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    use crate::codec::{RecvError, UserError};
    use crate::error::Reason;
    use crate::frame;
    use crate::proto::*;
    use std::task::{Context, Poll};

    #[derive(Debug)]
    pub(crate) struct Settings {
        /// Our local SETTINGS sync state with the remote.
        local: Local,
        /// Received SETTINGS frame pending processing. The ACK must be written to
        /// the socket first then the settings applied **before** receiving any
        /// further frames.
        remote: Option<frame::Settings>,
    }

    #[derive(Debug)]
    enum Local {
        /// We want to send these SETTINGS to the remote when the socket is ready.
        ToSend(frame::Settings),
        /// We have sent these SETTINGS and are waiting for the remote to ACK
        /// before we apply them.
        WaitingAck(frame::Settings),
        /// Our local settings are in sync with the remote.
        Synced,
    }

    impl Settings {
        pub(crate) fn new(local: frame::Settings) -> Self {
            Settings {
                // We assume the initial local SETTINGS were flushed during
                // the handshake process.
                local: Local::WaitingAck(local),
                remote: None,
            }
        }

        pub(crate) fn recv_settings<T, B, C, P>(
            &mut self,
            frame: frame::Settings,
            codec: &mut Codec<T, B>,
            streams: &mut Streams<C, P>,
        ) -> Result<(), RecvError>
        where
            T: AsyncWrite + Unpin,
            B: Buf,
            C: Buf,
            P: Peer,
        {
            if frame.is_ack() {
                match &self.local {
                    Local::WaitingAck(local) => {
                        tracing::debug!("received settings ACK; applying {:?}", local);

                        if let Some(max) = local.max_frame_size() {
                            codec.set_max_recv_frame_size(max as usize);
                        }

                        if let Some(max) = local.max_header_list_size() {
                            codec.set_max_recv_header_list_size(max as usize);
                        }

                        streams.apply_local_settings(local)?;
                        self.local = Local::Synced;
                        Ok(())
                    }
                    Local::ToSend(..) | Local::Synced => {
                        // We haven't sent any SETTINGS frames to be ACKed, so
                        // this is very bizarre! Remote is either buggy or malicious.
                        proto_err!(conn: "received unexpected settings ack");
                        Err(RecvError::Connection(Reason::PROTOCOL_ERROR))
                    }
                }
            } else {
                // We always ACK before reading more frames, so `remote` should
                // always be none!
                assert!(self.remote.is_none());
                self.remote = Some(frame);
                Ok(())
            }
        }

        pub(crate) fn send_settings(&mut self, frame: frame::Settings) -> Result<(), UserError> {
            assert!(!frame.is_ack());
            match &self.local {
                Local::ToSend(..) | Local::WaitingAck(..) => Err(UserError::SendSettingsWhilePending),
                Local::Synced => {
                    tracing::trace!("queue to send local settings: {:?}", frame);
                    self.local = Local::ToSend(frame);
                    Ok(())
                }
            }
        }

        pub(crate) fn poll_send<T, B, C, P>(
            &mut self,
            cx: &mut Context,
            dst: &mut Codec<T, B>,
            streams: &mut Streams<C, P>,
        ) -> Poll<Result<(), RecvError>>
        where
            T: AsyncWrite + Unpin,
            B: Buf,
            C: Buf,
            P: Peer,
        {
            if let Some(settings) = &self.remote {
                if !dst.poll_ready(cx)?.is_ready() {
                    return Poll::Pending;
                }

                // Create an ACK settings frame
                let frame = frame::Settings::ack();

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

                tracing::trace!("ACK sent; applying settings");

                if let Some(val) = settings.header_table_size() {
                    dst.set_send_header_table_size(val as usize);
                }

                if let Some(val) = settings.max_frame_size() {
                    dst.set_max_send_frame_size(val as usize);
                }

                streams.apply_remote_settings(settings)?;
            }

            self.remote = None;

            match &self.local {
                Local::ToSend(settings) => {
                    if !dst.poll_ready(cx)?.is_ready() {
                        return Poll::Pending;
                    }

                    // Buffer the settings frame
                    dst.buffer(settings.clone().into())
                        .expect("invalid settings frame");
                    tracing::trace!("local settings sent; waiting for ack: {:?}", settings);

                    self.local = Local::WaitingAck(settings.clone());
                }
                Local::WaitingAck(..) | Local::Synced => {}
            }

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