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    1103

    use super::*;
    use crate::codec::{RecvError, UserError};
    use crate::frame::{PushPromiseHeaderError, Reason, DEFAULT_INITIAL_WINDOW_SIZE};
    use crate::{frame, proto};
    use std::task::Context;

    use http::{HeaderMap, Request, Response};

    use std::io;
    use std::task::{Poll, Waker};
    use std::time::{Duration, Instant};

    #[derive(Debug)]
    pub(super) struct Recv {
        /// Initial window size of remote initiated streams
        init_window_sz: WindowSize,

        /// Connection level flow control governing received data
        flow: FlowControl,

        /// Amount of connection window capacity currently used by outstanding streams.
        in_flight_data: WindowSize,

        /// The lowest stream ID that is still idle
        next_stream_id: Result<StreamId, StreamIdOverflow>,

        /// The stream ID of the last processed stream
        last_processed_id: StreamId,

        /// Any streams with a higher ID are ignored.
        ///
        /// This starts as MAX, but is lowered when a GOAWAY is received.
        ///
        /// > After sending a GOAWAY frame, the sender can discard frames for
        /// > streams initiated by the receiver with identifiers higher than
        /// > the identified last stream.
        max_stream_id: StreamId,

        /// Streams that have pending window updates
        pending_window_updates: store::Queue<stream::NextWindowUpdate>,

        /// New streams to be accepted
        pending_accept: store::Queue<stream::NextAccept>,

        /// Locally reset streams that should be reaped when they expire
        pending_reset_expired: store::Queue<stream::NextResetExpire>,

        /// How long locally reset streams should ignore received frames
        reset_duration: Duration,

        /// Holds frames that are waiting to be read
        buffer: Buffer<Event>,

        /// Refused StreamId, this represents a frame that must be sent out.
        refused: Option<StreamId>,

        /// If push promises are allowed to be received.
        is_push_enabled: bool,
    }

    #[derive(Debug)]
    pub(super) enum Event {
        Headers(peer::PollMessage),
        Data(Bytes),
        Trailers(HeaderMap),
    }

    #[derive(Debug)]
    pub(super) enum RecvHeaderBlockError<T> {
        Oversize(T),
        State(RecvError),
    }

    #[derive(Debug)]
    pub(crate) enum Open {
        PushPromise,
        Headers,
    }

    #[derive(Debug, Clone, Copy)]
    struct Indices {
        head: store::Key,
        tail: store::Key,
    }

    impl Recv {
        pub fn new(peer: peer::Dyn, config: &Config) -> Self {
            let next_stream_id = if peer.is_server() { 1 } else { 2 };

            let mut flow = FlowControl::new();

            // connections always have the default window size, regardless of
            // settings
            flow.inc_window(DEFAULT_INITIAL_WINDOW_SIZE)
                .expect("invalid initial remote window size");
            flow.assign_capacity(DEFAULT_INITIAL_WINDOW_SIZE);

            Recv {
                init_window_sz: config.local_init_window_sz,
                flow,
                in_flight_data: 0 as WindowSize,
                next_stream_id: Ok(next_stream_id.into()),
                pending_window_updates: store::Queue::new(),
                last_processed_id: StreamId::ZERO,
                max_stream_id: StreamId::MAX,
                pending_accept: store::Queue::new(),
                pending_reset_expired: store::Queue::new(),
                reset_duration: config.local_reset_duration,
                buffer: Buffer::new(),
                refused: None,
                is_push_enabled: config.local_push_enabled,
            }
        }

        /// Returns the initial receive window size
        pub fn init_window_sz(&self) -> WindowSize {
            self.init_window_sz
        }

        /// Returns the ID of the last processed stream
        pub fn last_processed_id(&self) -> StreamId {
            self.last_processed_id
        }

        /// Update state reflecting a new, remotely opened stream
        ///
        /// Returns the stream state if successful. `None` if refused
        pub fn open(
            &mut self,
            id: StreamId,
            mode: Open,
            counts: &mut Counts,
        ) -> Result<Option<StreamId>, RecvError> {
            assert!(self.refused.is_none());

            counts.peer().ensure_can_open(id, mode)?;

            let next_id = self.next_stream_id()?;
            if id < next_id {
                proto_err!(conn: "id ({:?}) < next_id ({:?})", id, next_id);
                return Err(RecvError::Connection(Reason::PROTOCOL_ERROR));
            }

            self.next_stream_id = id.next_id();

            if !counts.can_inc_num_recv_streams() {
                self.refused = Some(id);
                return Ok(None);
            }

            Ok(Some(id))
        }

        /// Transition the stream state based on receiving headers
        ///
        /// The caller ensures that the frame represents headers and not trailers.
        pub fn recv_headers(
            &mut self,
            frame: frame::Headers,
            stream: &mut store::Ptr,
            counts: &mut Counts,
        ) -> Result<(), RecvHeaderBlockError<Option<frame::Headers>>> {
            tracing::trace!("opening stream; init_window={}", self.init_window_sz);
            let is_initial = stream.state.recv_open(&frame)?;

            if is_initial {
                // TODO: be smarter about this logic
                if frame.stream_id() > self.last_processed_id {
                    self.last_processed_id = frame.stream_id();
                }

                // Increment the number of concurrent streams
                counts.inc_num_recv_streams(stream);
            }

            if !stream.content_length.is_head() {
                use super::stream::ContentLength;
                use http::header;

                if let Some(content_length) = frame.fields().get(header::CONTENT_LENGTH) {
                    let content_length = match frame::parse_u64(content_length.as_bytes()) {
                        Ok(v) => v,
                        Err(()) => {
                            proto_err!(stream: "could not parse content-length; stream={:?}", stream.id);
                            return Err(RecvError::Stream {
                                id: stream.id,
                                reason: Reason::PROTOCOL_ERROR,
                            }
                            .into());
                        }
                    };

                    stream.content_length = ContentLength::Remaining(content_length);
                }
            }

            if frame.is_over_size() {
                // A frame is over size if the decoded header block was bigger than
                // SETTINGS_MAX_HEADER_LIST_SIZE.
                //
                // > A server that receives a larger header block than it is willing
                // > to handle can send an HTTP 431 (Request Header Fields Too
                // > Large) status code [RFC6585]. A client can discard responses
                // > that it cannot process.
                //
                // So, if peer is a server, we'll send a 431. In either case,
                // an error is recorded, which will send a REFUSED_STREAM,
                // since we don't want any of the data frames either.
                tracing::debug!(
                    "stream error REQUEST_HEADER_FIELDS_TOO_LARGE -- \
                     recv_headers: frame is over size; stream={:?}",
                    stream.id
                );
                return if counts.peer().is_server() && is_initial {
                    let mut res = frame::Headers::new(
                        stream.id,
                        frame::Pseudo::response(::http::StatusCode::REQUEST_HEADER_FIELDS_TOO_LARGE),
                        HeaderMap::new(),
                    );
                    res.set_end_stream();
                    Err(RecvHeaderBlockError::Oversize(Some(res)))
                } else {
                    Err(RecvHeaderBlockError::Oversize(None))
                };
            }

            let stream_id = frame.stream_id();
            let (pseudo, fields) = frame.into_parts();
            if !pseudo.is_informational() {
                let message = counts
                    .peer()
                    .convert_poll_message(pseudo, fields, stream_id)?;

                // Push the frame onto the stream's recv buffer
                stream
                    .pending_recv
                    .push_back(&mut self.buffer, Event::Headers(message));
                stream.notify_recv();
            }

            // Only servers can receive a headers frame that initiates the stream.
            // This is verified in `Streams` before calling this function.
            if counts.peer().is_server() {
                self.pending_accept.push(stream);
            }

            Ok(())
        }

        /// Called by the server to get the request
        ///
        /// TODO: Should this fn return `Result`?
        pub fn take_request(&mut self, stream: &mut store::Ptr) -> Request<()> {
            use super::peer::PollMessage::*;

            match stream.pending_recv.pop_front(&mut self.buffer) {
                Some(Event::Headers(Server(request))) => request,
                _ => panic!(),
            }
        }

        /// Called by the client to get pushed response
        pub fn poll_pushed(
            &mut self,
            cx: &Context,
            stream: &mut store::Ptr,
        ) -> Poll<Option<Result<(Request<()>, store::Key), proto::Error>>> {
            use super::peer::PollMessage::*;

            let mut ppp = stream.pending_push_promises.take();
            let pushed = ppp.pop(stream.store_mut()).map(|mut pushed| {
                match pushed.pending_recv.pop_front(&mut self.buffer) {
                    Some(Event::Headers(Server(headers))) => (headers, pushed.key()),
                    // When frames are pushed into the queue, it is verified that
                    // the first frame is a HEADERS frame.
                    _ => panic!("Headers not set on pushed stream"),
                }
            });
            stream.pending_push_promises = ppp;
            if let Some(p) = pushed {
                Poll::Ready(Some(Ok(p)))
            } else {
                let is_open = stream.state.ensure_recv_open()?;

                if is_open {
                    stream.recv_task = Some(cx.waker().clone());
                    Poll::Pending
                } else {
                    Poll::Ready(None)
                }
            }
        }

        /// Called by the client to get the response
        pub fn poll_response(
            &mut self,
            cx: &Context,
            stream: &mut store::Ptr,
        ) -> Poll<Result<Response<()>, proto::Error>> {
            use super::peer::PollMessage::*;

            // If the buffer is not empty, then the first frame must be a HEADERS
            // frame or the user violated the contract.
            match stream.pending_recv.pop_front(&mut self.buffer) {
                Some(Event::Headers(Client(response))) => Poll::Ready(Ok(response)),
                Some(_) => panic!("poll_response called after response returned"),
                None => {
                    stream.state.ensure_recv_open()?;

                    stream.recv_task = Some(cx.waker().clone());
                    Poll::Pending
                }
            }
        }

        /// Transition the stream based on receiving trailers
        pub fn recv_trailers(
            &mut self,
            frame: frame::Headers,
            stream: &mut store::Ptr,
        ) -> Result<(), RecvError> {
            // Transition the state
            stream.state.recv_close()?;

            if stream.ensure_content_length_zero().is_err() {
                proto_err!(stream: "recv_trailers: content-length is not zero; stream={:?};",  stream.id);
                return Err(RecvError::Stream {
                    id: stream.id,
                    reason: Reason::PROTOCOL_ERROR,
                });
            }

            let trailers = frame.into_fields();

            // Push the frame onto the stream's recv buffer
            stream
                .pending_recv
                .push_back(&mut self.buffer, Event::Trailers(trailers));
            stream.notify_recv();

            Ok(())
        }

        /// Releases capacity of the connection
        pub fn release_connection_capacity(&mut self, capacity: WindowSize, task: &mut Option<Waker>) {
            tracing::trace!(
                "release_connection_capacity; size={}, connection in_flight_data={}",
                capacity,
                self.in_flight_data,
            );

            // Decrement in-flight data
            self.in_flight_data -= capacity;

            // Assign capacity to connection
            self.flow.assign_capacity(capacity);

            if self.flow.unclaimed_capacity().is_some() {
                if let Some(task) = task.take() {
                    task.wake();
                }
            }
        }

        /// Releases capacity back to the connection & stream
        pub fn release_capacity(
            &mut self,
            capacity: WindowSize,
            stream: &mut store::Ptr,
            task: &mut Option<Waker>,
        ) -> Result<(), UserError> {
            tracing::trace!("release_capacity; size={}", capacity);

            if capacity > stream.in_flight_recv_data {
                return Err(UserError::ReleaseCapacityTooBig);
            }

            self.release_connection_capacity(capacity, task);

            // Decrement in-flight data
            stream.in_flight_recv_data -= capacity;

            // Assign capacity to stream
            stream.recv_flow.assign_capacity(capacity);

            if stream.recv_flow.unclaimed_capacity().is_some() {
                // Queue the stream for sending the WINDOW_UPDATE frame.
                self.pending_window_updates.push(stream);

                if let Some(task) = task.take() {
                    task.wake();
                }
            }

            Ok(())
        }

        /// Release any unclaimed capacity for a closed stream.
        pub fn release_closed_capacity(&mut self, stream: &mut store::Ptr, task: &mut Option<Waker>) {
            debug_assert_eq!(stream.ref_count, 0);

            if stream.in_flight_recv_data == 0 {
                return;
            }

            tracing::trace!(
                "auto-release closed stream ({:?}) capacity: {:?}",
                stream.id,
                stream.in_flight_recv_data,
            );

            self.release_connection_capacity(stream.in_flight_recv_data, task);
            stream.in_flight_recv_data = 0;

            self.clear_recv_buffer(stream);
        }

        /// Set the "target" connection window size.
        ///
        /// By default, all new connections start with 64kb of window size. As
        /// streams used and release capacity, we will send WINDOW_UPDATEs for the
        /// connection to bring it back up to the initial "target".
        ///
        /// Setting a target means that we will try to tell the peer about
        /// WINDOW_UPDATEs so the peer knows it has about `target` window to use
        /// for the whole connection.
        ///
        /// The `task` is an optional parked task for the `Connection` that might
        /// be blocked on needing more window capacity.
        pub fn set_target_connection_window(&mut self, target: WindowSize, task: &mut Option<Waker>) {
            tracing::trace!(
                "set_target_connection_window; target={}; available={}, reserved={}",
                target,
                self.flow.available(),
                self.in_flight_data,
            );

            // The current target connection window is our `available` plus any
            // in-flight data reserved by streams.
            //
            // Update the flow controller with the difference between the new
            // target and the current target.
            let current = (self.flow.available() + self.in_flight_data).checked_size();
            if target > current {
                self.flow.assign_capacity(target - current);
            } else {
                self.flow.claim_capacity(current - target);
            }

            // If changing the target capacity means we gained a bunch of capacity,
            // enough that we went over the update threshold, then schedule sending
            // a connection WINDOW_UPDATE.
            if self.flow.unclaimed_capacity().is_some() {
                if let Some(task) = task.take() {
                    task.wake();
                }
            }
        }

        pub(crate) fn apply_local_settings(
            &mut self,
            settings: &frame::Settings,
            store: &mut Store,
        ) -> Result<(), RecvError> {
            let target = if let Some(val) = settings.initial_window_size() {
                val
            } else {
                return Ok(());
            };

            let old_sz = self.init_window_sz;
            self.init_window_sz = target;

            tracing::trace!("update_initial_window_size; new={}; old={}", target, old_sz,);

            // Per RFC 7540 §6.9.2:
            //
            // In addition to changing the flow-control window for streams that are
            // not yet active, a SETTINGS frame can alter the initial flow-control
            // window size for streams with active flow-control windows (that is,
            // streams in the "open" or "half-closed (remote)" state). When the
            // value of SETTINGS_INITIAL_WINDOW_SIZE changes, a receiver MUST adjust
            // the size of all stream flow-control windows that it maintains by the
            // difference between the new value and the old value.
            //
            // A change to `SETTINGS_INITIAL_WINDOW_SIZE` can cause the available
            // space in a flow-control window to become negative. A sender MUST
            // track the negative flow-control window and MUST NOT send new
            // flow-controlled frames until it receives WINDOW_UPDATE frames that
            // cause the flow-control window to become positive.

            if target < old_sz {
                // We must decrease the (local) window on every open stream.
                let dec = old_sz - target;
                tracing::trace!("decrementing all windows; dec={}", dec);

                store.for_each(|mut stream| {
                    stream.recv_flow.dec_recv_window(dec);
                    Ok(())
                })
            } else if target > old_sz {
                // We must increase the (local) window on every open stream.
                let inc = target - old_sz;
                tracing::trace!("incrementing all windows; inc={}", inc);
                store.for_each(|mut stream| {
                    // XXX: Shouldn't the peer have already noticed our
                    // overflow and sent us a GOAWAY?
                    stream
                        .recv_flow
                        .inc_window(inc)
                        .map_err(RecvError::Connection)?;
                    stream.recv_flow.assign_capacity(inc);
                    Ok(())
                })
            } else {
                // size is the same... so do nothing
                Ok(())
            }
        }

        pub fn is_end_stream(&self, stream: &store::Ptr) -> bool {
            if !stream.state.is_recv_closed() {
                return false;
            }

            stream.pending_recv.is_empty()
        }

        pub fn recv_data(
            &mut self,
            frame: frame::Data,
            stream: &mut store::Ptr,
        ) -> Result<(), RecvError> {
            let sz = frame.payload().len();

            // This should have been enforced at the codec::FramedRead layer, so
            // this is just a sanity check.
            assert!(sz <= MAX_WINDOW_SIZE as usize);

            let sz = sz as WindowSize;

            let is_ignoring_frame = stream.state.is_local_reset();

            if !is_ignoring_frame && !stream.state.is_recv_streaming() {
                // TODO: There are cases where this can be a stream error of
                // STREAM_CLOSED instead...

                // Receiving a DATA frame when not expecting one is a protocol
                // error.
                proto_err!(conn: "unexpected DATA frame; stream={:?}", stream.id);
                return Err(RecvError::Connection(Reason::PROTOCOL_ERROR));
            }

            tracing::trace!(
                "recv_data; size={}; connection={}; stream={}",
                sz,
                self.flow.window_size(),
                stream.recv_flow.window_size()
            );

            if is_ignoring_frame {
                tracing::trace!(
                    "recv_data; frame ignored on locally reset {:?} for some time",
                    stream.id,
                );
                return self.ignore_data(sz);
            }

            // Ensure that there is enough capacity on the connection before acting
            // on the stream.
            self.consume_connection_window(sz)?;

            if stream.recv_flow.window_size() < sz {
                // http://httpwg.org/specs/rfc7540.html#WINDOW_UPDATE
                // > A receiver MAY respond with a stream error (Section 5.4.2) or
                // > connection error (Section 5.4.1) of type FLOW_CONTROL_ERROR if
                // > it is unable to accept a frame.
                //
                // So, for violating the **stream** window, we can send either a
                // stream or connection error. We've opted to send a stream
                // error.
                return Err(RecvError::Stream {
                    id: stream.id,
                    reason: Reason::FLOW_CONTROL_ERROR,
                });
            }

            if stream.dec_content_length(frame.payload().len()).is_err() {
                proto_err!(stream:
                    "recv_data: content-length overflow; stream={:?}; len={:?}",
                    stream.id,
                    frame.payload().len(),
                );
                return Err(RecvError::Stream {
                    id: stream.id,
                    reason: Reason::PROTOCOL_ERROR,
                });
            }

            if frame.is_end_stream() {
                if stream.ensure_content_length_zero().is_err() {
                    proto_err!(stream:
                        "recv_data: content-length underflow; stream={:?}; len={:?}",
                        stream.id,
                        frame.payload().len(),
                    );
                    return Err(RecvError::Stream {
                        id: stream.id,
                        reason: Reason::PROTOCOL_ERROR,
                    });
                }

                if stream.state.recv_close().is_err() {
                    proto_err!(conn: "recv_data: failed to transition to closed state; stream={:?}", stream.id);
                    return Err(RecvError::Connection(Reason::PROTOCOL_ERROR));
                }
            }

            // Update stream level flow control
            stream.recv_flow.send_data(sz);

            // Track the data as in-flight
            stream.in_flight_recv_data += sz;

            let event = Event::Data(frame.into_payload());

            // Push the frame onto the recv buffer
            stream.pending_recv.push_back(&mut self.buffer, event);
            stream.notify_recv();

            Ok(())
        }

        pub fn ignore_data(&mut self, sz: WindowSize) -> Result<(), RecvError> {
            // Ensure that there is enough capacity on the connection...
            self.consume_connection_window(sz)?;

            // Since we are ignoring this frame,
            // we aren't returning the frame to the user. That means they
            // have no way to release the capacity back to the connection. So
            // we have to release it automatically.
            //
            // This call doesn't send a WINDOW_UPDATE immediately, just marks
            // the capacity as available to be reclaimed. When the available
            // capacity meets a threshold, a WINDOW_UPDATE is then sent.
            self.release_connection_capacity(sz, &mut None);
            Ok(())
        }

        pub fn consume_connection_window(&mut self, sz: WindowSize) -> Result<(), RecvError> {
            if self.flow.window_size() < sz {
                tracing::debug!(
                    "connection error FLOW_CONTROL_ERROR -- window_size ({:?}) < sz ({:?});",
                    self.flow.window_size(),
                    sz,
                );
                return Err(RecvError::Connection(Reason::FLOW_CONTROL_ERROR));
            }

            // Update connection level flow control
            self.flow.send_data(sz);

            // Track the data as in-flight
            self.in_flight_data += sz;
            Ok(())
        }

        pub fn recv_push_promise(
            &mut self,
            frame: frame::PushPromise,
            stream: &mut store::Ptr,
        ) -> Result<(), RecvError> {
            stream.state.reserve_remote()?;
            if frame.is_over_size() {
                // A frame is over size if the decoded header block was bigger than
                // SETTINGS_MAX_HEADER_LIST_SIZE.
                //
                // > A server that receives a larger header block than it is willing
                // > to handle can send an HTTP 431 (Request Header Fields Too
                // > Large) status code [RFC6585]. A client can discard responses
                // > that it cannot process.
                //
                // So, if peer is a server, we'll send a 431. In either case,
                // an error is recorded, which will send a REFUSED_STREAM,
                // since we don't want any of the data frames either.
                tracing::debug!(
                    "stream error REFUSED_STREAM -- recv_push_promise: \
                     headers frame is over size; promised_id={:?};",
                    frame.promised_id(),
                );
                return Err(RecvError::Stream {
                    id: frame.promised_id(),
                    reason: Reason::REFUSED_STREAM,
                });
            }

            let promised_id = frame.promised_id();
            let (pseudo, fields) = frame.into_parts();
            let req = crate::server::Peer::convert_poll_message(pseudo, fields, promised_id)?;

            if let Err(e) = frame::PushPromise::validate_request(&req) {
                use PushPromiseHeaderError::*;
                match e {
                    NotSafeAndCacheable => proto_err!(
                        stream:
                        "recv_push_promise: method {} is not safe and cacheable; promised_id={:?}",
                        req.method(),
                        promised_id,
                    ),
                    InvalidContentLength(e) => proto_err!(
                        stream:
                        "recv_push_promise; promised request has invalid content-length {:?}; promised_id={:?}",
                        e,
                        promised_id,
                    ),
                }
                return Err(RecvError::Stream {
                    id: promised_id,
                    reason: Reason::PROTOCOL_ERROR,
                });
            }

            use super::peer::PollMessage::*;
            stream
                .pending_recv
                .push_back(&mut self.buffer, Event::Headers(Server(req)));
            stream.notify_recv();
            Ok(())
        }

        /// Ensures that `id` is not in the `Idle` state.
        pub fn ensure_not_idle(&self, id: StreamId) -> Result<(), Reason> {
            if let Ok(next) = self.next_stream_id {
                if id >= next {
                    tracing::debug!(
                        "stream ID implicitly closed, PROTOCOL_ERROR; stream={:?}",
                        id
                    );
                    return Err(Reason::PROTOCOL_ERROR);
                }
            }
            // if next_stream_id is overflowed, that's ok.

            Ok(())
        }

        /// Handle remote sending an explicit RST_STREAM.
        pub fn recv_reset(&mut self, frame: frame::Reset, stream: &mut Stream) {
            // Notify the stream
            stream
                .state
                .recv_reset(frame.reason(), stream.is_pending_send);

            stream.notify_send();
            stream.notify_recv();
        }

        /// Handle a received error
        pub fn recv_err(&mut self, err: &proto::Error, stream: &mut Stream) {
            // Receive an error
            stream.state.recv_err(err);

            // If a receiver is waiting, notify it
            stream.notify_send();
            stream.notify_recv();
        }

        pub fn go_away(&mut self, last_processed_id: StreamId) {
            assert!(self.max_stream_id >= last_processed_id);
            self.max_stream_id = last_processed_id;
        }

        pub fn recv_eof(&mut self, stream: &mut Stream) {
            stream.state.recv_eof();
            stream.notify_send();
            stream.notify_recv();
        }

        pub(super) fn clear_recv_buffer(&mut self, stream: &mut Stream) {
            while let Some(_) = stream.pending_recv.pop_front(&mut self.buffer) {
                // drop it
            }
        }

        /// Get the max ID of streams we can receive.
        ///
        /// This gets lowered if we send a GOAWAY frame.
        pub fn max_stream_id(&self) -> StreamId {
            self.max_stream_id
        }

        pub fn next_stream_id(&self) -> Result<StreamId, RecvError> {
            if let Ok(id) = self.next_stream_id {
                Ok(id)
            } else {
                Err(RecvError::Connection(Reason::PROTOCOL_ERROR))
            }
        }

        pub fn may_have_created_stream(&self, id: StreamId) -> bool {
            if let Ok(next_id) = self.next_stream_id {
                // Peer::is_local_init should have been called beforehand
                debug_assert_eq!(id.is_server_initiated(), next_id.is_server_initiated(),);
                id < next_id
            } else {
                true
            }
        }

        /// Returns true if the remote peer can reserve a stream with the given ID.
        pub fn ensure_can_reserve(&self) -> Result<(), RecvError> {
            if !self.is_push_enabled {
                proto_err!(conn: "recv_push_promise: push is disabled");
                return Err(RecvError::Connection(Reason::PROTOCOL_ERROR));
            }

            Ok(())
        }

        /// Add a locally reset stream to queue to be eventually reaped.
        pub fn enqueue_reset_expiration(&mut self, stream: &mut store::Ptr, counts: &mut Counts) {
            if !stream.state.is_local_reset() || stream.is_pending_reset_expiration() {
                return;
            }

            tracing::trace!("enqueue_reset_expiration; {:?}", stream.id);

            if !counts.can_inc_num_reset_streams() {
                // try to evict 1 stream if possible
                // if max allow is 0, this won't be able to evict,
                // and then we'll just bail after
                if let Some(evicted) = self.pending_reset_expired.pop(stream.store_mut()) {
                    counts.transition_after(evicted, true);
                }
            }

            if counts.can_inc_num_reset_streams() {
                counts.inc_num_reset_streams();
                self.pending_reset_expired.push(stream);
            }
        }

        /// Send any pending refusals.
        pub fn send_pending_refusal<T, B>(
            &mut self,
            cx: &mut Context,
            dst: &mut Codec<T, Prioritized<B>>,
        ) -> Poll<io::Result<()>>
        where
            T: AsyncWrite + Unpin,
            B: Buf,
        {
            if let Some(stream_id) = self.refused {
                ready!(dst.poll_ready(cx))?;

                // Create the RST_STREAM frame
                let frame = frame::Reset::new(stream_id, Reason::REFUSED_STREAM);

                // Buffer the frame
                dst.buffer(frame.into()).expect("invalid RST_STREAM frame");
            }

            self.refused = None;

            Poll::Ready(Ok(()))
        }

        pub fn clear_expired_reset_streams(&mut self, store: &mut Store, counts: &mut Counts) {
            let now = Instant::now();
            let reset_duration = self.reset_duration;
            while let Some(stream) = self.pending_reset_expired.pop_if(store, |stream| {
                let reset_at = stream.reset_at.expect("reset_at must be set if in queue");
                now - reset_at > reset_duration
            }) {
                counts.transition_after(stream, true);
            }
        }

        pub fn clear_queues(
            &mut self,
            clear_pending_accept: bool,
            store: &mut Store,
            counts: &mut Counts,
        ) {
            self.clear_stream_window_update_queue(store, counts);
            self.clear_all_reset_streams(store, counts);

            if clear_pending_accept {
                self.clear_all_pending_accept(store, counts);
            }
        }

        fn clear_stream_window_update_queue(&mut self, store: &mut Store, counts: &mut Counts) {
            while let Some(stream) = self.pending_window_updates.pop(store) {
                counts.transition(stream, |_, stream| {
                    tracing::trace!("clear_stream_window_update_queue; stream={:?}", stream.id);
                })
            }
        }

        /// Called on EOF
        fn clear_all_reset_streams(&mut self, store: &mut Store, counts: &mut Counts) {
            while let Some(stream) = self.pending_reset_expired.pop(store) {
                counts.transition_after(stream, true);
            }
        }

        fn clear_all_pending_accept(&mut self, store: &mut Store, counts: &mut Counts) {
            while let Some(stream) = self.pending_accept.pop(store) {
                counts.transition_after(stream, false);
            }
        }

        pub fn poll_complete<T, B>(
            &mut self,
            cx: &mut Context,
            store: &mut Store,
            counts: &mut Counts,
            dst: &mut Codec<T, Prioritized<B>>,
        ) -> Poll<io::Result<()>>
        where
            T: AsyncWrite + Unpin,
            B: Buf,
        {
            // Send any pending connection level window updates
            ready!(self.send_connection_window_update(cx, dst))?;

            // Send any pending stream level window updates
            ready!(self.send_stream_window_updates(cx, store, counts, dst))?;

            Poll::Ready(Ok(()))
        }

        /// Send connection level window update
        fn send_connection_window_update<T, B>(
            &mut self,
            cx: &mut Context,
            dst: &mut Codec<T, Prioritized<B>>,
        ) -> Poll<io::Result<()>>
        where
            T: AsyncWrite + Unpin,
            B: Buf,
        {
            if let Some(incr) = self.flow.unclaimed_capacity() {
                let frame = frame::WindowUpdate::new(StreamId::zero(), incr);

                // Ensure the codec has capacity
                ready!(dst.poll_ready(cx))?;

                // Buffer the WINDOW_UPDATE frame
                dst.buffer(frame.into())
                    .expect("invalid WINDOW_UPDATE frame");

                // Update flow control
                self.flow
                    .inc_window(incr)
                    .expect("unexpected flow control state");
            }

            Poll::Ready(Ok(()))
        }

        /// Send stream level window update
        pub fn send_stream_window_updates<T, B>(
            &mut self,
            cx: &mut Context,
            store: &mut Store,
            counts: &mut Counts,
            dst: &mut Codec<T, Prioritized<B>>,
        ) -> Poll<io::Result<()>>
        where
            T: AsyncWrite + Unpin,
            B: Buf,
        {
            loop {
                // Ensure the codec has capacity
                ready!(dst.poll_ready(cx))?;

                // Get the next stream
                let stream = match self.pending_window_updates.pop(store) {
                    Some(stream) => stream,
                    None => return Poll::Ready(Ok(())),
                };

                counts.transition(stream, |_, stream| {
                    tracing::trace!("pending_window_updates -- pop; stream={:?}", stream.id);
                    debug_assert!(!stream.is_pending_window_update);

                    if !stream.state.is_recv_streaming() {
                        // No need to send window updates on the stream if the stream is
                        // no longer receiving data.
                        //
                        // TODO: is this correct? We could possibly send a window
                        // update on a ReservedRemote stream if we already know
                        // we want to stream the data faster...
                        return;
                    }

                    // TODO: de-dup
                    if let Some(incr) = stream.recv_flow.unclaimed_capacity() {
                        // Create the WINDOW_UPDATE frame
                        let frame = frame::WindowUpdate::new(stream.id, incr);

                        // Buffer it
                        dst.buffer(frame.into())
                            .expect("invalid WINDOW_UPDATE frame");

                        // Update flow control
                        stream
                            .recv_flow
                            .inc_window(incr)
                            .expect("unexpected flow control state");
                    }
                })
            }
        }

        pub fn next_incoming(&mut self, store: &mut Store) -> Option<store::Key> {
            self.pending_accept.pop(store).map(|ptr| ptr.key())
        }

        pub fn poll_data(
            &mut self,
            cx: &Context,
            stream: &mut Stream,
        ) -> Poll<Option<Result<Bytes, proto::Error>>> {
            // TODO: Return error when the stream is reset
            match stream.pending_recv.pop_front(&mut self.buffer) {
                Some(Event::Data(payload)) => Poll::Ready(Some(Ok(payload))),
                Some(event) => {
                    // Frame is trailer
                    stream.pending_recv.push_front(&mut self.buffer, event);

                    // Notify the recv task. This is done just in case
                    // `poll_trailers` was called.
                    //
                    // It is very likely that `notify_recv` will just be a no-op (as
                    // the task will be None), so this isn't really much of a
                    // performance concern. It also means we don't have to track
                    // state to see if `poll_trailers` was called before `poll_data`
                    // returned `None`.
                    stream.notify_recv();

                    // No more data frames
                    Poll::Ready(None)
                }
                None => self.schedule_recv(cx, stream),
            }
        }

        pub fn poll_trailers(
            &mut self,
            cx: &Context,
            stream: &mut Stream,
        ) -> Poll<Option<Result<HeaderMap, proto::Error>>> {
            match stream.pending_recv.pop_front(&mut self.buffer) {
                Some(Event::Trailers(trailers)) => Poll::Ready(Some(Ok(trailers))),
                Some(event) => {
                    // Frame is not trailers.. not ready to poll trailers yet.
                    stream.pending_recv.push_front(&mut self.buffer, event);

                    Poll::Pending
                }
                None => self.schedule_recv(cx, stream),
            }
        }

        fn schedule_recv<T>(
            &mut self,
            cx: &Context,
            stream: &mut Stream,
        ) -> Poll<Option<Result<T, proto::Error>>> {
            if stream.state.ensure_recv_open()? {
                // Request to get notified once more frames arrive
                stream.recv_task = Some(cx.waker().clone());
                Poll::Pending
            } else {
                // No more frames will be received
                Poll::Ready(None)
            }
        }
    }

    // ===== impl Open =====

    impl Open {
        pub fn is_push_promise(&self) -> bool {
            use self::Open::*;

            match *self {
                PushPromise => true,
                _ => false,
            }
        }
    }

    // ===== impl RecvHeaderBlockError =====

    impl<T> From<RecvError> for RecvHeaderBlockError<T> {
        fn from(err: RecvError) -> Self {
            RecvHeaderBlockError::State(err)
        }
    }