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    use std::error::Error as StdError;
    #[cfg(feature = "runtime")]
    use std::time::Duration;

    use futures_channel::{mpsc, oneshot};
    use futures_util::future::{self, Either, FutureExt as _, TryFutureExt as _};
    use futures_util::stream::StreamExt as _;
    use h2::client::{Builder, SendRequest};
    use tokio::io::{AsyncRead, AsyncWrite};

    use super::{decode_content_length, ping, PipeToSendStream, SendBuf};
    use crate::body::HttpBody;
    use crate::common::{task, exec::Exec, Future, Never, Pin, Poll};
    use crate::headers;
    use crate::proto::Dispatched;
    use crate::{Body, Request, Response};

    type ClientRx<B> = crate::client::dispatch::Receiver<Request<B>, Response<Body>>;

    ///// An mpsc channel is used to help notify the `Connection` task when *all*
    ///// other handles to it have been dropped, so that it can shutdown.
    type ConnDropRef = mpsc::Sender<Never>;

    ///// A oneshot channel watches the `Connection` task, and when it completes,
    ///// the "dispatch" task will be notified and can shutdown sooner.
    type ConnEof = oneshot::Receiver<Never>;

    // Our defaults are chosen for the "majority" case, which usually are not
    // resource constrained, and so the spec default of 64kb can be too limiting
    // for performance.
    const DEFAULT_CONN_WINDOW: u32 = 1024 * 1024 * 5; // 5mb
    const DEFAULT_STREAM_WINDOW: u32 = 1024 * 1024 * 2; // 2mb
    const DEFAULT_MAX_FRAME_SIZE: u32 = 1024 * 16; // 16kb

    #[derive(Clone, Debug)]
    pub(crate) struct Config {
        pub(crate) adaptive_window: bool,
        pub(crate) initial_conn_window_size: u32,
        pub(crate) initial_stream_window_size: u32,
        pub(crate) max_frame_size: u32,
        #[cfg(feature = "runtime")]
        pub(crate) keep_alive_interval: Option<Duration>,
        #[cfg(feature = "runtime")]
        pub(crate) keep_alive_timeout: Duration,
        #[cfg(feature = "runtime")]
        pub(crate) keep_alive_while_idle: bool,
    }

    impl Default for Config {
        fn default() -> Config {
            Config {
                adaptive_window: false,
                initial_conn_window_size: DEFAULT_CONN_WINDOW,
                initial_stream_window_size: DEFAULT_STREAM_WINDOW,
                max_frame_size: DEFAULT_MAX_FRAME_SIZE,
                #[cfg(feature = "runtime")]
                keep_alive_interval: None,
                #[cfg(feature = "runtime")]
                keep_alive_timeout: Duration::from_secs(20),
                #[cfg(feature = "runtime")]
                keep_alive_while_idle: false,
            }
        }
    }

    fn new_builder(config: &Config) -> Builder {
        let mut builder = Builder::default();
        builder
            .initial_window_size(config.initial_stream_window_size)
            .initial_connection_window_size(config.initial_conn_window_size)
            .max_frame_size(config.max_frame_size)
            .enable_push(false);
        builder
    }

    fn new_ping_config(config: &Config) -> ping::Config {
        ping::Config {
            bdp_initial_window: if config.adaptive_window {
                Some(config.initial_stream_window_size)
            } else {
                None
            },
            #[cfg(feature = "runtime")]
            keep_alive_interval: config.keep_alive_interval,
            #[cfg(feature = "runtime")]
            keep_alive_timeout: config.keep_alive_timeout,
            #[cfg(feature = "runtime")]
            keep_alive_while_idle: config.keep_alive_while_idle,
        }
    }

    pub(crate) async fn handshake<T, B>(
        io: T,
        req_rx: ClientRx<B>,
        config: &Config,
        exec: Exec,
    ) -> crate::Result<ClientTask<B>>
    where
        T: AsyncRead + AsyncWrite + Send + Unpin + 'static,
        B: HttpBody,
        B::Data: Send + 'static,
    {
        let (h2_tx, mut conn) = new_builder(config)
            .handshake::<_, SendBuf<B::Data>>(io)
            .await
            .map_err(crate::Error::new_h2)?;

        // An mpsc channel is used entirely to detect when the
        // 'Client' has been dropped. This is to get around a bug
        // in h2 where dropping all SendRequests won't notify a
        // parked Connection.
        let (conn_drop_ref, rx) = mpsc::channel(1);
        let (cancel_tx, conn_eof) = oneshot::channel();

        let conn_drop_rx = rx.into_future().map(|(item, _rx)| {
            if let Some(never) = item {
                match never {}
            }
        });

        let ping_config = new_ping_config(&config);

        let (conn, ping) = if ping_config.is_enabled() {
            let pp = conn.ping_pong().expect("conn.ping_pong");
            let (recorder, mut ponger) = ping::channel(pp, ping_config);

            let conn = future::poll_fn(move |cx| {
                match ponger.poll(cx) {
                    Poll::Ready(ping::Ponged::SizeUpdate(wnd)) => {
                        conn.set_target_window_size(wnd);
                        conn.set_initial_window_size(wnd)?;
                    }
                    #[cfg(feature = "runtime")]
                    Poll::Ready(ping::Ponged::KeepAliveTimedOut) => {
                        debug!("connection keep-alive timed out");
                        return Poll::Ready(Ok(()));
                    }
                    Poll::Pending => {}
                }

                Pin::new(&mut conn).poll(cx)
            });
            (Either::Left(conn), recorder)
        } else {
            (Either::Right(conn), ping::disabled())
        };
        let conn = conn.map_err(|e| debug!("connection error: {}", e));

        exec.execute(conn_task(conn, conn_drop_rx, cancel_tx));

        Ok(ClientTask {
            ping,
            conn_drop_ref,
            conn_eof,
            executor: exec,
            h2_tx,
            req_rx,
        })
    }

    async fn conn_task<C, D>(conn: C, drop_rx: D, cancel_tx: oneshot::Sender<Never>)
    where
        C: Future + Unpin,
        D: Future<Output = ()> + Unpin,
    {
        match future::select(conn, drop_rx).await {
            Either::Left(_) => {
                // ok or err, the `conn` has finished
            }
            Either::Right(((), conn)) => {
                // mpsc has been dropped, hopefully polling
                // the connection some more should start shutdown
                // and then close
                trace!("send_request dropped, starting conn shutdown");
                drop(cancel_tx);
                let _ = conn.await;
            }
        }
    }

    pub(crate) struct ClientTask<B>
    where
        B: HttpBody,
    {
        ping: ping::Recorder,
        conn_drop_ref: ConnDropRef,
        conn_eof: ConnEof,
        executor: Exec,
        h2_tx: SendRequest<SendBuf<B::Data>>,
        req_rx: ClientRx<B>,
    }

    impl<B> Future for ClientTask<B>
    where
        B: HttpBody + Send + 'static,
        B::Data: Send,
        B::Error: Into<Box<dyn StdError + Send + Sync>>,
    {
        type Output = crate::Result<Dispatched>;

        fn poll(mut self: Pin<&mut Self>, cx: &mut task::Context<'_>) -> Poll<Self::Output> {
            loop {
                match ready!(self.h2_tx.poll_ready(cx)) {
                    Ok(()) => (),
                    Err(err) => {
                        self.ping.ensure_not_timed_out()?;
                        return if err.reason() == Some(::h2::Reason::NO_ERROR) {
                            trace!("connection gracefully shutdown");
                            Poll::Ready(Ok(Dispatched::Shutdown))
                        } else {
                            Poll::Ready(Err(crate::Error::new_h2(err)))
                        };
                    }
                };

                match self.req_rx.poll_recv(cx) {
                    Poll::Ready(Some((req, cb))) => {
                        // check that future hasn't been canceled already
                        if cb.is_canceled() {
                            trace!("request callback is canceled");
                            continue;
                        }
                        let (head, body) = req.into_parts();
                        let mut req = ::http::Request::from_parts(head, ());
                        super::strip_connection_headers(req.headers_mut(), true);
                        if let Some(len) = body.size_hint().exact() {
                            if len != 0 || headers::method_has_defined_payload_semantics(req.method()) {
                                headers::set_content_length_if_missing(req.headers_mut(), len);
                            }
                        }
                        let eos = body.is_end_stream();
                        let (fut, body_tx) = match self.h2_tx.send_request(req, eos) {
                            Ok(ok) => ok,
                            Err(err) => {
                                debug!("client send request error: {}", err);
                                cb.send(Err((crate::Error::new_h2(err), None)));
                                continue;
                            }
                        };

                        let ping = self.ping.clone();
                        if !eos {
                            let mut pipe = Box::pin(PipeToSendStream::new(body, body_tx)).map(|res| {
                                if let Err(e) = res {
                                    debug!("client request body error: {}", e);
                                }
                            });

                            // eagerly see if the body pipe is ready and
                            // can thus skip allocating in the executor
                            match Pin::new(&mut pipe).poll(cx) {
                                Poll::Ready(_) => (),
                                Poll::Pending => {
                                    let conn_drop_ref = self.conn_drop_ref.clone();
                                    // keep the ping recorder's knowledge of an
                                    // "open stream" alive while this body is
                                    // still sending...
                                    let ping = ping.clone();
                                    let pipe = pipe.map(move |x| {
                                        drop(conn_drop_ref);
                                        drop(ping);
                                        x
                                    });
                                    self.executor.execute(pipe);
                                }
                            }
                        }

                        let fut = fut.map(move |result| match result {
                            Ok(res) => {
                                // record that we got the response headers
                                ping.record_non_data();

                                let content_length = decode_content_length(res.headers());
                                let res = res.map(|stream| {
                                    let ping = ping.for_stream(&stream);
                                    crate::Body::h2(stream, content_length, ping)
                                });
                                Ok(res)
                            }
                            Err(err) => {
                                ping.ensure_not_timed_out().map_err(|e| (e, None))?;

                                debug!("client response error: {}", err);
                                Err((crate::Error::new_h2(err), None))
                            }
                        });
                        self.executor.execute(cb.send_when(fut));
                        continue;
                    }

                    Poll::Ready(None) => {
                        trace!("client::dispatch::Sender dropped");
                        return Poll::Ready(Ok(Dispatched::Shutdown));
                    }

                    Poll::Pending => match ready!(Pin::new(&mut self.conn_eof).poll(cx)) {
                        Ok(never) => match never {},
                        Err(_conn_is_eof) => {
                            trace!("connection task is closed, closing dispatch task");
                            return Poll::Ready(Ok(Dispatched::Shutdown));
                        }
                    },
                }
            }
        }
    }