hidpp/

channel.rs

//! Implements basic messaging across HID and HID++ channels.
//!
//! This includes mapping incoming messages to previously sent requests.

use std::{
    collections::{HashMap, VecDeque},
    error::Error,
    sync::{
        Arc, Mutex, Weak,
        atomic::{AtomicBool, AtomicU8, AtomicU64, Ordering},
    },
    thread::{self, JoinHandle},
    time::Duration,
};

use async_trait::async_trait;
use futures::{FutureExt, channel::oneshot, select};
use hidreport::{Field, Report, ReportDescriptor, Usage, UsageId, UsagePage};
use rand::Rng;
use thiserror::Error;
use tracing::trace;

use crate::nibble::U4;

/// hidapi defines this as the maximum EXPECTED size of report descriptors.
/// We will trust this for now, but a workaround may be required if devices do
/// in fact return longer descriptors.
const MAX_REPORT_DESCRIPTOR_LENGTH: usize = 4096;

/// This is the size of the buffer incoming reports are read into.
/// As we only care about HID++ reports, this equals to [`LONG_REPORT_LENGTH`].
const MAX_REPORT_LENGTH: usize = LONG_REPORT_LENGTH;

/// The default time budget for a [`HidppChannel::send`] request: the report
/// write plus the wait for a matching response. Callers that need a different
/// budget can use [`HidppChannel::send_with_timeout`].
pub const SEND_RESPONSE_TIMEOUT: Duration = Duration::from_secs(5);

/// The ID of the HID report that is used to transmit short HID++ messages.
pub const SHORT_REPORT_ID: u8 = 0x10;

/// The HID usage page ID of short HID++ message reports.
pub const SHORT_REPORT_USAGE_PAGE: u16 = 0xff00;

/// The HID usage ID of short HID++ message reports.
pub const SHORT_REPORT_USAGE: u16 = 0x0001;

/// The length of short HID++ message reports (including report ID).
pub const SHORT_REPORT_LENGTH: usize = 7;

/// The ID of the HID report that is used to transmit long HID++ messages.
pub const LONG_REPORT_ID: u8 = 0x11;

/// The HID usage page ID of long HID++ message reports.
pub const LONG_REPORT_USAGE_PAGE: u16 = 0xff00;

/// The HID usage ID of long HID++ message reports.
pub const LONG_REPORT_USAGE: u16 = 0x0002;

/// The length of long HID++ message reports (including report ID).
pub const LONG_REPORT_LENGTH: usize = 20;

/// Represents an arbitrary HID communication channel that is both readable and
/// writable. It has to support async I/O.
///
/// Any type this trait is implemented for can be used for HID(++)
/// communication. If a specific channel supports HID++ is determined at a later
/// stage and is not directly related to potential implementations of this
/// trait.
#[async_trait]
pub trait RawHidChannel: Sync + Send + 'static {
    /// Provides the vendor ID of the connected HID device.
    fn vendor_id(&self) -> u16;

    /// Provides the product ID of the connected HID device.
    fn product_id(&self) -> u16;

    /// Writes a raw report to the channel.
    ///
    /// Returns the exact amount of written bytes on success.
    async fn write_report(&self, src: &[u8]) -> Result<usize, Box<dyn Error + Sync + Send>>;

    /// Reads a raw report from the channel.
    ///
    /// If the buffer is not large enough to fit the whole report, its remainder
    /// should be discarded and must not be returned by any succeeding call to
    /// [`Self::read_report`].
    ///
    /// Returns the exact amount or read bytes on success. An `Err` is treated
    /// as transient: the [`HidppChannel`] read loop logs it and retries, so an
    /// implementation must not surface a condition that will never clear (it
    /// would busy-spin the loop). For a *permanent* failure — the device is
    /// gone and no report will ever arrive — the future may instead park
    /// forever. That is sound because the read loop always races this future
    /// against the channel's close signal in a `select!`; any other caller
    /// must do the same and must not await `read_report` bare.
    async fn read_report(&self, buf: &mut [u8]) -> Result<usize, Box<dyn Error + Sync + Send>>;

    /// If the implementation already knows whether the underlying HID channel
    /// supports HID++ messages, it should return `Some((supports_short,
    /// supports_long))` from this method.
    ///
    /// In this case, the report descriptor will not be read and parsed.
    fn supports_short_long_hidpp(&self) -> Option<(bool, bool)>;

    /// Retrieves the raw HID report descriptor from the channel.
    ///
    /// This is used to determine whether the channel supports HID++.
    ///
    /// Returns the exact size of the report descriptor on success.
    async fn get_report_descriptor(
        &self,
        buf: &mut [u8],
    ) -> Result<usize, Box<dyn Error + Sync + Send>>;
}

/// Checks whether a raw channel supports short or long HID++ messages.
async fn supports_short_long_hidpp(
    chan: &impl RawHidChannel,
) -> Result<(bool, bool), ChannelError> {
    if let Some((supports_short, supports_long)) = chan.supports_short_long_hidpp() {
        return Ok((supports_short, supports_long));
    }

    let mut raw_descriptor = vec![0u8; MAX_REPORT_DESCRIPTOR_LENGTH];
    let descriptor_size = chan.get_report_descriptor(&mut raw_descriptor).await?;

    let descriptor = match ReportDescriptor::try_from(&raw_descriptor[..descriptor_size]) {
        Ok(val) => val,
        Err(err) => return Err(ChannelError::ReportDescriptor(err)),
    };

    let supports_short = descriptor
        .find_input_report(&[SHORT_REPORT_ID])
        .and_then(|report| report.fields().first())
        .and_then(|field| match field {
            Field::Array(arr) => Some(arr.usage_range()),
            _ => None,
        })
        .is_some_and(|range| {
            range
                .lookup_usage(&Usage::from_page_and_id(
                    UsagePage::from(SHORT_REPORT_USAGE_PAGE),
                    UsageId::from(SHORT_REPORT_USAGE),
                ))
                .is_some()
        });

    let supports_long = descriptor
        .find_input_report(&[LONG_REPORT_ID])
        .and_then(|report| report.fields().first())
        .and_then(|field| match field {
            Field::Array(arr) => Some(arr.usage_range()),
            _ => None,
        })
        .is_some_and(|range| {
            range
                .lookup_usage(&Usage::from_page_and_id(
                    UsagePage::from(LONG_REPORT_USAGE_PAGE),
                    UsageId::from(LONG_REPORT_USAGE),
                ))
                .is_some()
        });

    Ok((supports_short, supports_long))
}

/// Represents an unversioned HID++ message.
#[derive(Clone, Copy, PartialEq, Eq, Hash, Debug)]
pub enum HidppMessage {
    /// Represents a short HID++ message.
    ///
    /// Please check [`HidppChannel::supports_short`] before sending this kind
    /// of message.
    Short([u8; SHORT_REPORT_LENGTH - 1]),

    /// Represents a long HID++ message.
    ///
    /// Please check [`HidppChannel::supports_long`] before sending this kind of
    /// message.
    Long([u8; LONG_REPORT_LENGTH - 1]),
}

impl HidppMessage {
    /// Tries to read a HID++ message from raw data.
    pub fn read_raw(data: &[u8]) -> Option<Self> {
        let (&report_id, rest) = data.split_first()?;

        // The empty-remainder patterns enforce the exact report lengths.
        if report_id == SHORT_REPORT_ID
            && let Some((&payload, [])) = rest.split_first_chunk()
        {
            Some(HidppMessage::Short(payload))
        } else if report_id == LONG_REPORT_ID
            && let Some((&payload, [])) = rest.split_first_chunk()
        {
            Some(HidppMessage::Long(payload))
        } else {
            None
        }
    }

    /// Writes a HID++ message in its raw byte form into a buffer.
    ///
    /// Returns the amount of written bytes.
    pub fn write_raw(&self, buf: &mut [u8]) -> usize {
        match self {
            Self::Short(payload) => {
                buf[0] = SHORT_REPORT_ID;
                buf[1..SHORT_REPORT_LENGTH].copy_from_slice(payload);
                SHORT_REPORT_LENGTH
            }
            Self::Long(payload) => {
                buf[0] = LONG_REPORT_ID;
                buf[1..LONG_REPORT_LENGTH].copy_from_slice(payload);
                LONG_REPORT_LENGTH
            }
        }
    }

    /// The HID++ addressing header `(device_index, feature_index, function)` —
    /// the first three payload bytes, present on both report kinds. Used only
    /// for wire tracing (OpenLogi-specific; not in upstream hidpp).
    fn header(&self) -> (u8, u8, u8) {
        let payload: &[u8] = match self {
            Self::Short(payload) => payload,
            Self::Long(payload) => payload,
        };
        (payload[0], payload[1], payload[2])
    }
}

type MessageListener = Arc<dyn Fn(HidppMessage, bool) + Send + Sync + 'static>;

/// Removes a HID++ message listener when dropped.
pub struct MessageListenerGuard {
    message_listeners: Weak<Mutex<HashMap<u32, MessageListener>>>,
    hdl: u32,
}

impl Drop for MessageListenerGuard {
    fn drop(&mut self) {
        if let Some(message_listeners) = self.message_listeners.upgrade() {
            message_listeners.lock().unwrap().remove(&self.hdl);
        }
    }
}

/// Represents a HID communication channel supporting HID++.
pub struct HidppChannel {
    /// Whether the channel supports short (7 bytes) HID++ messages.
    pub supports_short: bool,

    /// Whether the channel supports long (20 bytes) HID++ messages.
    pub supports_long: bool,

    /// The vendor ID of the connected HID device.
    pub vendor_id: u16,

    /// The product ID of the connected HID device.
    pub product_id: u16,

    /// The underlying raw HID channel.
    raw_channel: Arc<dyn RawHidChannel>,

    /// Whether to rotate the [`Self::software_id`].
    rotate_software_id: AtomicBool,

    /// The software ID to provide at the next call to [`Self::get_sw_id`].
    software_id: AtomicU8,

    /// All sent messages that are waiting for a response.
    pending_messages: Arc<Mutex<VecDeque<PendingMessage>>>,

    /// The request ID assigned to the next pending message.
    pending_message_id: AtomicU64,

    /// Registered listeners that will receive notifications about incoming
    /// messages.
    message_listeners: Arc<Mutex<HashMap<u32, MessageListener>>>,

    /// The sender signaling the read thread to stop.
    read_thread_close: Option<oneshot::Sender<()>>,

    /// The handle to the read thread. Should be joined after signaling
    /// [`Self::read_thread_close`].
    read_thread_hdl: Option<JoinHandle<()>>,
}

impl Drop for HidppChannel {
    fn drop(&mut self) {
        if let Some(read_thread_close) = self.read_thread_close.take() {
            // This only fails if the receiving end, which is owned by the read thread in
            // this case, is dropped.
            // This just means that the read thread is already stopped, so we can ignore the
            // error here.
            let _ = read_thread_close.send(());
        }

        if let Some(read_thread_hdl) = self.read_thread_hdl.take() {
            read_thread_hdl.join().unwrap();
        }
    }
}

/// Represents a message that was sent and is waiting for a response.
struct PendingMessage {
    /// Unique ID used to remove this request if it times out.
    id: u64,

    /// The predicate that has to match for an incoming message to be classified
    /// as the response.
    response_predicate: Box<dyn Fn(&HidppMessage) -> bool + Send>,

    /// The oneshot sender used to provide the response message to the receiving
    /// end.
    sender: oneshot::Sender<HidppMessage>,
}

impl HidppChannel {
    /// Tries to construct a HID++ channel from a raw HID channel.
    ///
    /// If the given HID channel does not support HID++,
    /// [`ChannelError::HidppNotSupported`] will be returned.
    pub async fn from_raw_channel(raw: impl RawHidChannel) -> Result<Self, ChannelError> {
        let (supports_short, supports_long) = supports_short_long_hidpp(&raw).await?;

        if !supports_short && !supports_long {
            return Err(ChannelError::HidppNotSupported);
        }

        let raw_channel_rc = Arc::new(raw);
        let pending_messages_rc = Arc::new(Mutex::new(VecDeque::<PendingMessage>::new()));
        let message_listeners_rc = Arc::new(Mutex::new(HashMap::<u32, MessageListener>::new()));

        let (close_sender, mut close_receiver) = oneshot::channel::<()>();

        let read_thread_hdl = thread::spawn({
            let raw_channel = Arc::clone(&raw_channel_rc);
            let pending_messages = Arc::clone(&pending_messages_rc);
            let message_listeners = Arc::clone(&message_listeners_rc);

            move || {
                futures::executor::block_on(async {
                    let mut buf = [0u8; MAX_REPORT_LENGTH];

                    loop {
                        let res = select! {
                            _ = close_receiver => {
                                break;
                            },
                            res = raw_channel.read_report(&mut buf).fuse() => res
                        };

                        let Ok(len) = res else {
                            continue;
                        };

                        let Some(msg) = HidppMessage::read_raw(&buf[..len]) else {
                            continue;
                        };

                        let mut matched = false;
                        {
                            let mut msgs = pending_messages.lock().unwrap();
                            if let Some(pos) =
                                msgs.iter().position(|elem| (elem.response_predicate)(&msg))
                            {
                                let waiting = msgs.remove(pos).unwrap();
                                let _ = waiting.sender.send(msg);
                                matched = true;
                            }
                        }

                        let listeners: Vec<_> = message_listeners
                            .lock()
                            .unwrap()
                            .values()
                            .cloned()
                            .collect();
                        for listener in listeners {
                            listener(msg, matched);
                        }
                    }
                });
            }
        });

        Ok(Self {
            supports_short,
            supports_long,
            vendor_id: raw_channel_rc.vendor_id(),
            product_id: raw_channel_rc.product_id(),
            raw_channel: raw_channel_rc,
            rotate_software_id: AtomicBool::new(false),
            software_id: AtomicU8::new(0x01),
            pending_messages: pending_messages_rc,
            pending_message_id: AtomicU64::new(1),
            message_listeners: message_listeners_rc,
            read_thread_close: Some(close_sender),
            read_thread_hdl: Some(read_thread_hdl),
        })
    }

    /// Sets the software ID that should be returned by the next call to
    /// [`Self::get_sw_id`].
    ///
    /// Using software ID `0` is highly discouraged as it is used for device
    /// notifications.
    pub fn set_sw_id(&self, sw_id: U4) {
        self.software_id.store(sw_id.to_lo(), Ordering::SeqCst);
    }

    /// Sets whether the software ID returned by a call to [`Self::get_sw_id`]
    /// should increment (and potentially wrap around) after each call.
    ///
    /// This comes in handy when trying to map responses to requests
    /// consistently.
    ///
    /// Software ID `0` will be skipped in the rotation process as it is
    /// reserved for device notifications.
    pub fn set_rotating_sw_id(&self, enable: bool) {
        self.rotate_software_id.store(enable, Ordering::SeqCst);
    }

    /// Provides a software ID that can be used to send a HID++ message across
    /// the channel.
    ///
    /// This method should be called separately for every message to send as it
    /// may rotate (as indicated by [`Self::set_rotating_sw_id`]).
    pub fn get_sw_id(&self) -> U4 {
        if self.rotate_software_id.load(Ordering::SeqCst) {
            U4::from_lo(
                self.software_id
                    .fetch_update(Ordering::SeqCst, Ordering::SeqCst, |old| {
                        Some(if old & 0x0f == 0x0f {
                            0x01
                        } else {
                            old.wrapping_add(1)
                        })
                    })
                    .unwrap(),
            )
        } else {
            U4::from_lo(self.software_id.load(Ordering::SeqCst))
        }
    }

    /// Checks whether the channel supports the given HID++ message.
    pub fn supports_msg(&self, msg: &HidppMessage) -> bool {
        match msg {
            HidppMessage::Short(_) => self.supports_short,
            HidppMessage::Long(_) => self.supports_long,
        }
    }

    /// Re-frames a short message as long on a long-only channel — a device that
    /// exposes only the long HID++ report (e.g. a Bluetooth-LE-direct mouse on
    /// macOS, where `IOHIDDeviceSetReport` rejects the short report). The HID++
    /// header bytes sit at the same offsets in both widths, so the only change
    /// is the report id plus zero-padding the extra payload; the device answers
    /// with a long report, which still matches the request by header. A no-op on
    /// channels that advertise short support.
    ///
    /// (OpenLogi local addition — candidate for upstreaming.)
    fn normalize_outgoing(&self, msg: HidppMessage) -> HidppMessage {
        match msg {
            HidppMessage::Short(payload) if !self.supports_short && self.supports_long => {
                HidppMessage::Long(short_payload_as_long(&payload))
            }
            other => other,
        }
    }

    /// Sends a HID++ message across the channel and waits for a response.
    ///
    /// If no response is expected/required, use [`Self::send_and_forget`].
    ///
    /// The whole request — the report write plus the wait for a matching
    /// response — is bounded by [`SEND_RESPONSE_TIMEOUT`]; the future resolves
    /// to [`ChannelError::Timeout`] on elapse. Use [`Self::send_with_timeout`]
    /// to choose a different budget.
    pub async fn send(
        &self,
        msg: HidppMessage,
        response_predicate: impl Fn(&HidppMessage) -> bool + Send + 'static,
    ) -> Result<HidppMessage, ChannelError> {
        self.send_with_timeout(msg, response_predicate, SEND_RESPONSE_TIMEOUT)
            .await
    }

    /// Sends a HID++ message across the channel and waits for a response,
    /// bounding the whole request — the report write plus the wait for a
    /// matching response — by `timeout`.
    ///
    /// On elapse the request's pending entry is removed (concurrent in-flight
    /// requests are unaffected) and [`ChannelError::Timeout`] is returned; a
    /// response that still arrives later reaches message listeners as an
    /// unmatched message.
    ///
    /// [`Self::send`] uses this with [`SEND_RESPONSE_TIMEOUT`], which suits
    /// requests to a device that may be asleep. Requests that should fail
    /// faster — e.g. probing a receiver that answers immediately or not at
    /// all — can pass a tighter budget.
    pub async fn send_with_timeout(
        &self,
        msg: HidppMessage,
        response_predicate: impl Fn(&HidppMessage) -> bool + Send + 'static,
        timeout: Duration,
    ) -> Result<HidppMessage, ChannelError> {
        let msg = self.normalize_outgoing(msg);
        if !self.supports_msg(&msg) {
            return Err(ChannelError::MessageTypeNotSupported);
        }

        // Wire trace (off by default; `OPENLOGI_LOG=hidpp=trace`). Capture the
        // header before `msg` is moved into the send future so the outcome line
        // below can name the same request.
        let (dev, feat, func) = msg.header();
        trace!(dev, feat, func, "hidpp request");

        let (sender, receiver) = oneshot::channel::<HidppMessage>();
        let pending_id = self.pending_message_id.fetch_add(1, Ordering::SeqCst);

        {
            let mut pending = self.pending_messages.lock().unwrap();
            // Drop abandoned requests before queuing this one. Timeouts and
            // write failures remove their entry eagerly below, but a caller
            // cancelled mid-flight (an outer `timeout(..)` dropping the whole
            // future) still leaves its `PendingMessage` behind. On a channel
            // reused across inventory ticks those would accumulate unboundedly
            // — and a late response could be mis-delivered to a recycled
            // software id. `is_canceled()` is true once the receiver is gone,
            // so this prunes exactly the give-ups.
            pending.retain(|m| !m.sender.is_canceled());
            pending.push_back(PendingMessage {
                id: pending_id,
                response_predicate: Box::new(response_predicate),
                sender,
            });
        }

        // The deadline covers the write as well: `write_report` has no
        // bounded-time contract of its own, so a wedged device could otherwise
        // park `send` forever before the response wait even starts.
        let mut request = std::pin::pin!(
            async {
                self.send_and_forget(msg).await?;
                receiver.await.map_err(|_| ChannelError::NoResponse)
            }
            .fuse()
        );

        let result = select! {
            result = request => result,
            _ = futures_timer::Delay::new(timeout).fuse() => Err(ChannelError::Timeout),
        };

        match &result {
            Ok(_) => trace!(dev, feat, "hidpp response"),
            Err(e) => trace!(dev, feat, error = ?e, "hidpp no response"),
        }

        if result.is_err() {
            // A timeout or write failure leaves the entry queued — remove it
            // eagerly. After a matched response the read thread has already
            // taken it, so this is a no-op then.
            self.remove_pending_message(pending_id);
        }

        result
    }

    fn remove_pending_message(&self, id: u64) {
        let mut pending = self.pending_messages.lock().unwrap();
        if let Some(pos) = pending.iter().position(|msg| msg.id == id) {
            pending.remove(pos);
        }
    }

    /// Sends a HID++ message across the channel and does not wait for a
    /// response.
    ///
    /// If a response is expected, use [`Self::send`],
    pub async fn send_and_forget(&self, msg: HidppMessage) -> Result<(), ChannelError> {
        let msg = self.normalize_outgoing(msg);
        if !self.supports_msg(&msg) {
            return Err(ChannelError::MessageTypeNotSupported);
        }

        let mut buf = [0u8; LONG_REPORT_LENGTH];
        let len = msg.write_raw(&mut buf);
        self.raw_channel
            .write_report(&buf[..len])
            .await
            .map(|_| ())
            .map_err(ChannelError::Implementation)
    }

    /// Registers a listener that will be called for every incoming message.
    ///
    /// Returns a handle that can be used to remove the listener using a call to
    /// [`Self::remove_msg_listener`].
    pub fn add_msg_listener(
        &self,
        listener: impl Fn(HidppMessage, bool) + Send + Sync + 'static,
    ) -> u32 {
        let mut listeners = self.message_listeners.lock().unwrap();

        let mut rng = rand::rng();
        let mut hdl = rng.random::<u32>();
        while listeners.contains_key(&hdl) {
            hdl = rng.random::<u32>();
        }

        listeners.insert(hdl, Arc::new(listener));
        hdl
    }

    /// Registers a listener that is automatically removed when the returned
    /// guard is dropped.
    pub fn add_msg_listener_guarded(
        &self,
        listener: impl Fn(HidppMessage, bool) + Send + Sync + 'static,
    ) -> MessageListenerGuard {
        let hdl = self.add_msg_listener(listener);
        MessageListenerGuard {
            message_listeners: Arc::downgrade(&self.message_listeners),
            hdl,
        }
    }

    /// Removes a previously registered message listener.
    ///
    /// Returns whether a listener was found using the given handle.
    pub fn remove_msg_listener(&self, hdl: u32) -> bool {
        self.message_listeners
            .lock()
            .unwrap()
            .remove(&hdl)
            .is_some()
    }
}

/// Represents an error that occurred when creating or interacting with a HID or
/// HID++ communication channel.
#[derive(Debug, Error)]
#[non_exhaustive]
pub enum ChannelError {
    /// Indicates that the concrete implementation of [`RawHidChannel`] returned
    /// an error.
    #[error("the HID channel implementation returned an error")]
    Implementation(#[from] Box<dyn Error + Sync + Send>),

    /// Indicates that the HID report descriptor could not be parsed.
    #[error("the report descriptor could not be parsed")]
    ReportDescriptor(hidreport::ParserError),

    /// Indicates that the channel in question does not support HID++.
    #[error("the HID channel does not support HID++")]
    HidppNotSupported,

    /// Indicates that the HID++ channel does not support messages of the given
    /// type (short/long).
    #[error("the channel does not support the given HID++ message type")]
    MessageTypeNotSupported,

    /// Indicates that no response was received following a request.
    #[error("the device did not respond to the request")]
    NoResponse,

    /// Indicates that a request did not complete within its time budget —
    /// typically the device is asleep, out of range or connected to another
    /// host. See [`HidppChannel::send_with_timeout`].
    #[error("the request timed out before the device responded")]
    Timeout,
}

/// Widen a short HID++ payload (6 bytes) to a long one (19 bytes): the HID++
/// header bytes (device / feature / function|sw) sit at the same offsets in
/// both widths, so the only change is zero-padding the trailing payload. Used
/// to re-frame short messages as long on a long-only channel — see
/// [`HidppChannel::normalize_outgoing`]. (OpenLogi local addition.)
fn short_payload_as_long(payload: &[u8; SHORT_REPORT_LENGTH - 1]) -> [u8; LONG_REPORT_LENGTH - 1] {
    let mut long = [0u8; LONG_REPORT_LENGTH - 1];
    long[..payload.len()].copy_from_slice(payload);
    long
}

#[cfg(test)]
mod tests {
    use super::*;
    use std::{
        io,
        sync::{
            Arc, Mutex,
            atomic::{AtomicUsize, Ordering},
        },
        time::{Duration, Instant},
    };

    use crate::{
        nibble,
        protocol::v20::{self, ErrorType, Hidpp20Error},
    };

    #[test]
    fn short_payload_widens_preserving_header_and_padding() {
        // [device, feature, function|sw, p0, p1, p2]
        let short = [0xff, 0x05, 0x1e, 0xaa, 0xbb, 0xcc];
        let long = short_payload_as_long(&short);
        assert_eq!(&long[..short.len()], &short[..]); // header + payload copied verbatim
        assert!(long[short.len()..].iter().all(|&b| b == 0)); // remainder zero-padded
        assert_eq!(long.len(), LONG_REPORT_LENGTH - 1);
    }

    #[test]
    fn send_returns_response_before_timeout() {
        futures::executor::block_on(async {
            let (raw, handle) = MockRawHidChannel::new();
            let channel = HidppChannel::from_raw_channel(raw).await.unwrap();

            let request = short_msg(0x10);
            let response = short_msg(0x20);
            handle.queue_response(response);

            let actual = channel
                .send_with_timeout(
                    request,
                    move |candidate| *candidate == response,
                    Duration::from_secs(1),
                )
                .await
                .unwrap();

            assert_eq!(actual, response);
            assert_eq!(handle.written_reports().len(), 1);
            assert_pending_empty(&channel);
        });
    }

    #[test]
    fn send_times_out_and_removes_pending_message() {
        futures::executor::block_on(async {
            let (raw, handle) = MockRawHidChannel::new();
            let channel = HidppChannel::from_raw_channel(raw).await.unwrap();
            let request = short_msg(0x10);
            let response = short_msg(0x20);

            let started = Instant::now();
            let err = channel
                .send_with_timeout(
                    request,
                    move |candidate| *candidate == response,
                    Duration::from_millis(25),
                )
                .await
                .unwrap_err();

            assert!(matches!(err, ChannelError::Timeout));
            assert!(started.elapsed() < Duration::from_secs(1));
            assert_eq!(handle.written_reports().len(), 1);
            assert_pending_empty(&channel);
        });
    }

    #[test]
    fn timeout_removes_only_its_own_pending_message() {
        futures::executor::block_on(async {
            let (raw, handle) = MockRawHidChannel::new();
            let channel = HidppChannel::from_raw_channel(raw).await.unwrap();

            let never_answered = short_msg(0x20);
            let slow_response = short_msg(0x21);

            let timed_out = channel.send_with_timeout(
                short_msg(0x10),
                move |candidate| *candidate == never_answered,
                Duration::from_millis(25),
            );
            let answered = channel.send_with_timeout(
                short_msg(0x11),
                move |candidate| *candidate == slow_response,
                Duration::from_secs(1),
            );
            // Answer the second request only after the first has timed out, so
            // a removal that took the wrong entry would fail this test.
            let respond_late = async {
                futures_timer::Delay::new(Duration::from_millis(100)).await;
                handle.send_incoming(slow_response).await;
            };

            let (timed_out, answered, ()) = futures::join!(timed_out, answered, respond_late);

            assert!(matches!(timed_out.unwrap_err(), ChannelError::Timeout));
            assert_eq!(answered.unwrap(), slow_response);
            assert_pending_empty(&channel);
        });
    }

    #[test]
    fn late_response_after_timeout_is_ignored() {
        futures::executor::block_on(async {
            let (raw, handle) = MockRawHidChannel::new();
            let channel = HidppChannel::from_raw_channel(raw).await.unwrap();
            let events = Arc::new(Mutex::new(Vec::new()));
            let listener_events = Arc::clone(&events);
            channel.add_msg_listener(move |msg, matched| {
                listener_events.lock().unwrap().push((msg, matched));
            });

            let request = short_msg(0x10);
            let late_response = short_msg(0x20);
            let err = channel
                .send_with_timeout(
                    request,
                    move |candidate| *candidate == late_response,
                    Duration::from_millis(25),
                )
                .await
                .unwrap_err();

            assert!(matches!(err, ChannelError::Timeout));
            assert_pending_empty(&channel);

            handle.send_incoming(late_response).await;
            wait_for_event_count(&events, 1).await;
            assert_eq!(events.lock().unwrap()[0], (late_response, false));
            assert_pending_empty(&channel);

            let later_request = short_msg(0x30);
            let later_response = short_msg(0x40);
            handle.queue_response(later_response);
            let actual = channel
                .send_with_timeout(
                    later_request,
                    move |candidate| *candidate == later_response,
                    Duration::from_secs(1),
                )
                .await
                .unwrap();

            assert_eq!(actual, later_response);
            wait_for_event_count(&events, 2).await;
            assert_eq!(events.lock().unwrap()[1], (later_response, true));
            assert_pending_empty(&channel);
        });
    }

    #[test]
    fn send_and_forget_writes_without_pending_message() {
        futures::executor::block_on(async {
            let (raw, handle) = MockRawHidChannel::new();
            let channel = HidppChannel::from_raw_channel(raw).await.unwrap();

            channel.send_and_forget(short_msg(0x10)).await.unwrap();

            assert_eq!(handle.written_reports().len(), 1);
            assert_pending_empty(&channel);
        });
    }

    #[test]
    fn listener_can_remove_another_listener_during_dispatch() {
        futures::executor::block_on(async {
            let (raw, handle) = MockRawHidChannel::new();
            let channel = Arc::new(HidppChannel::from_raw_channel(raw).await.unwrap());
            let removed_listener_calls = Arc::new(AtomicUsize::new(0));
            let removing_listener_calls = Arc::new(AtomicUsize::new(0));

            let removed_listener_calls_for_listener = Arc::clone(&removed_listener_calls);
            let removed_hdl = channel.add_msg_listener(move |_, _| {
                removed_listener_calls_for_listener.fetch_add(1, Ordering::SeqCst);
            });

            let channel_for_listener = Arc::clone(&channel);
            let removing_listener_calls_for_listener = Arc::clone(&removing_listener_calls);
            channel.add_msg_listener(move |_, _| {
                removing_listener_calls_for_listener.fetch_add(1, Ordering::SeqCst);
                channel_for_listener.remove_msg_listener(removed_hdl);
            });

            handle.send_incoming(short_msg(0x20)).await;
            wait_for_atomic_count(&removing_listener_calls, 1).await;
            wait_for_atomic_count(&removed_listener_calls, 1).await;

            handle.send_incoming(short_msg(0x21)).await;
            wait_for_atomic_count(&removing_listener_calls, 2).await;

            assert_eq!(removed_listener_calls.load(Ordering::SeqCst), 1);
        });
    }

    // --- HID++2.0 (v20) send/matcher characterization tests -----------------
    //
    // `HidppChannel::send`/`send_with_timeout` above are protocol-agnostic:
    // they match on an arbitrary predicate over raw `HidppMessage`s. The
    // v20-specific correlation logic (matching by header, splitting out error
    // frames) lives in `protocol::v20::HidppChannel::send_v20`, which is built
    // directly on top of `send`. These tests pin that logic's current
    // behaviour using the same mock transport as the tests above.

    #[test]
    fn send_v20_matches_response_by_header_ignoring_unrelated_messages() {
        futures::executor::block_on(async {
            let (raw, handle) = MockRawHidChannel::new();
            let channel = HidppChannel::from_raw_channel(raw).await.unwrap();

            let header = v20::MessageHeader {
                device_index: 0x01,
                feature_index: 0x05,
                function_id: U4::from_lo(0x2),
                software_id: U4::from_lo(0x3),
            };
            let request = v20::Message::Short(header, [0x00, 0x00, 0x00]);
            let response = v20::Message::Short(header, [0xaa, 0xbb, 0xcc]);

            // Each decoy differs from the request in exactly one header field, so
            // none of them may be mistaken for its response.
            let wrong_device = v20::Message::Short(
                v20::MessageHeader {
                    device_index: 0x02,
                    ..header
                },
                [0, 0, 0],
            );
            let wrong_feature = v20::Message::Short(
                v20::MessageHeader {
                    feature_index: 0x06,
                    ..header
                },
                [0, 0, 0],
            );
            let wrong_sw_id = v20::Message::Short(
                v20::MessageHeader {
                    software_id: U4::from_lo(0x4),
                    ..header
                },
                [0, 0, 0],
            );

            let send_fut = channel.send_v20(request);
            let feed_fut = async {
                handle.send_incoming(wrong_device.into()).await;
                handle.send_incoming(wrong_feature.into()).await;
                handle.send_incoming(wrong_sw_id.into()).await;
                handle.send_incoming(response.into()).await;
            };

            let (result, ()) = futures::join!(send_fut, feed_fut);

            assert_eq!(result.unwrap(), response);
            assert_pending_empty(&channel);
        });
    }

    #[test]
    fn send_v20_broadcast_event_does_not_resolve_pending_request() {
        futures::executor::block_on(async {
            let (raw, handle) = MockRawHidChannel::new();
            let channel = HidppChannel::from_raw_channel(raw).await.unwrap();
            let events = Arc::new(Mutex::new(Vec::new()));
            let listener_events = Arc::clone(&events);
            channel.add_msg_listener(move |msg, matched| {
                listener_events.lock().unwrap().push((msg, matched));
            });

            let header = v20::MessageHeader {
                device_index: 0x01,
                feature_index: 0x05,
                function_id: U4::from_lo(0x2),
                software_id: U4::from_lo(0x3),
            };
            let request = v20::Message::Short(header, [0, 0, 0]);
            let response = v20::Message::Short(header, [0xaa, 0xbb, 0xcc]);

            // Software ID 0 is reserved for unsolicited device notifications
            // (see `feature::event_payload`). The request above uses a non-zero
            // ID, so an incoming broadcast sharing device/feature but using ID 0
            // must be routed to listeners, not consumed as this request's
            // response.
            let event = v20::Message::Short(
                v20::MessageHeader {
                    software_id: U4::from_lo(0x0),
                    ..header
                },
                [0x01, 0x02, 0x03],
            );

            let send_fut = channel.send_v20(request);
            let feed_fut = async {
                handle.send_incoming(event.into()).await;
                wait_for_event_count(&events, 1).await;
                handle.send_incoming(response.into()).await;
            };

            let (result, ()) = futures::join!(send_fut, feed_fut);

            assert_eq!(result.unwrap(), response);
            // The oneshot resolves before the listener loop runs on the read
            // thread; wait for both deliveries before asserting on them.
            wait_for_event_count(&events, 2).await;
            let recorded = events.lock().unwrap().clone();
            assert_eq!(
                recorded,
                vec![
                    (HidppMessage::from(event), false),
                    (HidppMessage::from(response), true),
                ]
            );
            assert_pending_empty(&channel);
        });
    }

    #[test]
    fn send_v20_response_may_arrive_as_a_different_report_width() {
        futures::executor::block_on(async {
            let (raw, handle) = MockRawHidChannel::new();
            let channel = HidppChannel::from_raw_channel(raw).await.unwrap();

            let header = v20::MessageHeader {
                device_index: 0x01,
                feature_index: 0x05,
                function_id: U4::from_lo(0x2),
                software_id: U4::from_lo(0x3),
            };
            let request = v20::Message::Short(header, [0, 0, 0]);
            // Quirk: `send_v20`'s response predicate compares only the parsed
            // v20 header, not the underlying report width. A device replying
            // with a long report to a short request — same header, wider
            // payload — is still accepted as the response.
            let response = v20::Message::Long(header, [0xaa; 16]);
            handle.queue_response(response.into());

            let result = channel.send_v20(request).await.unwrap();

            assert_eq!(result, response);
            assert_pending_empty(&channel);
        });
    }

    #[test]
    fn send_v20_error_frame_resolves_to_feature_error() {
        futures::executor::block_on(async {
            let (raw, handle) = MockRawHidChannel::new();
            let channel = HidppChannel::from_raw_channel(raw).await.unwrap();

            let header = v20::MessageHeader {
                device_index: 0x01,
                feature_index: 0x05,
                function_id: U4::from_lo(0x2),
                software_id: U4::from_lo(0x3),
            };
            let request = v20::Message::Short(header, [0, 0, 0]);
            let error_response = v20_error_frame(header, ErrorType::InvalidArgument.into());
            handle.queue_response(error_response.into());

            let err = channel.send_v20(request).await.unwrap_err();

            assert!(matches!(
                err,
                Hidpp20Error::Feature(ErrorType::InvalidArgument)
            ));
            assert_pending_empty(&channel);
        });
    }

    #[test]
    fn send_v20_error_frame_with_unmapped_code_is_unsupported_response() {
        futures::executor::block_on(async {
            let (raw, handle) = MockRawHidChannel::new();
            let channel = HidppChannel::from_raw_channel(raw).await.unwrap();

            let header = v20::MessageHeader {
                device_index: 0x01,
                feature_index: 0x05,
                function_id: U4::from_lo(0x2),
                software_id: U4::from_lo(0x3),
            };
            let request = v20::Message::Short(header, [0, 0, 0]);
            // 0xfe is not a defined `ErrorType` variant.
            let error_response = v20_error_frame(header, 0xfe);
            handle.queue_response(error_response.into());

            let err = channel.send_v20(request).await.unwrap_err();

            assert!(matches!(err, Hidpp20Error::UnsupportedResponse));
            assert_pending_empty(&channel);
        });
    }

    /// Builds the HID++2.0 error-frame encoding for `request_header`: feature
    /// index 0xFF, with the original feature index and function|software byte
    /// shifted one byte to the right (see `v20::HidppChannel::send_v20`'s
    /// `is_error` predicate for the reverse mapping).
    fn v20_error_frame(request_header: v20::MessageHeader, error_code: u8) -> v20::Message {
        let error_header = v20::MessageHeader {
            device_index: request_header.device_index,
            feature_index: 0xff,
            function_id: U4::from_hi(request_header.feature_index),
            software_id: U4::from_lo(request_header.feature_index),
        };
        let mut payload = [0u8; 3];
        payload[0] = nibble::combine(request_header.function_id, request_header.software_id);
        payload[1] = error_code;
        v20::Message::Short(error_header, payload)
    }

    #[derive(Clone)]
    struct MockRawHidHandle {
        incoming_tx: async_channel::Sender<Vec<u8>>,
        written_reports: Arc<Mutex<Vec<Vec<u8>>>>,
        responses_on_write: Arc<Mutex<VecDeque<Vec<u8>>>>,
    }

    impl MockRawHidHandle {
        fn queue_response(&self, msg: HidppMessage) {
            self.responses_on_write
                .lock()
                .unwrap()
                .push_back(raw_report(msg));
        }

        async fn send_incoming(&self, msg: HidppMessage) {
            self.incoming_tx.send(raw_report(msg)).await.unwrap();
        }

        fn written_reports(&self) -> Vec<Vec<u8>> {
            self.written_reports.lock().unwrap().clone()
        }
    }

    struct MockRawHidChannel {
        incoming_tx: async_channel::Sender<Vec<u8>>,
        incoming_rx: async_channel::Receiver<Vec<u8>>,
        written_reports: Arc<Mutex<Vec<Vec<u8>>>>,
        responses_on_write: Arc<Mutex<VecDeque<Vec<u8>>>>,
    }

    impl MockRawHidChannel {
        fn new() -> (Self, MockRawHidHandle) {
            let (incoming_tx, incoming_rx) = async_channel::unbounded();
            let written_reports = Arc::new(Mutex::new(Vec::new()));
            let responses_on_write = Arc::new(Mutex::new(VecDeque::new()));

            let handle = MockRawHidHandle {
                incoming_tx: incoming_tx.clone(),
                written_reports: Arc::clone(&written_reports),
                responses_on_write: Arc::clone(&responses_on_write),
            };

            (
                Self {
                    incoming_tx,
                    incoming_rx,
                    written_reports,
                    responses_on_write,
                },
                handle,
            )
        }
    }

    #[async_trait]
    impl RawHidChannel for MockRawHidChannel {
        fn vendor_id(&self) -> u16 {
            0x046d
        }

        fn product_id(&self) -> u16 {
            0xc539
        }

        async fn write_report(&self, src: &[u8]) -> Result<usize, Box<dyn Error + Sync + Send>> {
            self.written_reports.lock().unwrap().push(src.to_vec());
            let response = self.responses_on_write.lock().unwrap().pop_front();
            if let Some(response) = response {
                self.incoming_tx.send(response).await.unwrap();
            }

            Ok(src.len())
        }

        async fn read_report(&self, buf: &mut [u8]) -> Result<usize, Box<dyn Error + Sync + Send>> {
            let report = self.incoming_rx.recv().await.map_err(|_| mock_error())?;
            let len = report.len().min(buf.len());
            buf[..len].copy_from_slice(&report[..len]);
            Ok(len)
        }

        fn supports_short_long_hidpp(&self) -> Option<(bool, bool)> {
            Some((true, true))
        }

        async fn get_report_descriptor(
            &self,
            _buf: &mut [u8],
        ) -> Result<usize, Box<dyn Error + Sync + Send>> {
            unreachable!("mock declares HID++ support")
        }
    }

    fn short_msg(marker: u8) -> HidppMessage {
        HidppMessage::Short([0xff, marker, 0x10, marker, marker, marker])
    }

    fn raw_report(msg: HidppMessage) -> Vec<u8> {
        let mut buf = [0u8; LONG_REPORT_LENGTH];
        let len = msg.write_raw(&mut buf);
        buf[..len].to_vec()
    }

    fn assert_pending_empty(channel: &HidppChannel) {
        assert!(channel.pending_messages.lock().unwrap().is_empty());
    }

    async fn wait_for_event_count(events: &Arc<Mutex<Vec<(HidppMessage, bool)>>>, count: usize) {
        let started = Instant::now();
        while started.elapsed() < Duration::from_secs(1) {
            if events.lock().unwrap().len() >= count {
                return;
            }
            futures_timer::Delay::new(Duration::from_millis(10)).await;
        }

        panic!("timed out waiting for {count} listener events");
    }

    async fn wait_for_atomic_count(count: &AtomicUsize, expected: usize) {
        let started = Instant::now();
        while started.elapsed() < Duration::from_secs(1) {
            if count.load(Ordering::SeqCst) >= expected {
                return;
            }
            futures_timer::Delay::new(Duration::from_millis(10)).await;
        }

        panic!("timed out waiting for atomic count {expected}");
    }

    fn mock_error() -> Box<dyn Error + Sync + Send> {
        Box::new(io::Error::new(
            io::ErrorKind::BrokenPipe,
            "mock channel closed",
        ))
    }
}