Vendor things

third-party/vendor/crossbeam-channel/src/flavors/array.rs

@@ -0,0 +1,637 @@
+//! Bounded channel based on a preallocated array.
+//!
+//! This flavor has a fixed, positive capacity.
+//!
+//! The implementation is based on Dmitry Vyukov's bounded MPMC queue.
+//!
+//! Source:
+//!   - <http://www.1024cores.net/home/lock-free-algorithms/queues/bounded-mpmc-queue>
+//!   - <https://docs.google.com/document/d/1yIAYmbvL3JxOKOjuCyon7JhW4cSv1wy5hC0ApeGMV9s/pub>
+
+use std::boxed::Box;
+use std::cell::UnsafeCell;
+use std::mem::{self, MaybeUninit};
+use std::ptr;
+use std::sync::atomic::{self, AtomicUsize, Ordering};
+use std::time::Instant;
+
+use crossbeam_utils::{Backoff, CachePadded};
+
+use crate::context::Context;
+use crate::err::{RecvTimeoutError, SendTimeoutError, TryRecvError, TrySendError};
+use crate::select::{Operation, SelectHandle, Selected, Token};
+use crate::waker::SyncWaker;
+
+/// A slot in a channel.
+struct Slot<T> {
+    /// The current stamp.
+    stamp: AtomicUsize,
+
+    /// The message in this slot.
+    msg: UnsafeCell<MaybeUninit<T>>,
+}
+
+/// The token type for the array flavor.
+#[derive(Debug)]
+pub(crate) struct ArrayToken {
+    /// Slot to read from or write to.
+    slot: *const u8,
+
+    /// Stamp to store into the slot after reading or writing.
+    stamp: usize,
+}
+
+impl Default for ArrayToken {
+    #[inline]
+    fn default() -> Self {
+        ArrayToken {
+            slot: ptr::null(),
+            stamp: 0,
+        }
+    }
+}
+
+/// Bounded channel based on a preallocated array.
+pub(crate) struct Channel<T> {
+    /// The head of the channel.
+    ///
+    /// This value is a "stamp" consisting of an index into the buffer, a mark bit, and a lap, but
+    /// packed into a single `usize`. The lower bits represent the index, while the upper bits
+    /// represent the lap. The mark bit in the head is always zero.
+    ///
+    /// Messages are popped from the head of the channel.
+    head: CachePadded<AtomicUsize>,
+
+    /// The tail of the channel.
+    ///
+    /// This value is a "stamp" consisting of an index into the buffer, a mark bit, and a lap, but
+    /// packed into a single `usize`. The lower bits represent the index, while the upper bits
+    /// represent the lap. The mark bit indicates that the channel is disconnected.
+    ///
+    /// Messages are pushed into the tail of the channel.
+    tail: CachePadded<AtomicUsize>,
+
+    /// The buffer holding slots.
+    buffer: Box<[Slot<T>]>,
+
+    /// The channel capacity.
+    cap: usize,
+
+    /// A stamp with the value of `{ lap: 1, mark: 0, index: 0 }`.
+    one_lap: usize,
+
+    /// If this bit is set in the tail, that means the channel is disconnected.
+    mark_bit: usize,
+
+    /// Senders waiting while the channel is full.
+    senders: SyncWaker,
+
+    /// Receivers waiting while the channel is empty and not disconnected.
+    receivers: SyncWaker,
+}
+
+impl<T> Channel<T> {
+    /// Creates a bounded channel of capacity `cap`.
+    pub(crate) fn with_capacity(cap: usize) -> Self {
+        assert!(cap > 0, "capacity must be positive");
+
+        // Compute constants `mark_bit` and `one_lap`.
+        let mark_bit = (cap + 1).next_power_of_two();
+        let one_lap = mark_bit * 2;
+
+        // Head is initialized to `{ lap: 0, mark: 0, index: 0 }`.
+        let head = 0;
+        // Tail is initialized to `{ lap: 0, mark: 0, index: 0 }`.
+        let tail = 0;
+
+        // Allocate a buffer of `cap` slots initialized
+        // with stamps.
+        let buffer: Box<[Slot<T>]> = (0..cap)
+            .map(|i| {
+                // Set the stamp to `{ lap: 0, mark: 0, index: i }`.
+                Slot {
+                    stamp: AtomicUsize::new(i),
+                    msg: UnsafeCell::new(MaybeUninit::uninit()),
+                }
+            })
+            .collect();
+
+        Channel {
+            buffer,
+            cap,
+            one_lap,
+            mark_bit,
+            head: CachePadded::new(AtomicUsize::new(head)),
+            tail: CachePadded::new(AtomicUsize::new(tail)),
+            senders: SyncWaker::new(),
+            receivers: SyncWaker::new(),
+        }
+    }
+
+    /// Returns a receiver handle to the channel.
+    pub(crate) fn receiver(&self) -> Receiver<'_, T> {
+        Receiver(self)
+    }
+
+    /// Returns a sender handle to the channel.
+    pub(crate) fn sender(&self) -> Sender<'_, T> {
+        Sender(self)
+    }
+
+    /// Attempts to reserve a slot for sending a message.
+    fn start_send(&self, token: &mut Token) -> bool {
+        let backoff = Backoff::new();
+        let mut tail = self.tail.load(Ordering::Relaxed);
+
+        loop {
+            // Check if the channel is disconnected.
+            if tail & self.mark_bit != 0 {
+                token.array.slot = ptr::null();
+                token.array.stamp = 0;
+                return true;
+            }
+
+            // Deconstruct the tail.
+            let index = tail & (self.mark_bit - 1);
+            let lap = tail & !(self.one_lap - 1);
+
+            // Inspect the corresponding slot.
+            debug_assert!(index < self.buffer.len());
+            let slot = unsafe { self.buffer.get_unchecked(index) };
+            let stamp = slot.stamp.load(Ordering::Acquire);
+
+            // If the tail and the stamp match, we may attempt to push.
+            if tail == stamp {
+                let new_tail = if index + 1 < self.cap {
+                    // Same lap, incremented index.
+                    // Set to `{ lap: lap, mark: 0, index: index + 1 }`.
+                    tail + 1
+                } else {
+                    // One lap forward, index wraps around to zero.
+                    // Set to `{ lap: lap.wrapping_add(1), mark: 0, index: 0 }`.
+                    lap.wrapping_add(self.one_lap)
+                };
+
+                // Try moving the tail.
+                match self.tail.compare_exchange_weak(
+                    tail,
+                    new_tail,
+                    Ordering::SeqCst,
+                    Ordering::Relaxed,
+                ) {
+                    Ok(_) => {
+                        // Prepare the token for the follow-up call to `write`.
+                        token.array.slot = slot as *const Slot<T> as *const u8;
+                        token.array.stamp = tail + 1;
+                        return true;
+                    }
+                    Err(t) => {
+                        tail = t;
+                        backoff.spin();
+                    }
+                }
+            } else if stamp.wrapping_add(self.one_lap) == tail + 1 {
+                atomic::fence(Ordering::SeqCst);
+                let head = self.head.load(Ordering::Relaxed);
+
+                // If the head lags one lap behind the tail as well...
+                if head.wrapping_add(self.one_lap) == tail {
+                    // ...then the channel is full.
+                    return false;
+                }
+
+                backoff.spin();
+                tail = self.tail.load(Ordering::Relaxed);
+            } else {
+                // Snooze because we need to wait for the stamp to get updated.
+                backoff.snooze();
+                tail = self.tail.load(Ordering::Relaxed);
+            }
+        }
+    }
+
+    /// Writes a message into the channel.
+    pub(crate) unsafe fn write(&self, token: &mut Token, msg: T) -> Result<(), T> {
+        // If there is no slot, the channel is disconnected.
+        if token.array.slot.is_null() {
+            return Err(msg);
+        }
+
+        let slot: &Slot<T> = &*token.array.slot.cast::<Slot<T>>();
+
+        // Write the message into the slot and update the stamp.
+        slot.msg.get().write(MaybeUninit::new(msg));
+        slot.stamp.store(token.array.stamp, Ordering::Release);
+
+        // Wake a sleeping receiver.
+        self.receivers.notify();
+        Ok(())
+    }
+
+    /// Attempts to reserve a slot for receiving a message.
+    fn start_recv(&self, token: &mut Token) -> bool {
+        let backoff = Backoff::new();
+        let mut head = self.head.load(Ordering::Relaxed);
+
+        loop {
+            // Deconstruct the head.
+            let index = head & (self.mark_bit - 1);
+            let lap = head & !(self.one_lap - 1);
+
+            // Inspect the corresponding slot.
+            debug_assert!(index < self.buffer.len());
+            let slot = unsafe { self.buffer.get_unchecked(index) };
+            let stamp = slot.stamp.load(Ordering::Acquire);
+
+            // If the the stamp is ahead of the head by 1, we may attempt to pop.
+            if head + 1 == stamp {
+                let new = if index + 1 < self.cap {
+                    // Same lap, incremented index.
+                    // Set to `{ lap: lap, mark: 0, index: index + 1 }`.
+                    head + 1
+                } else {
+                    // One lap forward, index wraps around to zero.
+                    // Set to `{ lap: lap.wrapping_add(1), mark: 0, index: 0 }`.
+                    lap.wrapping_add(self.one_lap)
+                };
+
+                // Try moving the head.
+                match self.head.compare_exchange_weak(
+                    head,
+                    new,
+                    Ordering::SeqCst,
+                    Ordering::Relaxed,
+                ) {
+                    Ok(_) => {
+                        // Prepare the token for the follow-up call to `read`.
+                        token.array.slot = slot as *const Slot<T> as *const u8;
+                        token.array.stamp = head.wrapping_add(self.one_lap);
+                        return true;
+                    }
+                    Err(h) => {
+                        head = h;
+                        backoff.spin();
+                    }
+                }
+            } else if stamp == head {
+                atomic::fence(Ordering::SeqCst);
+                let tail = self.tail.load(Ordering::Relaxed);
+
+                // If the tail equals the head, that means the channel is empty.
+                if (tail & !self.mark_bit) == head {
+                    // If the channel is disconnected...
+                    if tail & self.mark_bit != 0 {
+                        // ...then receive an error.
+                        token.array.slot = ptr::null();
+                        token.array.stamp = 0;
+                        return true;
+                    } else {
+                        // Otherwise, the receive operation is not ready.
+                        return false;
+                    }
+                }
+
+                backoff.spin();
+                head = self.head.load(Ordering::Relaxed);
+            } else {
+                // Snooze because we need to wait for the stamp to get updated.
+                backoff.snooze();
+                head = self.head.load(Ordering::Relaxed);
+            }
+        }
+    }
+
+    /// Reads a message from the channel.
+    pub(crate) unsafe fn read(&self, token: &mut Token) -> Result<T, ()> {
+        if token.array.slot.is_null() {
+            // The channel is disconnected.
+            return Err(());
+        }
+
+        let slot: &Slot<T> = &*token.array.slot.cast::<Slot<T>>();
+
+        // Read the message from the slot and update the stamp.
+        let msg = slot.msg.get().read().assume_init();
+        slot.stamp.store(token.array.stamp, Ordering::Release);
+
+        // Wake a sleeping sender.
+        self.senders.notify();
+        Ok(msg)
+    }
+
+    /// Attempts to send a message into the channel.
+    pub(crate) fn try_send(&self, msg: T) -> Result<(), TrySendError<T>> {
+        let token = &mut Token::default();
+        if self.start_send(token) {
+            unsafe { self.write(token, msg).map_err(TrySendError::Disconnected) }
+        } else {
+            Err(TrySendError::Full(msg))
+        }
+    }
+
+    /// Sends a message into the channel.
+    pub(crate) fn send(
+        &self,
+        msg: T,
+        deadline: Option<Instant>,
+    ) -> Result<(), SendTimeoutError<T>> {
+        let token = &mut Token::default();
+        loop {
+            // Try sending a message several times.
+            let backoff = Backoff::new();
+            loop {
+                if self.start_send(token) {
+                    let res = unsafe { self.write(token, msg) };
+                    return res.map_err(SendTimeoutError::Disconnected);
+                }
+
+                if backoff.is_completed() {
+                    break;
+                } else {
+                    backoff.snooze();
+                }
+            }
+
+            if let Some(d) = deadline {
+                if Instant::now() >= d {
+                    return Err(SendTimeoutError::Timeout(msg));
+                }
+            }
+
+            Context::with(|cx| {
+                // Prepare for blocking until a receiver wakes us up.
+                let oper = Operation::hook(token);
+                self.senders.register(oper, cx);
+
+                // Has the channel become ready just now?
+                if !self.is_full() || self.is_disconnected() {
+                    let _ = cx.try_select(Selected::Aborted);
+                }
+
+                // Block the current thread.
+                let sel = cx.wait_until(deadline);
+
+                match sel {
+                    Selected::Waiting => unreachable!(),
+                    Selected::Aborted | Selected::Disconnected => {
+                        self.senders.unregister(oper).unwrap();
+                    }
+                    Selected::Operation(_) => {}
+                }
+            });
+        }
+    }
+
+    /// Attempts to receive a message without blocking.
+    pub(crate) fn try_recv(&self) -> Result<T, TryRecvError> {
+        let token = &mut Token::default();
+
+        if self.start_recv(token) {
+            unsafe { self.read(token).map_err(|_| TryRecvError::Disconnected) }
+        } else {
+            Err(TryRecvError::Empty)
+        }
+    }
+
+    /// Receives a message from the channel.
+    pub(crate) fn recv(&self, deadline: Option<Instant>) -> Result<T, RecvTimeoutError> {
+        let token = &mut Token::default();
+        loop {
+            // Try receiving a message several times.
+            let backoff = Backoff::new();
+            loop {
+                if self.start_recv(token) {
+                    let res = unsafe { self.read(token) };
+                    return res.map_err(|_| RecvTimeoutError::Disconnected);
+                }
+
+                if backoff.is_completed() {
+                    break;
+                } else {
+                    backoff.snooze();
+                }
+            }
+
+            if let Some(d) = deadline {
+                if Instant::now() >= d {
+                    return Err(RecvTimeoutError::Timeout);
+                }
+            }
+
+            Context::with(|cx| {
+                // Prepare for blocking until a sender wakes us up.
+                let oper = Operation::hook(token);
+                self.receivers.register(oper, cx);
+
+                // Has the channel become ready just now?
+                if !self.is_empty() || self.is_disconnected() {
+                    let _ = cx.try_select(Selected::Aborted);
+                }
+
+                // Block the current thread.
+                let sel = cx.wait_until(deadline);
+
+                match sel {
+                    Selected::Waiting => unreachable!(),
+                    Selected::Aborted | Selected::Disconnected => {
+                        self.receivers.unregister(oper).unwrap();
+                        // If the channel was disconnected, we still have to check for remaining
+                        // messages.
+                    }
+                    Selected::Operation(_) => {}
+                }
+            });
+        }
+    }
+
+    /// Returns the current number of messages inside the channel.
+    pub(crate) fn len(&self) -> usize {
+        loop {
+            // Load the tail, then load the head.
+            let tail = self.tail.load(Ordering::SeqCst);
+            let head = self.head.load(Ordering::SeqCst);
+
+            // If the tail didn't change, we've got consistent values to work with.
+            if self.tail.load(Ordering::SeqCst) == tail {
+                let hix = head & (self.mark_bit - 1);
+                let tix = tail & (self.mark_bit - 1);
+
+                return if hix < tix {
+                    tix - hix
+                } else if hix > tix {
+                    self.cap - hix + tix
+                } else if (tail & !self.mark_bit) == head {
+                    0
+                } else {
+                    self.cap
+                };
+            }
+        }
+    }
+
+    /// Returns the capacity of the channel.
+    pub(crate) fn capacity(&self) -> Option<usize> {
+        Some(self.cap)
+    }
+
+    /// Disconnects the channel and wakes up all blocked senders and receivers.
+    ///
+    /// Returns `true` if this call disconnected the channel.
+    pub(crate) fn disconnect(&self) -> bool {
+        let tail = self.tail.fetch_or(self.mark_bit, Ordering::SeqCst);
+
+        if tail & self.mark_bit == 0 {
+            self.senders.disconnect();
+            self.receivers.disconnect();
+            true
+        } else {
+            false
+        }
+    }
+
+    /// Returns `true` if the channel is disconnected.
+    pub(crate) fn is_disconnected(&self) -> bool {
+        self.tail.load(Ordering::SeqCst) & self.mark_bit != 0
+    }
+
+    /// Returns `true` if the channel is empty.
+    pub(crate) fn is_empty(&self) -> bool {
+        let head = self.head.load(Ordering::SeqCst);
+        let tail = self.tail.load(Ordering::SeqCst);
+
+        // Is the tail equal to the head?
+        //
+        // Note: If the head changes just before we load the tail, that means there was a moment
+        // when the channel was not empty, so it is safe to just return `false`.
+        (tail & !self.mark_bit) == head
+    }
+
+    /// Returns `true` if the channel is full.
+    pub(crate) fn is_full(&self) -> bool {
+        let tail = self.tail.load(Ordering::SeqCst);
+        let head = self.head.load(Ordering::SeqCst);
+
+        // Is the head lagging one lap behind tail?
+        //
+        // Note: If the tail changes just before we load the head, that means there was a moment
+        // when the channel was not full, so it is safe to just return `false`.
+        head.wrapping_add(self.one_lap) == tail & !self.mark_bit
+    }
+}
+
+impl<T> Drop for Channel<T> {
+    fn drop(&mut self) {
+        if mem::needs_drop::<T>() {
+            // Get the index of the head.
+            let head = *self.head.get_mut();
+            let tail = *self.tail.get_mut();
+
+            let hix = head & (self.mark_bit - 1);
+            let tix = tail & (self.mark_bit - 1);
+
+            let len = if hix < tix {
+                tix - hix
+            } else if hix > tix {
+                self.cap - hix + tix
+            } else if (tail & !self.mark_bit) == head {
+                0
+            } else {
+                self.cap
+            };
+
+            // Loop over all slots that hold a message and drop them.
+            for i in 0..len {
+                // Compute the index of the next slot holding a message.
+                let index = if hix + i < self.cap {
+                    hix + i
+                } else {
+                    hix + i - self.cap
+                };
+
+                unsafe {
+                    debug_assert!(index < self.buffer.len());
+                    let slot = self.buffer.get_unchecked_mut(index);
+                    (*slot.msg.get()).assume_init_drop();
+                }
+            }
+        }
+    }
+}
+
+/// Receiver handle to a channel.
+pub(crate) struct Receiver<'a, T>(&'a Channel<T>);
+
+/// Sender handle to a channel.
+pub(crate) struct Sender<'a, T>(&'a Channel<T>);
+
+impl<T> SelectHandle for Receiver<'_, T> {
+    fn try_select(&self, token: &mut Token) -> bool {
+        self.0.start_recv(token)
+    }
+
+    fn deadline(&self) -> Option<Instant> {
+        None
+    }
+
+    fn register(&self, oper: Operation, cx: &Context) -> bool {
+        self.0.receivers.register(oper, cx);
+        self.is_ready()
+    }
+
+    fn unregister(&self, oper: Operation) {
+        self.0.receivers.unregister(oper);
+    }
+
+    fn accept(&self, token: &mut Token, _cx: &Context) -> bool {
+        self.try_select(token)
+    }
+
+    fn is_ready(&self) -> bool {
+        !self.0.is_empty() || self.0.is_disconnected()
+    }
+
+    fn watch(&self, oper: Operation, cx: &Context) -> bool {
+        self.0.receivers.watch(oper, cx);
+        self.is_ready()
+    }
+
+    fn unwatch(&self, oper: Operation) {
+        self.0.receivers.unwatch(oper);
+    }
+}
+
+impl<T> SelectHandle for Sender<'_, T> {
+    fn try_select(&self, token: &mut Token) -> bool {
+        self.0.start_send(token)
+    }
+
+    fn deadline(&self) -> Option<Instant> {
+        None
+    }
+
+    fn register(&self, oper: Operation, cx: &Context) -> bool {
+        self.0.senders.register(oper, cx);
+        self.is_ready()
+    }
+
+    fn unregister(&self, oper: Operation) {
+        self.0.senders.unregister(oper);
+    }
+
+    fn accept(&self, token: &mut Token, _cx: &Context) -> bool {
+        self.try_select(token)
+    }
+
+    fn is_ready(&self) -> bool {
+        !self.0.is_full() || self.0.is_disconnected()
+    }
+
+    fn watch(&self, oper: Operation, cx: &Context) -> bool {
+        self.0.senders.watch(oper, cx);
+        self.is_ready()
+    }
+
+    fn unwatch(&self, oper: Operation) {
+        self.0.senders.unwatch(oper);
+    }
+}

third-party/vendor/crossbeam-channel/src/flavors/at.rs

@@ -0,0 +1,197 @@
+//! Channel that delivers a message at a certain moment in time.
+//!
+//! Messages cannot be sent into this kind of channel; they are materialized on demand.
+
+use std::sync::atomic::{AtomicBool, Ordering};
+use std::thread;
+use std::time::Instant;
+
+use crate::context::Context;
+use crate::err::{RecvTimeoutError, TryRecvError};
+use crate::select::{Operation, SelectHandle, Token};
+use crate::utils;
+
+/// Result of a receive operation.
+pub(crate) type AtToken = Option<Instant>;
+
+/// Channel that delivers a message at a certain moment in time
+pub(crate) struct Channel {
+    /// The instant at which the message will be delivered.
+    delivery_time: Instant,
+
+    /// `true` if the message has been received.
+    received: AtomicBool,
+}
+
+impl Channel {
+    /// Creates a channel that delivers a message at a certain instant in time.
+    #[inline]
+    pub(crate) fn new_deadline(when: Instant) -> Self {
+        Channel {
+            delivery_time: when,
+            received: AtomicBool::new(false),
+        }
+    }
+
+    /// Attempts to receive a message without blocking.
+    #[inline]
+    pub(crate) fn try_recv(&self) -> Result<Instant, TryRecvError> {
+        // We use relaxed ordering because this is just an optional optimistic check.
+        if self.received.load(Ordering::Relaxed) {
+            // The message has already been received.
+            return Err(TryRecvError::Empty);
+        }
+
+        if Instant::now() < self.delivery_time {
+            // The message was not delivered yet.
+            return Err(TryRecvError::Empty);
+        }
+
+        // Try receiving the message if it is still available.
+        if !self.received.swap(true, Ordering::SeqCst) {
+            // Success! Return delivery time as the message.
+            Ok(self.delivery_time)
+        } else {
+            // The message was already received.
+            Err(TryRecvError::Empty)
+        }
+    }
+
+    /// Receives a message from the channel.
+    #[inline]
+    pub(crate) fn recv(&self, deadline: Option<Instant>) -> Result<Instant, RecvTimeoutError> {
+        // We use relaxed ordering because this is just an optional optimistic check.
+        if self.received.load(Ordering::Relaxed) {
+            // The message has already been received.
+            utils::sleep_until(deadline);
+            return Err(RecvTimeoutError::Timeout);
+        }
+
+        // Wait until the message is received or the deadline is reached.
+        loop {
+            let now = Instant::now();
+
+            let deadline = match deadline {
+                // Check if we can receive the next message.
+                _ if now >= self.delivery_time => break,
+                // Check if the timeout deadline has been reached.
+                Some(d) if now >= d => return Err(RecvTimeoutError::Timeout),
+
+                // Sleep until one of the above happens
+                Some(d) if d < self.delivery_time => d,
+                _ => self.delivery_time,
+            };
+
+            thread::sleep(deadline - now);
+        }
+
+        // Try receiving the message if it is still available.
+        if !self.received.swap(true, Ordering::SeqCst) {
+            // Success! Return the message, which is the instant at which it was delivered.
+            Ok(self.delivery_time)
+        } else {
+            // The message was already received. Block forever.
+            utils::sleep_until(None);
+            unreachable!()
+        }
+    }
+
+    /// Reads a message from the channel.
+    #[inline]
+    pub(crate) unsafe fn read(&self, token: &mut Token) -> Result<Instant, ()> {
+        token.at.ok_or(())
+    }
+
+    /// Returns `true` if the channel is empty.
+    #[inline]
+    pub(crate) fn is_empty(&self) -> bool {
+        // We use relaxed ordering because this is just an optional optimistic check.
+        if self.received.load(Ordering::Relaxed) {
+            return true;
+        }
+
+        // If the delivery time hasn't been reached yet, the channel is empty.
+        if Instant::now() < self.delivery_time {
+            return true;
+        }
+
+        // The delivery time has been reached. The channel is empty only if the message has already
+        // been received.
+        self.received.load(Ordering::SeqCst)
+    }
+
+    /// Returns `true` if the channel is full.
+    #[inline]
+    pub(crate) fn is_full(&self) -> bool {
+        !self.is_empty()
+    }
+
+    /// Returns the number of messages in the channel.
+    #[inline]
+    pub(crate) fn len(&self) -> usize {
+        if self.is_empty() {
+            0
+        } else {
+            1
+        }
+    }
+
+    /// Returns the capacity of the channel.
+    #[inline]
+    pub(crate) fn capacity(&self) -> Option<usize> {
+        Some(1)
+    }
+}
+
+impl SelectHandle for Channel {
+    #[inline]
+    fn try_select(&self, token: &mut Token) -> bool {
+        match self.try_recv() {
+            Ok(msg) => {
+                token.at = Some(msg);
+                true
+            }
+            Err(TryRecvError::Disconnected) => {
+                token.at = None;
+                true
+            }
+            Err(TryRecvError::Empty) => false,
+        }
+    }
+
+    #[inline]
+    fn deadline(&self) -> Option<Instant> {
+        // We use relaxed ordering because this is just an optional optimistic check.
+        if self.received.load(Ordering::Relaxed) {
+            None
+        } else {
+            Some(self.delivery_time)
+        }
+    }
+
+    #[inline]
+    fn register(&self, _oper: Operation, _cx: &Context) -> bool {
+        self.is_ready()
+    }
+
+    #[inline]
+    fn unregister(&self, _oper: Operation) {}
+
+    #[inline]
+    fn accept(&self, token: &mut Token, _cx: &Context) -> bool {
+        self.try_select(token)
+    }
+
+    #[inline]
+    fn is_ready(&self) -> bool {
+        !self.is_empty()
+    }
+
+    #[inline]
+    fn watch(&self, _oper: Operation, _cx: &Context) -> bool {
+        self.is_ready()
+    }
+
+    #[inline]
+    fn unwatch(&self, _oper: Operation) {}
+}

third-party/vendor/crossbeam-channel/src/flavors/list.rs

@@ -0,0 +1,755 @@
+//! Unbounded channel implemented as a linked list.
+
+use std::boxed::Box;
+use std::cell::UnsafeCell;
+use std::marker::PhantomData;
+use std::mem::MaybeUninit;
+use std::ptr;
+use std::sync::atomic::{self, AtomicPtr, AtomicUsize, Ordering};
+use std::time::Instant;
+
+use crossbeam_utils::{Backoff, CachePadded};
+
+use crate::context::Context;
+use crate::err::{RecvTimeoutError, SendTimeoutError, TryRecvError, TrySendError};
+use crate::select::{Operation, SelectHandle, Selected, Token};
+use crate::waker::SyncWaker;
+
+// TODO(stjepang): Once we bump the minimum required Rust version to 1.28 or newer, re-apply the
+// following changes by @kleimkuhler:
+//
+// 1. https://github.com/crossbeam-rs/crossbeam-channel/pull/100
+// 2. https://github.com/crossbeam-rs/crossbeam-channel/pull/101
+
+// Bits indicating the state of a slot:
+// * If a message has been written into the slot, `WRITE` is set.
+// * If a message has been read from the slot, `READ` is set.
+// * If the block is being destroyed, `DESTROY` is set.
+const WRITE: usize = 1;
+const READ: usize = 2;
+const DESTROY: usize = 4;
+
+// Each block covers one "lap" of indices.
+const LAP: usize = 32;
+// The maximum number of messages a block can hold.
+const BLOCK_CAP: usize = LAP - 1;
+// How many lower bits are reserved for metadata.
+const SHIFT: usize = 1;
+// Has two different purposes:
+// * If set in head, indicates that the block is not the last one.
+// * If set in tail, indicates that the channel is disconnected.
+const MARK_BIT: usize = 1;
+
+/// A slot in a block.
+struct Slot<T> {
+    /// The message.
+    msg: UnsafeCell<MaybeUninit<T>>,
+
+    /// The state of the slot.
+    state: AtomicUsize,
+}
+
+impl<T> Slot<T> {
+    const UNINIT: Self = Self {
+        msg: UnsafeCell::new(MaybeUninit::uninit()),
+        state: AtomicUsize::new(0),
+    };
+
+    /// Waits until a message is written into the slot.
+    fn wait_write(&self) {
+        let backoff = Backoff::new();
+        while self.state.load(Ordering::Acquire) & WRITE == 0 {
+            backoff.snooze();
+        }
+    }
+}
+
+/// A block in a linked list.
+///
+/// Each block in the list can hold up to `BLOCK_CAP` messages.
+struct Block<T> {
+    /// The next block in the linked list.
+    next: AtomicPtr<Block<T>>,
+
+    /// Slots for messages.
+    slots: [Slot<T>; BLOCK_CAP],
+}
+
+impl<T> Block<T> {
+    /// Creates an empty block.
+    fn new() -> Block<T> {
+        Self {
+            next: AtomicPtr::new(ptr::null_mut()),
+            slots: [Slot::UNINIT; BLOCK_CAP],
+        }
+    }
+
+    /// Waits until the next pointer is set.
+    fn wait_next(&self) -> *mut Block<T> {
+        let backoff = Backoff::new();
+        loop {
+            let next = self.next.load(Ordering::Acquire);
+            if !next.is_null() {
+                return next;
+            }
+            backoff.snooze();
+        }
+    }
+
+    /// Sets the `DESTROY` bit in slots starting from `start` and destroys the block.
+    unsafe fn destroy(this: *mut Block<T>, start: usize) {
+        // It is not necessary to set the `DESTROY` bit in the last slot because that slot has
+        // begun destruction of the block.
+        for i in start..BLOCK_CAP - 1 {
+            let slot = (*this).slots.get_unchecked(i);
+
+            // Mark the `DESTROY` bit if a thread is still using the slot.
+            if slot.state.load(Ordering::Acquire) & READ == 0
+                && slot.state.fetch_or(DESTROY, Ordering::AcqRel) & READ == 0
+            {
+                // If a thread is still using the slot, it will continue destruction of the block.
+                return;
+            }
+        }
+
+        // No thread is using the block, now it is safe to destroy it.
+        drop(Box::from_raw(this));
+    }
+}
+
+/// A position in a channel.
+#[derive(Debug)]
+struct Position<T> {
+    /// The index in the channel.
+    index: AtomicUsize,
+
+    /// The block in the linked list.
+    block: AtomicPtr<Block<T>>,
+}
+
+/// The token type for the list flavor.
+#[derive(Debug)]
+pub(crate) struct ListToken {
+    /// The block of slots.
+    block: *const u8,
+
+    /// The offset into the block.
+    offset: usize,
+}
+
+impl Default for ListToken {
+    #[inline]
+    fn default() -> Self {
+        ListToken {
+            block: ptr::null(),
+            offset: 0,
+        }
+    }
+}
+
+/// Unbounded channel implemented as a linked list.
+///
+/// Each message sent into the channel is assigned a sequence number, i.e. an index. Indices are
+/// represented as numbers of type `usize` and wrap on overflow.
+///
+/// Consecutive messages are grouped into blocks in order to put less pressure on the allocator and
+/// improve cache efficiency.
+pub(crate) struct Channel<T> {
+    /// The head of the channel.
+    head: CachePadded<Position<T>>,
+
+    /// The tail of the channel.
+    tail: CachePadded<Position<T>>,
+
+    /// Receivers waiting while the channel is empty and not disconnected.
+    receivers: SyncWaker,
+
+    /// Indicates that dropping a `Channel<T>` may drop messages of type `T`.
+    _marker: PhantomData<T>,
+}
+
+impl<T> Channel<T> {
+    /// Creates a new unbounded channel.
+    pub(crate) fn new() -> Self {
+        Channel {
+            head: CachePadded::new(Position {
+                block: AtomicPtr::new(ptr::null_mut()),
+                index: AtomicUsize::new(0),
+            }),
+            tail: CachePadded::new(Position {
+                block: AtomicPtr::new(ptr::null_mut()),
+                index: AtomicUsize::new(0),
+            }),
+            receivers: SyncWaker::new(),
+            _marker: PhantomData,
+        }
+    }
+
+    /// Returns a receiver handle to the channel.
+    pub(crate) fn receiver(&self) -> Receiver<'_, T> {
+        Receiver(self)
+    }
+
+    /// Returns a sender handle to the channel.
+    pub(crate) fn sender(&self) -> Sender<'_, T> {
+        Sender(self)
+    }
+
+    /// Attempts to reserve a slot for sending a message.
+    fn start_send(&self, token: &mut Token) -> bool {
+        let backoff = Backoff::new();
+        let mut tail = self.tail.index.load(Ordering::Acquire);
+        let mut block = self.tail.block.load(Ordering::Acquire);
+        let mut next_block = None;
+
+        loop {
+            // Check if the channel is disconnected.
+            if tail & MARK_BIT != 0 {
+                token.list.block = ptr::null();
+                return true;
+            }
+
+            // Calculate the offset of the index into the block.
+            let offset = (tail >> SHIFT) % LAP;
+
+            // If we reached the end of the block, wait until the next one is installed.
+            if offset == BLOCK_CAP {
+                backoff.snooze();
+                tail = self.tail.index.load(Ordering::Acquire);
+                block = self.tail.block.load(Ordering::Acquire);
+                continue;
+            }
+
+            // If we're going to have to install the next block, allocate it in advance in order to
+            // make the wait for other threads as short as possible.
+            if offset + 1 == BLOCK_CAP && next_block.is_none() {
+                next_block = Some(Box::new(Block::<T>::new()));
+            }
+
+            // If this is the first message to be sent into the channel, we need to allocate the
+            // first block and install it.
+            if block.is_null() {
+                let new = Box::into_raw(Box::new(Block::<T>::new()));
+
+                if self
+                    .tail
+                    .block
+                    .compare_exchange(block, new, Ordering::Release, Ordering::Relaxed)
+                    .is_ok()
+                {
+                    self.head.block.store(new, Ordering::Release);
+                    block = new;
+                } else {
+                    next_block = unsafe { Some(Box::from_raw(new)) };
+                    tail = self.tail.index.load(Ordering::Acquire);
+                    block = self.tail.block.load(Ordering::Acquire);
+                    continue;
+                }
+            }
+
+            let new_tail = tail + (1 << SHIFT);
+
+            // Try advancing the tail forward.
+            match self.tail.index.compare_exchange_weak(
+                tail,
+                new_tail,
+                Ordering::SeqCst,
+                Ordering::Acquire,
+            ) {
+                Ok(_) => unsafe {
+                    // If we've reached the end of the block, install the next one.
+                    if offset + 1 == BLOCK_CAP {
+                        let next_block = Box::into_raw(next_block.unwrap());
+                        self.tail.block.store(next_block, Ordering::Release);
+                        self.tail.index.fetch_add(1 << SHIFT, Ordering::Release);
+                        (*block).next.store(next_block, Ordering::Release);
+                    }
+
+                    token.list.block = block as *const u8;
+                    token.list.offset = offset;
+                    return true;
+                },
+                Err(t) => {
+                    tail = t;
+                    block = self.tail.block.load(Ordering::Acquire);
+                    backoff.spin();
+                }
+            }
+        }
+    }
+
+    /// Writes a message into the channel.
+    pub(crate) unsafe fn write(&self, token: &mut Token, msg: T) -> Result<(), T> {
+        // If there is no slot, the channel is disconnected.
+        if token.list.block.is_null() {
+            return Err(msg);
+        }
+
+        // Write the message into the slot.
+        let block = token.list.block.cast::<Block<T>>();
+        let offset = token.list.offset;
+        let slot = (*block).slots.get_unchecked(offset);
+        slot.msg.get().write(MaybeUninit::new(msg));
+        slot.state.fetch_or(WRITE, Ordering::Release);
+
+        // Wake a sleeping receiver.
+        self.receivers.notify();
+        Ok(())
+    }
+
+    /// Attempts to reserve a slot for receiving a message.
+    fn start_recv(&self, token: &mut Token) -> bool {
+        let backoff = Backoff::new();
+        let mut head = self.head.index.load(Ordering::Acquire);
+        let mut block = self.head.block.load(Ordering::Acquire);
+
+        loop {
+            // Calculate the offset of the index into the block.
+            let offset = (head >> SHIFT) % LAP;
+
+            // If we reached the end of the block, wait until the next one is installed.
+            if offset == BLOCK_CAP {
+                backoff.snooze();
+                head = self.head.index.load(Ordering::Acquire);
+                block = self.head.block.load(Ordering::Acquire);
+                continue;
+            }
+
+            let mut new_head = head + (1 << SHIFT);
+
+            if new_head & MARK_BIT == 0 {
+                atomic::fence(Ordering::SeqCst);
+                let tail = self.tail.index.load(Ordering::Relaxed);
+
+                // If the tail equals the head, that means the channel is empty.
+                if head >> SHIFT == tail >> SHIFT {
+                    // If the channel is disconnected...
+                    if tail & MARK_BIT != 0 {
+                        // ...then receive an error.
+                        token.list.block = ptr::null();
+                        return true;
+                    } else {
+                        // Otherwise, the receive operation is not ready.
+                        return false;
+                    }
+                }
+
+                // If head and tail are not in the same block, set `MARK_BIT` in head.
+                if (head >> SHIFT) / LAP != (tail >> SHIFT) / LAP {
+                    new_head |= MARK_BIT;
+                }
+            }
+
+            // The block can be null here only if the first message is being sent into the channel.
+            // In that case, just wait until it gets initialized.
+            if block.is_null() {
+                backoff.snooze();
+                head = self.head.index.load(Ordering::Acquire);
+                block = self.head.block.load(Ordering::Acquire);
+                continue;
+            }
+
+            // Try moving the head index forward.
+            match self.head.index.compare_exchange_weak(
+                head,
+                new_head,
+                Ordering::SeqCst,
+                Ordering::Acquire,
+            ) {
+                Ok(_) => unsafe {
+                    // If we've reached the end of the block, move to the next one.
+                    if offset + 1 == BLOCK_CAP {
+                        let next = (*block).wait_next();
+                        let mut next_index = (new_head & !MARK_BIT).wrapping_add(1 << SHIFT);
+                        if !(*next).next.load(Ordering::Relaxed).is_null() {
+                            next_index |= MARK_BIT;
+                        }
+
+                        self.head.block.store(next, Ordering::Release);
+                        self.head.index.store(next_index, Ordering::Release);
+                    }
+
+                    token.list.block = block as *const u8;
+                    token.list.offset = offset;
+                    return true;
+                },
+                Err(h) => {
+                    head = h;
+                    block = self.head.block.load(Ordering::Acquire);
+                    backoff.spin();
+                }
+            }
+        }
+    }
+
+    /// Reads a message from the channel.
+    pub(crate) unsafe fn read(&self, token: &mut Token) -> Result<T, ()> {
+        if token.list.block.is_null() {
+            // The channel is disconnected.
+            return Err(());
+        }
+
+        // Read the message.
+        let block = token.list.block as *mut Block<T>;
+        let offset = token.list.offset;
+        let slot = (*block).slots.get_unchecked(offset);
+        slot.wait_write();
+        let msg = slot.msg.get().read().assume_init();
+
+        // Destroy the block if we've reached the end, or if another thread wanted to destroy but
+        // couldn't because we were busy reading from the slot.
+        if offset + 1 == BLOCK_CAP {
+            Block::destroy(block, 0);
+        } else if slot.state.fetch_or(READ, Ordering::AcqRel) & DESTROY != 0 {
+            Block::destroy(block, offset + 1);
+        }
+
+        Ok(msg)
+    }
+
+    /// Attempts to send a message into the channel.
+    pub(crate) fn try_send(&self, msg: T) -> Result<(), TrySendError<T>> {
+        self.send(msg, None).map_err(|err| match err {
+            SendTimeoutError::Disconnected(msg) => TrySendError::Disconnected(msg),
+            SendTimeoutError::Timeout(_) => unreachable!(),
+        })
+    }
+
+    /// Sends a message into the channel.
+    pub(crate) fn send(
+        &self,
+        msg: T,
+        _deadline: Option<Instant>,
+    ) -> Result<(), SendTimeoutError<T>> {
+        let token = &mut Token::default();
+        assert!(self.start_send(token));
+        unsafe {
+            self.write(token, msg)
+                .map_err(SendTimeoutError::Disconnected)
+        }
+    }
+
+    /// Attempts to receive a message without blocking.
+    pub(crate) fn try_recv(&self) -> Result<T, TryRecvError> {
+        let token = &mut Token::default();
+
+        if self.start_recv(token) {
+            unsafe { self.read(token).map_err(|_| TryRecvError::Disconnected) }
+        } else {
+            Err(TryRecvError::Empty)
+        }
+    }
+
+    /// Receives a message from the channel.
+    pub(crate) fn recv(&self, deadline: Option<Instant>) -> Result<T, RecvTimeoutError> {
+        let token = &mut Token::default();
+        loop {
+            // Try receiving a message several times.
+            let backoff = Backoff::new();
+            loop {
+                if self.start_recv(token) {
+                    unsafe {
+                        return self.read(token).map_err(|_| RecvTimeoutError::Disconnected);
+                    }
+                }
+
+                if backoff.is_completed() {
+                    break;
+                } else {
+                    backoff.snooze();
+                }
+            }
+
+            if let Some(d) = deadline {
+                if Instant::now() >= d {
+                    return Err(RecvTimeoutError::Timeout);
+                }
+            }
+
+            // Prepare for blocking until a sender wakes us up.
+            Context::with(|cx| {
+                let oper = Operation::hook(token);
+                self.receivers.register(oper, cx);
+
+                // Has the channel become ready just now?
+                if !self.is_empty() || self.is_disconnected() {
+                    let _ = cx.try_select(Selected::Aborted);
+                }
+
+                // Block the current thread.
+                let sel = cx.wait_until(deadline);
+
+                match sel {
+                    Selected::Waiting => unreachable!(),
+                    Selected::Aborted | Selected::Disconnected => {
+                        self.receivers.unregister(oper).unwrap();
+                        // If the channel was disconnected, we still have to check for remaining
+                        // messages.
+                    }
+                    Selected::Operation(_) => {}
+                }
+            });
+        }
+    }
+
+    /// Returns the current number of messages inside the channel.
+    pub(crate) fn len(&self) -> usize {
+        loop {
+            // Load the tail index, then load the head index.
+            let mut tail = self.tail.index.load(Ordering::SeqCst);
+            let mut head = self.head.index.load(Ordering::SeqCst);
+
+            // If the tail index didn't change, we've got consistent indices to work with.
+            if self.tail.index.load(Ordering::SeqCst) == tail {
+                // Erase the lower bits.
+                tail &= !((1 << SHIFT) - 1);
+                head &= !((1 << SHIFT) - 1);
+
+                // Fix up indices if they fall onto block ends.
+                if (tail >> SHIFT) & (LAP - 1) == LAP - 1 {
+                    tail = tail.wrapping_add(1 << SHIFT);
+                }
+                if (head >> SHIFT) & (LAP - 1) == LAP - 1 {
+                    head = head.wrapping_add(1 << SHIFT);
+                }
+
+                // Rotate indices so that head falls into the first block.
+                let lap = (head >> SHIFT) / LAP;
+                tail = tail.wrapping_sub((lap * LAP) << SHIFT);
+                head = head.wrapping_sub((lap * LAP) << SHIFT);
+
+                // Remove the lower bits.
+                tail >>= SHIFT;
+                head >>= SHIFT;
+
+                // Return the difference minus the number of blocks between tail and head.
+                return tail - head - tail / LAP;
+            }
+        }
+    }
+
+    /// Returns the capacity of the channel.
+    pub(crate) fn capacity(&self) -> Option<usize> {
+        None
+    }
+
+    /// Disconnects senders and wakes up all blocked receivers.
+    ///
+    /// Returns `true` if this call disconnected the channel.
+    pub(crate) fn disconnect_senders(&self) -> bool {
+        let tail = self.tail.index.fetch_or(MARK_BIT, Ordering::SeqCst);
+
+        if tail & MARK_BIT == 0 {
+            self.receivers.disconnect();
+            true
+        } else {
+            false
+        }
+    }
+
+    /// Disconnects receivers.
+    ///
+    /// Returns `true` if this call disconnected the channel.
+    pub(crate) fn disconnect_receivers(&self) -> bool {
+        let tail = self.tail.index.fetch_or(MARK_BIT, Ordering::SeqCst);
+
+        if tail & MARK_BIT == 0 {
+            // If receivers are dropped first, discard all messages to free
+            // memory eagerly.
+            self.discard_all_messages();
+            true
+        } else {
+            false
+        }
+    }
+
+    /// Discards all messages.
+    ///
+    /// This method should only be called when all receivers are dropped.
+    fn discard_all_messages(&self) {
+        let backoff = Backoff::new();
+        let mut tail = self.tail.index.load(Ordering::Acquire);
+        loop {
+            let offset = (tail >> SHIFT) % LAP;
+            if offset != BLOCK_CAP {
+                break;
+            }
+
+            // New updates to tail will be rejected by MARK_BIT and aborted unless it's
+            // at boundary. We need to wait for the updates take affect otherwise there
+            // can be memory leaks.
+            backoff.snooze();
+            tail = self.tail.index.load(Ordering::Acquire);
+        }
+
+        let mut head = self.head.index.load(Ordering::Acquire);
+        let mut block = self.head.block.swap(ptr::null_mut(), Ordering::AcqRel);
+
+        // If we're going to be dropping messages we need to synchronize with initialization
+        if head >> SHIFT != tail >> SHIFT {
+            // The block can be null here only if a sender is in the process of initializing the
+            // channel while another sender managed to send a message by inserting it into the
+            // semi-initialized channel and advanced the tail.
+            // In that case, just wait until it gets initialized.
+            while block.is_null() {
+                backoff.snooze();
+                block = self.head.block.load(Ordering::Acquire);
+            }
+        }
+
+        unsafe {
+            // Drop all messages between head and tail and deallocate the heap-allocated blocks.
+            while head >> SHIFT != tail >> SHIFT {
+                let offset = (head >> SHIFT) % LAP;
+
+                if offset < BLOCK_CAP {
+                    // Drop the message in the slot.
+                    let slot = (*block).slots.get_unchecked(offset);
+                    slot.wait_write();
+                    (*slot.msg.get()).assume_init_drop();
+                } else {
+                    (*block).wait_next();
+                    // Deallocate the block and move to the next one.
+                    let next = (*block).next.load(Ordering::Acquire);
+                    drop(Box::from_raw(block));
+                    block = next;
+                }
+
+                head = head.wrapping_add(1 << SHIFT);
+            }
+
+            // Deallocate the last remaining block.
+            if !block.is_null() {
+                drop(Box::from_raw(block));
+            }
+        }
+        head &= !MARK_BIT;
+        self.head.index.store(head, Ordering::Release);
+    }
+
+    /// Returns `true` if the channel is disconnected.
+    pub(crate) fn is_disconnected(&self) -> bool {
+        self.tail.index.load(Ordering::SeqCst) & MARK_BIT != 0
+    }
+
+    /// Returns `true` if the channel is empty.
+    pub(crate) fn is_empty(&self) -> bool {
+        let head = self.head.index.load(Ordering::SeqCst);
+        let tail = self.tail.index.load(Ordering::SeqCst);
+        head >> SHIFT == tail >> SHIFT
+    }
+
+    /// Returns `true` if the channel is full.
+    pub(crate) fn is_full(&self) -> bool {
+        false
+    }
+}
+
+impl<T> Drop for Channel<T> {
+    fn drop(&mut self) {
+        let mut head = *self.head.index.get_mut();
+        let mut tail = *self.tail.index.get_mut();
+        let mut block = *self.head.block.get_mut();
+
+        // Erase the lower bits.
+        head &= !((1 << SHIFT) - 1);
+        tail &= !((1 << SHIFT) - 1);
+
+        unsafe {
+            // Drop all messages between head and tail and deallocate the heap-allocated blocks.
+            while head != tail {
+                let offset = (head >> SHIFT) % LAP;
+
+                if offset < BLOCK_CAP {
+                    // Drop the message in the slot.
+                    let slot = (*block).slots.get_unchecked(offset);
+                    (*slot.msg.get()).assume_init_drop();
+                } else {
+                    // Deallocate the block and move to the next one.
+                    let next = *(*block).next.get_mut();
+                    drop(Box::from_raw(block));
+                    block = next;
+                }
+
+                head = head.wrapping_add(1 << SHIFT);
+            }
+
+            // Deallocate the last remaining block.
+            if !block.is_null() {
+                drop(Box::from_raw(block));
+            }
+        }
+    }
+}
+
+/// Receiver handle to a channel.
+pub(crate) struct Receiver<'a, T>(&'a Channel<T>);
+
+/// Sender handle to a channel.
+pub(crate) struct Sender<'a, T>(&'a Channel<T>);
+
+impl<T> SelectHandle for Receiver<'_, T> {
+    fn try_select(&self, token: &mut Token) -> bool {
+        self.0.start_recv(token)
+    }
+
+    fn deadline(&self) -> Option<Instant> {
+        None
+    }
+
+    fn register(&self, oper: Operation, cx: &Context) -> bool {
+        self.0.receivers.register(oper, cx);
+        self.is_ready()
+    }
+
+    fn unregister(&self, oper: Operation) {
+        self.0.receivers.unregister(oper);
+    }
+
+    fn accept(&self, token: &mut Token, _cx: &Context) -> bool {
+        self.try_select(token)
+    }
+
+    fn is_ready(&self) -> bool {
+        !self.0.is_empty() || self.0.is_disconnected()
+    }
+
+    fn watch(&self, oper: Operation, cx: &Context) -> bool {
+        self.0.receivers.watch(oper, cx);
+        self.is_ready()
+    }
+
+    fn unwatch(&self, oper: Operation) {
+        self.0.receivers.unwatch(oper);
+    }
+}
+
+impl<T> SelectHandle for Sender<'_, T> {
+    fn try_select(&self, token: &mut Token) -> bool {
+        self.0.start_send(token)
+    }
+
+    fn deadline(&self) -> Option<Instant> {
+        None
+    }
+
+    fn register(&self, _oper: Operation, _cx: &Context) -> bool {
+        self.is_ready()
+    }
+
+    fn unregister(&self, _oper: Operation) {}
+
+    fn accept(&self, token: &mut Token, _cx: &Context) -> bool {
+        self.try_select(token)
+    }
+
+    fn is_ready(&self) -> bool {
+        true
+    }
+
+    fn watch(&self, _oper: Operation, _cx: &Context) -> bool {
+        self.is_ready()
+    }
+
+    fn unwatch(&self, _oper: Operation) {}
+}

third-party/vendor/crossbeam-channel/src/flavors/mod.rs

@@ -0,0 +1,17 @@
+//! Channel flavors.
+//!
+//! There are six flavors:
+//!
+//! 1. `at` - Channel that delivers a message after a certain amount of time.
+//! 2. `array` - Bounded channel based on a preallocated array.
+//! 3. `list` - Unbounded channel implemented as a linked list.
+//! 4. `never` - Channel that never delivers messages.
+//! 5. `tick` - Channel that delivers messages periodically.
+//! 6. `zero` - Zero-capacity channel.
+
+pub(crate) mod array;
+pub(crate) mod at;
+pub(crate) mod list;
+pub(crate) mod never;
+pub(crate) mod tick;
+pub(crate) mod zero;

third-party/vendor/crossbeam-channel/src/flavors/never.rs

@@ -0,0 +1,110 @@
+//! Channel that never delivers messages.
+//!
+//! Messages cannot be sent into this kind of channel.
+
+use std::marker::PhantomData;
+use std::time::Instant;
+
+use crate::context::Context;
+use crate::err::{RecvTimeoutError, TryRecvError};
+use crate::select::{Operation, SelectHandle, Token};
+use crate::utils;
+
+/// This flavor doesn't need a token.
+pub(crate) type NeverToken = ();
+
+/// Channel that never delivers messages.
+pub(crate) struct Channel<T> {
+    _marker: PhantomData<T>,
+}
+
+impl<T> Channel<T> {
+    /// Creates a channel that never delivers messages.
+    #[inline]
+    pub(crate) fn new() -> Self {
+        Channel {
+            _marker: PhantomData,
+        }
+    }
+
+    /// Attempts to receive a message without blocking.
+    #[inline]
+    pub(crate) fn try_recv(&self) -> Result<T, TryRecvError> {
+        Err(TryRecvError::Empty)
+    }
+
+    /// Receives a message from the channel.
+    #[inline]
+    pub(crate) fn recv(&self, deadline: Option<Instant>) -> Result<T, RecvTimeoutError> {
+        utils::sleep_until(deadline);
+        Err(RecvTimeoutError::Timeout)
+    }
+
+    /// Reads a message from the channel.
+    #[inline]
+    pub(crate) unsafe fn read(&self, _token: &mut Token) -> Result<T, ()> {
+        Err(())
+    }
+
+    /// Returns `true` if the channel is empty.
+    #[inline]
+    pub(crate) fn is_empty(&self) -> bool {
+        true
+    }
+
+    /// Returns `true` if the channel is full.
+    #[inline]
+    pub(crate) fn is_full(&self) -> bool {
+        true
+    }
+
+    /// Returns the number of messages in the channel.
+    #[inline]
+    pub(crate) fn len(&self) -> usize {
+        0
+    }
+
+    /// Returns the capacity of the channel.
+    #[inline]
+    pub(crate) fn capacity(&self) -> Option<usize> {
+        Some(0)
+    }
+}
+
+impl<T> SelectHandle for Channel<T> {
+    #[inline]
+    fn try_select(&self, _token: &mut Token) -> bool {
+        false
+    }
+
+    #[inline]
+    fn deadline(&self) -> Option<Instant> {
+        None
+    }
+
+    #[inline]
+    fn register(&self, _oper: Operation, _cx: &Context) -> bool {
+        self.is_ready()
+    }
+
+    #[inline]
+    fn unregister(&self, _oper: Operation) {}
+
+    #[inline]
+    fn accept(&self, token: &mut Token, _cx: &Context) -> bool {
+        self.try_select(token)
+    }
+
+    #[inline]
+    fn is_ready(&self) -> bool {
+        false
+    }
+
+    #[inline]
+    fn watch(&self, _oper: Operation, _cx: &Context) -> bool {
+        self.is_ready()
+    }
+
+    #[inline]
+    fn unwatch(&self, _oper: Operation) {}
+}

third-party/vendor/crossbeam-channel/src/flavors/tick.rs

@@ -0,0 +1,167 @@
+//! Channel that delivers messages periodically.
+//!
+//! Messages cannot be sent into this kind of channel; they are materialized on demand.
+
+use std::thread;
+use std::time::{Duration, Instant};
+
+use crossbeam_utils::atomic::AtomicCell;
+
+use crate::context::Context;
+use crate::err::{RecvTimeoutError, TryRecvError};
+use crate::select::{Operation, SelectHandle, Token};
+
+/// Result of a receive operation.
+pub(crate) type TickToken = Option<Instant>;
+
+/// Channel that delivers messages periodically.
+pub(crate) struct Channel {
+    /// The instant at which the next message will be delivered.
+    delivery_time: AtomicCell<Instant>,
+
+    /// The time interval in which messages get delivered.
+    duration: Duration,
+}
+
+impl Channel {
+    /// Creates a channel that delivers messages periodically.
+    #[inline]
+    pub(crate) fn new(delivery_time: Instant, dur: Duration) -> Self {
+        Channel {
+            delivery_time: AtomicCell::new(delivery_time),
+            duration: dur,
+        }
+    }
+
+    /// Attempts to receive a message without blocking.
+    #[inline]
+    pub(crate) fn try_recv(&self) -> Result<Instant, TryRecvError> {
+        loop {
+            let now = Instant::now();
+            let delivery_time = self.delivery_time.load();
+
+            if now < delivery_time {
+                return Err(TryRecvError::Empty);
+            }
+
+            if self
+                .delivery_time
+                .compare_exchange(delivery_time, now + self.duration)
+                .is_ok()
+            {
+                return Ok(delivery_time);
+            }
+        }
+    }
+
+    /// Receives a message from the channel.
+    #[inline]
+    pub(crate) fn recv(&self, deadline: Option<Instant>) -> Result<Instant, RecvTimeoutError> {
+        loop {
+            let delivery_time = self.delivery_time.load();
+            let now = Instant::now();
+
+            if let Some(d) = deadline {
+                if d < delivery_time {
+                    if now < d {
+                        thread::sleep(d - now);
+                    }
+                    return Err(RecvTimeoutError::Timeout);
+                }
+            }
+
+            if self
+                .delivery_time
+                .compare_exchange(delivery_time, delivery_time.max(now) + self.duration)
+                .is_ok()
+            {
+                if now < delivery_time {
+                    thread::sleep(delivery_time - now);
+                }
+                return Ok(delivery_time);
+            }
+        }
+    }
+
+    /// Reads a message from the channel.
+    #[inline]
+    pub(crate) unsafe fn read(&self, token: &mut Token) -> Result<Instant, ()> {
+        token.tick.ok_or(())
+    }
+
+    /// Returns `true` if the channel is empty.
+    #[inline]
+    pub(crate) fn is_empty(&self) -> bool {
+        Instant::now() < self.delivery_time.load()
+    }
+
+    /// Returns `true` if the channel is full.
+    #[inline]
+    pub(crate) fn is_full(&self) -> bool {
+        !self.is_empty()
+    }
+
+    /// Returns the number of messages in the channel.
+    #[inline]
+    pub(crate) fn len(&self) -> usize {
+        if self.is_empty() {
+            0
+        } else {
+            1
+        }
+    }
+
+    /// Returns the capacity of the channel.
+    #[inline]
+    pub(crate) fn capacity(&self) -> Option<usize> {
+        Some(1)
+    }
+}
+
+impl SelectHandle for Channel {
+    #[inline]
+    fn try_select(&self, token: &mut Token) -> bool {
+        match self.try_recv() {
+            Ok(msg) => {
+                token.tick = Some(msg);
+                true
+            }
+            Err(TryRecvError::Disconnected) => {
+                token.tick = None;
+                true
+            }
+            Err(TryRecvError::Empty) => false,
+        }
+    }
+
+    #[inline]
+    fn deadline(&self) -> Option<Instant> {
+        Some(self.delivery_time.load())
+    }
+
+    #[inline]
+    fn register(&self, _oper: Operation, _cx: &Context) -> bool {
+        self.is_ready()
+    }
+
+    #[inline]
+    fn unregister(&self, _oper: Operation) {}
+
+    #[inline]
+    fn accept(&self, token: &mut Token, _cx: &Context) -> bool {
+        self.try_select(token)
+    }
+
+    #[inline]
+    fn is_ready(&self) -> bool {
+        !self.is_empty()
+    }
+
+    #[inline]
+    fn watch(&self, _oper: Operation, _cx: &Context) -> bool {
+        self.is_ready()
+    }
+
+    #[inline]
+    fn unwatch(&self, _oper: Operation) {}
+}

third-party/vendor/crossbeam-channel/src/flavors/zero.rs

@@ -0,0 +1,496 @@
+//! Zero-capacity channel.
+//!
+//! This kind of channel is also known as *rendezvous* channel.
+
+use std::boxed::Box;
+use std::cell::UnsafeCell;
+use std::marker::PhantomData;
+use std::sync::atomic::{AtomicBool, Ordering};
+use std::sync::Mutex;
+use std::time::Instant;
+use std::{fmt, ptr};
+
+use crossbeam_utils::Backoff;
+
+use crate::context::Context;
+use crate::err::{RecvTimeoutError, SendTimeoutError, TryRecvError, TrySendError};
+use crate::select::{Operation, SelectHandle, Selected, Token};
+use crate::waker::Waker;
+
+/// A pointer to a packet.
+pub(crate) struct ZeroToken(*mut ());
+
+impl Default for ZeroToken {
+    fn default() -> Self {
+        Self(ptr::null_mut())
+    }
+}
+
+impl fmt::Debug for ZeroToken {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Debug::fmt(&(self.0 as usize), f)
+    }
+}
+
+/// A slot for passing one message from a sender to a receiver.
+struct Packet<T> {
+    /// Equals `true` if the packet is allocated on the stack.
+    on_stack: bool,
+
+    /// Equals `true` once the packet is ready for reading or writing.
+    ready: AtomicBool,
+
+    /// The message.
+    msg: UnsafeCell<Option<T>>,
+}
+
+impl<T> Packet<T> {
+    /// Creates an empty packet on the stack.
+    fn empty_on_stack() -> Packet<T> {
+        Packet {
+            on_stack: true,
+            ready: AtomicBool::new(false),
+            msg: UnsafeCell::new(None),
+        }
+    }
+
+    /// Creates an empty packet on the heap.
+    fn empty_on_heap() -> Box<Packet<T>> {
+        Box::new(Packet {
+            on_stack: false,
+            ready: AtomicBool::new(false),
+            msg: UnsafeCell::new(None),
+        })
+    }
+
+    /// Creates a packet on the stack, containing a message.
+    fn message_on_stack(msg: T) -> Packet<T> {
+        Packet {
+            on_stack: true,
+            ready: AtomicBool::new(false),
+            msg: UnsafeCell::new(Some(msg)),
+        }
+    }
+
+    /// Waits until the packet becomes ready for reading or writing.
+    fn wait_ready(&self) {
+        let backoff = Backoff::new();
+        while !self.ready.load(Ordering::Acquire) {
+            backoff.snooze();
+        }
+    }
+}
+
+/// Inner representation of a zero-capacity channel.
+struct Inner {
+    /// Senders waiting to pair up with a receive operation.
+    senders: Waker,
+
+    /// Receivers waiting to pair up with a send operation.
+    receivers: Waker,
+
+    /// Equals `true` when the channel is disconnected.
+    is_disconnected: bool,
+}
+
+/// Zero-capacity channel.
+pub(crate) struct Channel<T> {
+    /// Inner representation of the channel.
+    inner: Mutex<Inner>,
+
+    /// Indicates that dropping a `Channel<T>` may drop values of type `T`.
+    _marker: PhantomData<T>,
+}
+
+impl<T> Channel<T> {
+    /// Constructs a new zero-capacity channel.
+    pub(crate) fn new() -> Self {
+        Channel {
+            inner: Mutex::new(Inner {
+                senders: Waker::new(),
+                receivers: Waker::new(),
+                is_disconnected: false,
+            }),
+            _marker: PhantomData,
+        }
+    }
+
+    /// Returns a receiver handle to the channel.
+    pub(crate) fn receiver(&self) -> Receiver<'_, T> {
+        Receiver(self)
+    }
+
+    /// Returns a sender handle to the channel.
+    pub(crate) fn sender(&self) -> Sender<'_, T> {
+        Sender(self)
+    }
+
+    /// Attempts to reserve a slot for sending a message.
+    fn start_send(&self, token: &mut Token) -> bool {
+        let mut inner = self.inner.lock().unwrap();
+
+        // If there's a waiting receiver, pair up with it.
+        if let Some(operation) = inner.receivers.try_select() {
+            token.zero.0 = operation.packet;
+            true
+        } else if inner.is_disconnected {
+            token.zero.0 = ptr::null_mut();
+            true
+        } else {
+            false
+        }
+    }
+
+    /// Writes a message into the packet.
+    pub(crate) unsafe fn write(&self, token: &mut Token, msg: T) -> Result<(), T> {
+        // If there is no packet, the channel is disconnected.
+        if token.zero.0.is_null() {
+            return Err(msg);
+        }
+
+        let packet = &*(token.zero.0 as *const Packet<T>);
+        packet.msg.get().write(Some(msg));
+        packet.ready.store(true, Ordering::Release);
+        Ok(())
+    }
+
+    /// Attempts to pair up with a sender.
+    fn start_recv(&self, token: &mut Token) -> bool {
+        let mut inner = self.inner.lock().unwrap();
+
+        // If there's a waiting sender, pair up with it.
+        if let Some(operation) = inner.senders.try_select() {
+            token.zero.0 = operation.packet;
+            true
+        } else if inner.is_disconnected {
+            token.zero.0 = ptr::null_mut();
+            true
+        } else {
+            false
+        }
+    }
+
+    /// Reads a message from the packet.
+    pub(crate) unsafe fn read(&self, token: &mut Token) -> Result<T, ()> {
+        // If there is no packet, the channel is disconnected.
+        if token.zero.0.is_null() {
+            return Err(());
+        }
+
+        let packet = &*(token.zero.0 as *const Packet<T>);
+
+        if packet.on_stack {
+            // The message has been in the packet from the beginning, so there is no need to wait
+            // for it. However, after reading the message, we need to set `ready` to `true` in
+            // order to signal that the packet can be destroyed.
+            let msg = packet.msg.get().replace(None).unwrap();
+            packet.ready.store(true, Ordering::Release);
+            Ok(msg)
+        } else {
+            // Wait until the message becomes available, then read it and destroy the
+            // heap-allocated packet.
+            packet.wait_ready();
+            let msg = packet.msg.get().replace(None).unwrap();
+            drop(Box::from_raw(token.zero.0.cast::<Packet<T>>()));
+            Ok(msg)
+        }
+    }
+
+    /// Attempts to send a message into the channel.
+    pub(crate) fn try_send(&self, msg: T) -> Result<(), TrySendError<T>> {
+        let token = &mut Token::default();
+        let mut inner = self.inner.lock().unwrap();
+
+        // If there's a waiting receiver, pair up with it.
+        if let Some(operation) = inner.receivers.try_select() {
+            token.zero.0 = operation.packet;
+            drop(inner);
+            unsafe {
+                self.write(token, msg).ok().unwrap();
+            }
+            Ok(())
+        } else if inner.is_disconnected {
+            Err(TrySendError::Disconnected(msg))
+        } else {
+            Err(TrySendError::Full(msg))
+        }
+    }
+
+    /// Sends a message into the channel.
+    pub(crate) fn send(
+        &self,
+        msg: T,
+        deadline: Option<Instant>,
+    ) -> Result<(), SendTimeoutError<T>> {
+        let token = &mut Token::default();
+        let mut inner = self.inner.lock().unwrap();
+
+        // If there's a waiting receiver, pair up with it.
+        if let Some(operation) = inner.receivers.try_select() {
+            token.zero.0 = operation.packet;
+            drop(inner);
+            unsafe {
+                self.write(token, msg).ok().unwrap();
+            }
+            return Ok(());
+        }
+
+        if inner.is_disconnected {
+            return Err(SendTimeoutError::Disconnected(msg));
+        }
+
+        Context::with(|cx| {
+            // Prepare for blocking until a receiver wakes us up.
+            let oper = Operation::hook(token);
+            let mut packet = Packet::<T>::message_on_stack(msg);
+            inner
+                .senders
+                .register_with_packet(oper, &mut packet as *mut Packet<T> as *mut (), cx);
+            inner.receivers.notify();
+            drop(inner);
+
+            // Block the current thread.
+            let sel = cx.wait_until(deadline);
+
+            match sel {
+                Selected::Waiting => unreachable!(),
+                Selected::Aborted => {
+                    self.inner.lock().unwrap().senders.unregister(oper).unwrap();
+                    let msg = unsafe { packet.msg.get().replace(None).unwrap() };
+                    Err(SendTimeoutError::Timeout(msg))
+                }
+                Selected::Disconnected => {
+                    self.inner.lock().unwrap().senders.unregister(oper).unwrap();
+                    let msg = unsafe { packet.msg.get().replace(None).unwrap() };
+                    Err(SendTimeoutError::Disconnected(msg))
+                }
+                Selected::Operation(_) => {
+                    // Wait until the message is read, then drop the packet.
+                    packet.wait_ready();
+                    Ok(())
+                }
+            }
+        })
+    }
+
+    /// Attempts to receive a message without blocking.
+    pub(crate) fn try_recv(&self) -> Result<T, TryRecvError> {
+        let token = &mut Token::default();
+        let mut inner = self.inner.lock().unwrap();
+
+        // If there's a waiting sender, pair up with it.
+        if let Some(operation) = inner.senders.try_select() {
+            token.zero.0 = operation.packet;
+            drop(inner);
+            unsafe { self.read(token).map_err(|_| TryRecvError::Disconnected) }
+        } else if inner.is_disconnected {
+            Err(TryRecvError::Disconnected)
+        } else {
+            Err(TryRecvError::Empty)
+        }
+    }
+
+    /// Receives a message from the channel.
+    pub(crate) fn recv(&self, deadline: Option<Instant>) -> Result<T, RecvTimeoutError> {
+        let token = &mut Token::default();
+        let mut inner = self.inner.lock().unwrap();
+
+        // If there's a waiting sender, pair up with it.
+        if let Some(operation) = inner.senders.try_select() {
+            token.zero.0 = operation.packet;
+            drop(inner);
+            unsafe {
+                return self.read(token).map_err(|_| RecvTimeoutError::Disconnected);
+            }
+        }
+
+        if inner.is_disconnected {
+            return Err(RecvTimeoutError::Disconnected);
+        }
+
+        Context::with(|cx| {
+            // Prepare for blocking until a sender wakes us up.
+            let oper = Operation::hook(token);
+            let mut packet = Packet::<T>::empty_on_stack();
+            inner.receivers.register_with_packet(
+                oper,
+                &mut packet as *mut Packet<T> as *mut (),
+                cx,
+            );
+            inner.senders.notify();
+            drop(inner);
+
+            // Block the current thread.
+            let sel = cx.wait_until(deadline);
+
+            match sel {
+                Selected::Waiting => unreachable!(),
+                Selected::Aborted => {
+                    self.inner
+                        .lock()
+                        .unwrap()
+                        .receivers
+                        .unregister(oper)
+                        .unwrap();
+                    Err(RecvTimeoutError::Timeout)
+                }
+                Selected::Disconnected => {
+                    self.inner
+                        .lock()
+                        .unwrap()
+                        .receivers
+                        .unregister(oper)
+                        .unwrap();
+                    Err(RecvTimeoutError::Disconnected)
+                }
+                Selected::Operation(_) => {
+                    // Wait until the message is provided, then read it.
+                    packet.wait_ready();
+                    unsafe { Ok(packet.msg.get().replace(None).unwrap()) }
+                }
+            }
+        })
+    }
+
+    /// Disconnects the channel and wakes up all blocked senders and receivers.
+    ///
+    /// Returns `true` if this call disconnected the channel.
+    pub(crate) fn disconnect(&self) -> bool {
+        let mut inner = self.inner.lock().unwrap();
+
+        if !inner.is_disconnected {
+            inner.is_disconnected = true;
+            inner.senders.disconnect();
+            inner.receivers.disconnect();
+            true
+        } else {
+            false
+        }
+    }
+
+    /// Returns the current number of messages inside the channel.
+    pub(crate) fn len(&self) -> usize {
+        0
+    }
+
+    /// Returns the capacity of the channel.
+    pub(crate) fn capacity(&self) -> Option<usize> {
+        Some(0)
+    }
+
+    /// Returns `true` if the channel is empty.
+    pub(crate) fn is_empty(&self) -> bool {
+        true
+    }
+
+    /// Returns `true` if the channel is full.
+    pub(crate) fn is_full(&self) -> bool {
+        true
+    }
+}
+
+/// Receiver handle to a channel.
+pub(crate) struct Receiver<'a, T>(&'a Channel<T>);
+
+/// Sender handle to a channel.
+pub(crate) struct Sender<'a, T>(&'a Channel<T>);
+
+impl<T> SelectHandle for Receiver<'_, T> {
+    fn try_select(&self, token: &mut Token) -> bool {
+        self.0.start_recv(token)
+    }
+
+    fn deadline(&self) -> Option<Instant> {
+        None
+    }
+
+    fn register(&self, oper: Operation, cx: &Context) -> bool {
+        let packet = Box::into_raw(Packet::<T>::empty_on_heap());
+
+        let mut inner = self.0.inner.lock().unwrap();
+        inner
+            .receivers
+            .register_with_packet(oper, packet.cast::<()>(), cx);
+        inner.senders.notify();
+        inner.senders.can_select() || inner.is_disconnected
+    }
+
+    fn unregister(&self, oper: Operation) {
+        if let Some(operation) = self.0.inner.lock().unwrap().receivers.unregister(oper) {
+            unsafe {
+                drop(Box::from_raw(operation.packet.cast::<Packet<T>>()));
+            }
+        }
+    }
+
+    fn accept(&self, token: &mut Token, cx: &Context) -> bool {
+        token.zero.0 = cx.wait_packet();
+        true
+    }
+
+    fn is_ready(&self) -> bool {
+        let inner = self.0.inner.lock().unwrap();
+        inner.senders.can_select() || inner.is_disconnected
+    }
+
+    fn watch(&self, oper: Operation, cx: &Context) -> bool {
+        let mut inner = self.0.inner.lock().unwrap();
+        inner.receivers.watch(oper, cx);
+        inner.senders.can_select() || inner.is_disconnected
+    }
+
+    fn unwatch(&self, oper: Operation) {
+        let mut inner = self.0.inner.lock().unwrap();
+        inner.receivers.unwatch(oper);
+    }
+}
+
+impl<T> SelectHandle for Sender<'_, T> {
+    fn try_select(&self, token: &mut Token) -> bool {
+        self.0.start_send(token)
+    }
+
+    fn deadline(&self) -> Option<Instant> {
+        None
+    }
+
+    fn register(&self, oper: Operation, cx: &Context) -> bool {
+        let packet = Box::into_raw(Packet::<T>::empty_on_heap());
+
+        let mut inner = self.0.inner.lock().unwrap();
+        inner
+            .senders
+            .register_with_packet(oper, packet.cast::<()>(), cx);
+        inner.receivers.notify();
+        inner.receivers.can_select() || inner.is_disconnected
+    }
+
+    fn unregister(&self, oper: Operation) {
+        if let Some(operation) = self.0.inner.lock().unwrap().senders.unregister(oper) {
+            unsafe {
+                drop(Box::from_raw(operation.packet.cast::<Packet<T>>()));
+            }
+        }
+    }
+
+    fn accept(&self, token: &mut Token, cx: &Context) -> bool {
+        token.zero.0 = cx.wait_packet();
+        true
+    }
+
+    fn is_ready(&self) -> bool {
+        let inner = self.0.inner.lock().unwrap();
+        inner.receivers.can_select() || inner.is_disconnected
+    }
+
+    fn watch(&self, oper: Operation, cx: &Context) -> bool {
+        let mut inner = self.0.inner.lock().unwrap();
+        inner.senders.watch(oper, cx);
+        inner.receivers.can_select() || inner.is_disconnected
+    }
+
+    fn unwatch(&self, oper: Operation) {
+        let mut inner = self.0.inner.lock().unwrap();
+        inner.senders.unwatch(oper);
+    }
+}