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    /// HTTP2 Ping usage
    ///
    /// hyper uses HTTP2 pings for two purposes:
    ///
    /// 1. Adaptive flow control using BDP
    /// 2. Connection keep-alive
    ///
    /// Both cases are optional.
    ///
    /// # BDP Algorithm
    ///
    /// 1. When receiving a DATA frame, if a BDP ping isn't outstanding:
    ///   1a. Record current time.
    ///   1b. Send a BDP ping.
    /// 2. Increment the number of received bytes.
    /// 3. When the BDP ping ack is received:
    ///   3a. Record duration from sent time.
    ///   3b. Merge RTT with a running average.
    ///   3c. Calculate bdp as bytes/rtt.
    ///   3d. If bdp is over 2/3 max, set new max to bdp and update windows.

    #[cfg(feature = "runtime")]
    use std::fmt;
    #[cfg(feature = "runtime")]
    use std::future::Future;
    #[cfg(feature = "runtime")]
    use std::pin::Pin;
    use std::sync::{Arc, Mutex};
    use std::task::{self, Poll};
    use std::time::Duration;
    #[cfg(not(feature = "runtime"))]
    use std::time::Instant;

    use h2::{Ping, PingPong};
    #[cfg(feature = "runtime")]
    use tokio::time::{Instant, Sleep};

    type WindowSize = u32;

    pub(super) fn disabled() -> Recorder {
        Recorder { shared: None }
    }

    pub(super) fn channel(ping_pong: PingPong, config: Config) -> (Recorder, Ponger) {
        debug_assert!(
            config.is_enabled(),
            "ping channel requires bdp or keep-alive config",
        );

        let bdp = config.bdp_initial_window.map(|wnd| Bdp {
            bdp: wnd,
            max_bandwidth: 0.0,
            rtt: 0.0,
        });

        let bytes = bdp.as_ref().map(|_| 0);

        #[cfg(feature = "runtime")]
        let keep_alive = config.keep_alive_interval.map(|interval| KeepAlive {
            interval,
            timeout: config.keep_alive_timeout,
            while_idle: config.keep_alive_while_idle,
            timer: Box::pin(tokio::time::sleep(interval)),
            state: KeepAliveState::Init,
        });

        #[cfg(feature = "runtime")]
        let last_read_at = keep_alive.as_ref().map(|_| Instant::now());

        let shared = Arc::new(Mutex::new(Shared {
            bytes,
            #[cfg(feature = "runtime")]
            last_read_at,
            #[cfg(feature = "runtime")]
            is_keep_alive_timed_out: false,
            ping_pong,
            ping_sent_at: None,
        }));

        (
            Recorder {
                shared: Some(shared.clone()),
            },
            Ponger {
                bdp,
                #[cfg(feature = "runtime")]
                keep_alive,
                shared,
            },
        )
    }

    #[derive(Clone)]
    pub(super) struct Config {
        pub(super) bdp_initial_window: Option<WindowSize>,
        /// If no frames are received in this amount of time, a PING frame is sent.
        #[cfg(feature = "runtime")]
        pub(super) keep_alive_interval: Option<Duration>,
        /// After sending a keepalive PING, the connection will be closed if
        /// a pong is not received in this amount of time.
        #[cfg(feature = "runtime")]
        pub(super) keep_alive_timeout: Duration,
        /// If true, sends pings even when there are no active streams.
        #[cfg(feature = "runtime")]
        pub(super) keep_alive_while_idle: bool,
    }

    #[derive(Clone)]
    pub(crate) struct Recorder {
        shared: Option<Arc<Mutex<Shared>>>,
    }

    pub(super) struct Ponger {
        bdp: Option<Bdp>,
        #[cfg(feature = "runtime")]
        keep_alive: Option<KeepAlive>,
        shared: Arc<Mutex<Shared>>,
    }

    struct Shared {
        ping_pong: PingPong,
        ping_sent_at: Option<Instant>,

        // bdp
        /// If `Some`, bdp is enabled, and this tracks how many bytes have been
        /// read during the current sample.
        bytes: Option<usize>,

        // keep-alive
        /// If `Some`, keep-alive is enabled, and the Instant is how long ago
        /// the connection read the last frame.
        #[cfg(feature = "runtime")]
        last_read_at: Option<Instant>,

        #[cfg(feature = "runtime")]
        is_keep_alive_timed_out: bool,
    }

    struct Bdp {
        /// Current BDP in bytes
        bdp: u32,
        /// Largest bandwidth we've seen so far.
        max_bandwidth: f64,
        /// Round trip time in seconds
        rtt: f64,
    }

    #[cfg(feature = "runtime")]
    struct KeepAlive {
        /// If no frames are received in this amount of time, a PING frame is sent.
        interval: Duration,
        /// After sending a keepalive PING, the connection will be closed if
        /// a pong is not received in this amount of time.
        timeout: Duration,
        /// If true, sends pings even when there are no active streams.
        while_idle: bool,

        state: KeepAliveState,
        timer: Pin<Box<Sleep>>,
    }

    #[cfg(feature = "runtime")]
    enum KeepAliveState {
        Init,
        Scheduled,
        PingSent,
    }

    pub(super) enum Ponged {
        SizeUpdate(WindowSize),
        #[cfg(feature = "runtime")]
        KeepAliveTimedOut,
    }

    #[cfg(feature = "runtime")]
    #[derive(Debug)]
    pub(super) struct KeepAliveTimedOut;

    // ===== impl Config =====

    impl Config {
        pub(super) fn is_enabled(&self) -> bool {
            #[cfg(feature = "runtime")]
            {
                self.bdp_initial_window.is_some() || self.keep_alive_interval.is_some()
            }

            #[cfg(not(feature = "runtime"))]
            {
                self.bdp_initial_window.is_some()
            }
        }
    }

    // ===== impl Recorder =====

    impl Recorder {
        pub(crate) fn record_data(&self, len: usize) {
            let shared = if let Some(ref shared) = self.shared {
                shared
            } else {
                return;
            };

            let mut locked = shared.lock().unwrap();

            #[cfg(feature = "runtime")]
            locked.update_last_read_at();

            if let Some(ref mut bytes) = locked.bytes {
                *bytes += len;
            } else {
                // no need to send bdp ping if bdp is disabled
                return;
            }

            if !locked.is_ping_sent() {
                locked.send_ping();
            }
        }

        pub(crate) fn record_non_data(&self) {
            #[cfg(feature = "runtime")]
            {
                let shared = if let Some(ref shared) = self.shared {
                    shared
                } else {
                    return;
                };

                let mut locked = shared.lock().unwrap();

                locked.update_last_read_at();
            }
        }

        /// If the incoming stream is already closed, convert self into
        /// a disabled reporter.
        #[cfg(feature = "client")]
        pub(super) fn for_stream(self, stream: &h2::RecvStream) -> Self {
            if stream.is_end_stream() {
                disabled()
            } else {
                self
            }
        }

        pub(super) fn ensure_not_timed_out(&self) -> crate::Result<()> {
            #[cfg(feature = "runtime")]
            {
                if let Some(ref shared) = self.shared {
                    let locked = shared.lock().unwrap();
                    if locked.is_keep_alive_timed_out {
                        return Err(KeepAliveTimedOut.crate_error());
                    }
                }
            }

            // else
            Ok(())
        }
    }

    // ===== impl Ponger =====

    impl Ponger {
        pub(super) fn poll(&mut self, cx: &mut task::Context<'_>) -> Poll<Ponged> {
            let mut locked = self.shared.lock().unwrap();
            #[cfg(feature = "runtime")]
            let is_idle = self.is_idle();

            #[cfg(feature = "runtime")]
            {
                if let Some(ref mut ka) = self.keep_alive {
                    ka.schedule(is_idle, &locked);
                    ka.maybe_ping(cx, &mut locked);
                }
            }

            if !locked.is_ping_sent() {
                // XXX: this doesn't register a waker...?
                return Poll::Pending;
            }

            let (bytes, rtt) = match locked.ping_pong.poll_pong(cx) {
                Poll::Ready(Ok(_pong)) => {
                    let rtt = locked
                        .ping_sent_at
                        .expect("pong received implies ping_sent_at")
                        .elapsed();
                    locked.ping_sent_at = None;
                    trace!("recv pong");

                    #[cfg(feature = "runtime")]
                    {
                        if let Some(ref mut ka) = self.keep_alive {
                            locked.update_last_read_at();
                            ka.schedule(is_idle, &locked);
                        }
                    }

                    if self.bdp.is_some() {
                        let bytes = locked.bytes.expect("bdp enabled implies bytes");
                        locked.bytes = Some(0); // reset
                        trace!("received BDP ack; bytes = {}, rtt = {:?}", bytes, rtt);
                        (bytes, rtt)
                    } else {
                        // no bdp, done!
                        return Poll::Pending;
                    }
                }
                Poll::Ready(Err(e)) => {
                    debug!("pong error: {}", e);
                    return Poll::Pending;
                }
                Poll::Pending => {
                    #[cfg(feature = "runtime")]
                    {
                        if let Some(ref mut ka) = self.keep_alive {
                            if let Err(KeepAliveTimedOut) = ka.maybe_timeout(cx) {
                                self.keep_alive = None;
                                locked.is_keep_alive_timed_out = true;
                                return Poll::Ready(Ponged::KeepAliveTimedOut);
                            }
                        }
                    }

                    return Poll::Pending;
                }
            };

            drop(locked);

            if let Some(bdp) = self.bdp.as_mut().and_then(|bdp| bdp.calculate(bytes, rtt)) {
                Poll::Ready(Ponged::SizeUpdate(bdp))
            } else {
                // XXX: this doesn't register a waker...?
                Poll::Pending
            }
        }

        #[cfg(feature = "runtime")]
        fn is_idle(&self) -> bool {
            Arc::strong_count(&self.shared) <= 2
        }
    }

    // ===== impl Shared =====

    impl Shared {
        fn send_ping(&mut self) {
            match self.ping_pong.send_ping(Ping::opaque()) {
                Ok(()) => {
                    self.ping_sent_at = Some(Instant::now());
                    trace!("sent ping");
                }
                Err(err) => {
                    debug!("error sending ping: {}", err);
                }
            }
        }

        fn is_ping_sent(&self) -> bool {
            self.ping_sent_at.is_some()
        }

        #[cfg(feature = "runtime")]
        fn update_last_read_at(&mut self) {
            if self.last_read_at.is_some() {
                self.last_read_at = Some(Instant::now());
            }
        }

        #[cfg(feature = "runtime")]
        fn last_read_at(&self) -> Instant {
            self.last_read_at.expect("keep_alive expects last_read_at")
        }
    }

    // ===== impl Bdp =====

    /// Any higher than this likely will be hitting the TCP flow control.
    const BDP_LIMIT: usize = 1024 * 1024 * 16;

    impl Bdp {
        fn calculate(&mut self, bytes: usize, rtt: Duration) -> Option<WindowSize> {
            // No need to do any math if we're at the limit.
            if self.bdp as usize == BDP_LIMIT {
                return None;
            }

            // average the rtt
            let rtt = seconds(rtt);
            if self.rtt == 0.0 {
                // First sample means rtt is first rtt.
                self.rtt = rtt;
            } else {
                // Weigh this rtt as 1/8 for a moving average.
                self.rtt += (rtt - self.rtt) * 0.125;
            }

            // calculate the current bandwidth
            let bw = (bytes as f64) / (self.rtt * 1.5);
            trace!("current bandwidth = {:.1}B/s", bw);

            if bw < self.max_bandwidth {
                // not a faster bandwidth, so don't update
                return None;
            } else {
                self.max_bandwidth = bw;
            }

            // if the current `bytes` sample is at least 2/3 the previous
            // bdp, increase to double the current sample.
            if bytes >= self.bdp as usize * 2 / 3 {
                self.bdp = (bytes * 2).min(BDP_LIMIT) as WindowSize;
                trace!("BDP increased to {}", self.bdp);
                Some(self.bdp)
            } else {
                None
            }
        }
    }

    fn seconds(dur: Duration) -> f64 {
        const NANOS_PER_SEC: f64 = 1_000_000_000.0;
        let secs = dur.as_secs() as f64;
        secs + (dur.subsec_nanos() as f64) / NANOS_PER_SEC
    }

    // ===== impl KeepAlive =====

    #[cfg(feature = "runtime")]
    impl KeepAlive {
        fn schedule(&mut self, is_idle: bool, shared: &Shared) {
            match self.state {
                KeepAliveState::Init => {
                    if !self.while_idle && is_idle {
                        return;
                    }

                    self.state = KeepAliveState::Scheduled;
                    let interval = shared.last_read_at() + self.interval;
                    self.timer.as_mut().reset(interval);
                }
                KeepAliveState::PingSent => {
                    if shared.is_ping_sent() {
                        return;
                    }

                    self.state = KeepAliveState::Scheduled;
                    let interval = shared.last_read_at() + self.interval;
                    self.timer.as_mut().reset(interval);
                }
                KeepAliveState::Scheduled => (),
            }
        }

        fn maybe_ping(&mut self, cx: &mut task::Context<'_>, shared: &mut Shared) {
            match self.state {
                KeepAliveState::Scheduled => {
                    if Pin::new(&mut self.timer).poll(cx).is_pending() {
                        return;
                    }
                    // check if we've received a frame while we were scheduled
                    if shared.last_read_at() + self.interval > self.timer.deadline() {
                        self.state = KeepAliveState::Init;
                        cx.waker().wake_by_ref(); // schedule us again
                        return;
                    }
                    trace!("keep-alive interval ({:?}) reached", self.interval);
                    shared.send_ping();
                    self.state = KeepAliveState::PingSent;
                    let timeout = Instant::now() + self.timeout;
                    self.timer.as_mut().reset(timeout);
                }
                KeepAliveState::Init | KeepAliveState::PingSent => (),
            }
        }

        fn maybe_timeout(&mut self, cx: &mut task::Context<'_>) -> Result<(), KeepAliveTimedOut> {
            match self.state {
                KeepAliveState::PingSent => {
                    if Pin::new(&mut self.timer).poll(cx).is_pending() {
                        return Ok(());
                    }
                    trace!("keep-alive timeout ({:?}) reached", self.timeout);
                    Err(KeepAliveTimedOut)
                }
                KeepAliveState::Init | KeepAliveState::Scheduled => Ok(()),
            }
        }
    }

    // ===== impl KeepAliveTimedOut =====

    #[cfg(feature = "runtime")]
    impl KeepAliveTimedOut {
        pub(super) fn crate_error(self) -> crate::Error {
            crate::Error::new(crate::error::Kind::Http2).with(self)
        }
    }

    #[cfg(feature = "runtime")]
    impl fmt::Display for KeepAliveTimedOut {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.write_str("keep-alive timed out")
        }
    }

    #[cfg(feature = "runtime")]
    impl std::error::Error for KeepAliveTimedOut {
        fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
            Some(&crate::error::TimedOut)
        }
    }