Fut1, Fut2> {
    #[pin]
    try_chain: TryChain<Fut1, Fut2>,
}

impl<Fut1, Fut2> AndThenIdent<Fut1, Fut2>
where
    Fut1: TryFuture<Ok = Fut2>,
    Fut2: TryFuture,
{
    /// Creates a new `Then`.
    fn new(future: Fut1) -> AndThenIdent<Fut1, Fut2> {
        AndThenIdent {
            try_chain: TryChain::new(future),
        }
    }
}

impl<Fut1, Fut2> Future for AndThenIdent<Fut1, Fut2>
where
    Fut1: TryFuture<Ok = Fut2>,
    Fut2: TryFuture<Error = Fut1::Error>,
{
    type Output = Result<Fut2::Ok, Fut2::Error>;

    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        self.project().try_chain.poll(cx, |result| match result {
            Ok(ok) => TryChainAction::Future(ok),
            Err(err) => TryChainAction::Output(Err(err)),
        })
    }
}

#[must_use = "futures do nothing unless polled"]
#[derive(Debug)]
enum TryChain<Fut1, Fut2> {
    First(Fut1),
    Second(Fut2),
    Empty,
}

enum TryChainAction<Fut2>
where
    Fut2: TryFuture,
{
    Future(Fut2),
    Output(Result<Fut2::Ok, Fut2::Error>),
}

impl<Fut1, Fut2> TryChain<Fut1, Fut2>
where
    Fut1: TryFuture<Ok = Fut2>,
    Fut2: TryFuture,
{
    fn new(fut1: Fut1) -> TryChain<Fut1, Fut2> {
        TryChain::First(fut1)
    }

    fn poll<F>(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        f: F,
    ) -> Poll<Result<Fut2::Ok, Fut2::Error>>
    where
        F: FnOnce(Result<Fut1::Ok, Fut1::Error>) -> TryChainAction<Fut2>,
    {
        let mut f = Some(f);

        // Safe to call `get_unchecked_mut` because we won't move the futures.
        let this = unsafe { Pin::get_unchecked_mut(self) };

        loop {
            let output = match this {
                TryChain::First(fut1) => {
                    // Poll the first future
                    match unsafe { Pin::new_unchecked(fut1) }.try_poll(cx) {
                        Poll::Pending => return Poll::Pending,
                        Poll::Ready(output) => output,
                    }
                }
                TryChain::Second(fut2) => {
                    // Poll the second future
                    return unsafe { Pin::new_unchecked(fut2) }.try_poll(cx);
                }
                TryChain::Empty => {
                    panic!("future must not be polled after it returned `Poll::Ready`");
                }
            };

            *this = TryChain::Empty; // Drop fut1
            let f = f.take().unwrap();
            match f(output) {
                TryChainAction::Future(fut2) => *this = TryChain::Second(fut2),
                TryChainAction::Output(output) => return Poll::Ready(output),
            }
        }
    }
}

#[cfg(test)]
mod tests {
    use super::{
        cancellations, CanceledRequests, Channel, DispatchResponse, RequestCancellation,
        RequestDispatch,
    };
    use crate::{
        client::Config,
        context,
        transport::{self, channel::UnboundedChannel},
        ClientMessage, Response,
    };
    use fnv::FnvHashMap;
    use futures::{
        channel::{mpsc, oneshot},
        prelude::*,
        task::Context,
        Poll,
    };
    use futures_test::task::noop_waker_ref;
    use std::time::Duration;
    use std::{pin::Pin, sync::atomic::AtomicU64, sync::Arc};
    use tokio::runtime::current_thread;
    use tokio_timer::Timeout;

    #[test]
    fn dispatch_response_cancels_on_timeout() {
        let (_response_completion, response) = oneshot::channel();
        let (cancellation, mut canceled_requests) = cancellations();
        let resp = DispatchResponse::<u64> {
            // Timeout in the past should cause resp to error out when polled.
            response: Timeout::new(response, Duration::from_secs(0)),
            complete: false,
            request_id: 3,
            cancellation,
            ctx: context::current(),
        };
        {
            pin_utils::pin_mut!(resp);
            let timer = tokio_timer::Timer::default();
            let handle = timer.handle();
            let _guard = tokio_timer::set_default(&handle);

            let _ = resp
                .as_mut()
                .poll(&mut Context::from_waker(&noop_waker_ref()));
            // End of block should cause resp.drop() to run, which should send a cancel message.
        }
        assert!(canceled_requests.0.try_next().unwrap() == Some(3));
    }

    #[test]
    fn stage_request() {
        let (mut dispatch, mut channel, _server_channel) = set_up();
        let dispatch = Pin::new(&mut dispatch);
        let cx = &mut Context::from_waker(&noop_waker_ref());

        let _resp = send_request(&mut channel, "hi");

        let req = dispatch.poll_next_request(cx).ready();
        assert!(req.is_some());

        let req = req.unwrap();
        assert_eq!(req.request_id, 0);
        assert_eq!(req.request, "hi".to_string());
    }

    fn block_on<F: Future>(f: F) -> F::Output {
        current_thread::Runtime::new().unwrap().block_on(f)
    }

    // Regression test for  https://github.com/google/tarpc/issues/220
    #[test]
    fn stage_request_channel_dropped_doesnt_panic() {
        let (mut dispatch, mut channel, mut server_channel) = set_up();
        let mut dispatch = Pin::new(&mut dispatch);
        let cx = &mut Context::from_waker(&noop_waker_ref());

        let _ = send_request(&mut channel, "hi");
        drop(channel);

        assert!(dispatch.as_mut().poll(cx).is_ready());
        send_response(
            &mut server_channel,
            Response {
                request_id: 0,
                message: Ok("hello".into()),
                _non_exhaustive: (),
            },
        );
        block_on(dispatch).unwrap();
    }

    #[test]
    fn stage_request_response_future_dropped_is_canceled_before_sending() {
        let (mut dispatch, mut channel, _server_channel) = set_up();
        let dispatch = Pin::new(&mut dispatch);
        let cx = &mut Context::from_waker(&noop_waker_ref());

        let _ = send_request(&mut channel, "hi");

        // Drop the channel so polling returns none if no requests are currently ready.
        drop(channel);
        // Test that a request future dropped before it's processed by dispatch will cause the request
        // to not be added to the in-flight request map.
        assert!(dispatch.poll_next_request(cx).ready().is_none());
    }

    #[test]
    fn stage_request_response_future_dropped_is_canceled_after_sending() {
        let (mut dispatch, mut channel, _server_channel) = set_up();
        let cx = &mut Context::from_waker(&noop_waker_ref());
        let mut dispatch = Pin::new(&mut dispatch);

        let req = send_request(&mut channel, "hi");

        assert!(dispatch.as_mut().pump_write(cx).ready().is_some());
        assert!(!dispatch.as_mut().project().in_flight_requests.is_empty());

        // Test that a request future dropped after it's processed by dispatch will cause the request
        // to be removed from the in-flight request map.
        drop(req);
        if let Poll::Ready(Some(_)) = dispatch.as_mut().poll_next_cancellation(cx).unwrap() {
            // ok
        } else {
            panic!("Expected request to be cancelled")
        };
        assert!(dispatch.project().in_flight_requests.is_empty());
    }

    #[test]
    fn stage_request_response_closed_skipped() {
        let (mut dispatch, mut channel, _server_channel) = set_up();
        let dispatch = Pin::new(&mut dispatch);
        let cx = &mut Context::from_waker(&noop_waker_ref());

        // Test that a request future that's closed its receiver but not yet canceled its request --
        // i.e. still in `drop fn` -- will cause the request to not be added to the in-flight request
        // map.
        let mut resp = send_request(&mut channel, "hi");
        resp.response.get_mut().close();

        assert!(dispatch.poll_next_request(cx).is_pending());
    }

    fn set_up() -> (
        RequestDispatch<String, String, UnboundedChannel<Response<String>, ClientMessage<String>>>,
        Channel<String, String>,
        UnboundedChannel<ClientMessage<String>, Response<String>>,
    ) {
        let _ = env_logger::try_init();

        let (to_dispatch, pending_requests) = mpsc::channel(1);
        let (cancel_tx, canceled_requests) = mpsc::unbounded();
        let (client_channel, server_channel) = transport::channel::unbounded();

        let dispatch = RequestDispatch::<String, String, _> {
            transport: client_channel.fuse(),
            pending_requests: pending_requests.fuse(),
            canceled_requests: CanceledRequests(canceled_requests).fuse(),
            in_flight_requests: FnvHashMap::default(),
            config: Config::default(),
        };

        let cancellation = RequestCancellation(cancel_tx);
        let channel = Channel {
            to_dispatch,
            cancellation,
            next_request_id: Arc::new(AtomicU64::new(0)),
        };

        (dispatch, channel, server_channel)
    }

    fn send_request(
        channel: &mut Channel<String, String>,
        request: &str,
    ) -> DispatchResponse<String> {
        block_on(channel.send(context::current(), request.to_string())).unwrap()
    }

    fn send_response(
        channel: &mut UnboundedChannel<ClientMessage<String>, Response<String>>,
        response: Response<String>,
    ) {
        block_on(channel.send(response)).unwrap();
    }

    trait PollTest {
        type T;
        fn unwrap(self) -> Poll<Self::T>;
        fn ready(self) -> Self::T;
    }

    impl<T, E> PollTest for Poll<Option<Result<T, E>>>
    where
        E: ::std::fmt::Display,
    {
        type T = Option<T>;

        fn unwrap(self) -> Poll<Option<T>> {
            match self {
                Poll::Ready(Some(Ok(t))) => Poll::Ready(Some(t)),
                Poll::Ready(None) => Poll::Ready(None),
                Poll::Ready(Some(Err(e))) => panic!(e.to_string()),
                Poll::Pending => Poll::Pending,
            }
        }

        fn ready(self) -> Option<T> {
            match self {
                Poll::Ready(Some(Ok(t))) => Some(t),
                Poll::Ready(None) => None,
                Poll::Ready(Some(Err(e))) => panic!(e.to_string()),
                Poll::Pending => panic!("Pending"),
            }
        }
    }
}