Vendor dependencies for 0.3.0 release

vendor/crossbeam-channel/src/context.rs

@@ -0,0 +1,178 @@
+//! Thread-local context used in select.
+
+use std::cell::Cell;
+use std::ptr;
+use std::sync::atomic::{AtomicPtr, AtomicUsize, Ordering};
+use std::sync::Arc;
+use std::thread::{self, Thread, ThreadId};
+use std::time::Instant;
+
+use crossbeam_utils::Backoff;
+
+use crate::select::Selected;
+
+/// Thread-local context used in select.
+// This is a private API that is used by the select macro.
+#[derive(Debug, Clone)]
+pub struct Context {
+    inner: Arc<Inner>,
+}
+
+/// Inner representation of `Context`.
+#[derive(Debug)]
+struct Inner {
+    /// Selected operation.
+    select: AtomicUsize,
+
+    /// A slot into which another thread may store a pointer to its `Packet`.
+    packet: AtomicPtr<()>,
+
+    /// Thread handle.
+    thread: Thread,
+
+    /// Thread id.
+    thread_id: ThreadId,
+}
+
+impl Context {
+    /// Creates a new context for the duration of the closure.
+    #[inline]
+    pub fn with<F, R>(f: F) -> R
+    where
+        F: FnOnce(&Context) -> R,
+    {
+        std::thread_local! {
+            /// Cached thread-local context.
+            static CONTEXT: Cell<Option<Context>> = Cell::new(Some(Context::new()));
+        }
+
+        let mut f = Some(f);
+        let mut f = |cx: &Context| -> R {
+            let f = f.take().unwrap();
+            f(cx)
+        };
+
+        CONTEXT
+            .try_with(|cell| match cell.take() {
+                None => f(&Context::new()),
+                Some(cx) => {
+                    cx.reset();
+                    let res = f(&cx);
+                    cell.set(Some(cx));
+                    res
+                }
+            })
+            .unwrap_or_else(|_| f(&Context::new()))
+    }
+
+    /// Creates a new `Context`.
+    #[cold]
+    fn new() -> Context {
+        Context {
+            inner: Arc::new(Inner {
+                select: AtomicUsize::new(Selected::Waiting.into()),
+                packet: AtomicPtr::new(ptr::null_mut()),
+                thread: thread::current(),
+                thread_id: thread::current().id(),
+            }),
+        }
+    }
+
+    /// Resets `select` and `packet`.
+    #[inline]
+    fn reset(&self) {
+        self.inner
+            .select
+            .store(Selected::Waiting.into(), Ordering::Release);
+        self.inner.packet.store(ptr::null_mut(), Ordering::Release);
+    }
+
+    /// Attempts to select an operation.
+    ///
+    /// On failure, the previously selected operation is returned.
+    #[inline]
+    pub fn try_select(&self, select: Selected) -> Result<(), Selected> {
+        self.inner
+            .select
+            .compare_exchange(
+                Selected::Waiting.into(),
+                select.into(),
+                Ordering::AcqRel,
+                Ordering::Acquire,
+            )
+            .map(|_| ())
+            .map_err(|e| e.into())
+    }
+
+    /// Returns the selected operation.
+    #[inline]
+    pub fn selected(&self) -> Selected {
+        Selected::from(self.inner.select.load(Ordering::Acquire))
+    }
+
+    /// Stores a packet.
+    ///
+    /// This method must be called after `try_select` succeeds and there is a packet to provide.
+    #[inline]
+    pub fn store_packet(&self, packet: *mut ()) {
+        if !packet.is_null() {
+            self.inner.packet.store(packet, Ordering::Release);
+        }
+    }
+
+    /// Waits until a packet is provided and returns it.
+    #[inline]
+    pub fn wait_packet(&self) -> *mut () {
+        let backoff = Backoff::new();
+        loop {
+            let packet = self.inner.packet.load(Ordering::Acquire);
+            if !packet.is_null() {
+                return packet;
+            }
+            backoff.snooze();
+        }
+    }
+
+    /// Waits until an operation is selected and returns it.
+    ///
+    /// If the deadline is reached, `Selected::Aborted` will be selected.
+    #[inline]
+    pub fn wait_until(&self, deadline: Option<Instant>) -> Selected {
+        loop {
+            // Check whether an operation has been selected.
+            let sel = Selected::from(self.inner.select.load(Ordering::Acquire));
+            if sel != Selected::Waiting {
+                return sel;
+            }
+
+            // If there's a deadline, park the current thread until the deadline is reached.
+            if let Some(end) = deadline {
+                let now = Instant::now();
+
+                if now < end {
+                    thread::park_timeout(end - now);
+                } else {
+                    // The deadline has been reached. Try aborting select.
+                    return match self.try_select(Selected::Aborted) {
+                        Ok(()) => Selected::Aborted,
+                        Err(s) => s,
+                    };
+                }
+            } else {
+                thread::park();
+            }
+        }
+    }
+
+    /// Unparks the thread this context belongs to.
+    #[inline]
+    pub fn unpark(&self) {
+        self.inner.thread.unpark();
+    }
+
+    /// Returns the id of the thread this context belongs to.
+    #[inline]
+    pub fn thread_id(&self) -> ThreadId {
+        self.inner.thread_id
+    }
+}

vendor/crossbeam-channel/src/counter.rs

@@ -0,0 +1,145 @@
+//! Reference counter for channels.
+
+use std::boxed::Box;
+use std::isize;
+use std::ops;
+use std::process;
+use std::sync::atomic::{AtomicBool, AtomicUsize, Ordering};
+
+/// Reference counter internals.
+struct Counter<C> {
+    /// The number of senders associated with the channel.
+    senders: AtomicUsize,
+
+    /// The number of receivers associated with the channel.
+    receivers: AtomicUsize,
+
+    /// Set to `true` if the last sender or the last receiver reference deallocates the channel.
+    destroy: AtomicBool,
+
+    /// The internal channel.
+    chan: C,
+}
+
+/// Wraps a channel into the reference counter.
+pub(crate) fn new<C>(chan: C) -> (Sender<C>, Receiver<C>) {
+    let counter = Box::into_raw(Box::new(Counter {
+        senders: AtomicUsize::new(1),
+        receivers: AtomicUsize::new(1),
+        destroy: AtomicBool::new(false),
+        chan,
+    }));
+    let s = Sender { counter };
+    let r = Receiver { counter };
+    (s, r)
+}
+
+/// The sending side.
+pub(crate) struct Sender<C> {
+    counter: *mut Counter<C>,
+}
+
+impl<C> Sender<C> {
+    /// Returns the internal `Counter`.
+    fn counter(&self) -> &Counter<C> {
+        unsafe { &*self.counter }
+    }
+
+    /// Acquires another sender reference.
+    pub(crate) fn acquire(&self) -> Sender<C> {
+        let count = self.counter().senders.fetch_add(1, Ordering::Relaxed);
+
+        // Cloning senders and calling `mem::forget` on the clones could potentially overflow the
+        // counter. It's very difficult to recover sensibly from such degenerate scenarios so we
+        // just abort when the count becomes very large.
+        if count > isize::MAX as usize {
+            process::abort();
+        }
+
+        Sender {
+            counter: self.counter,
+        }
+    }
+
+    /// Releases the sender reference.
+    ///
+    /// Function `disconnect` will be called if this is the last sender reference.
+    pub(crate) unsafe fn release<F: FnOnce(&C) -> bool>(&self, disconnect: F) {
+        if self.counter().senders.fetch_sub(1, Ordering::AcqRel) == 1 {
+            disconnect(&self.counter().chan);
+
+            if self.counter().destroy.swap(true, Ordering::AcqRel) {
+                drop(Box::from_raw(self.counter));
+            }
+        }
+    }
+}
+
+impl<C> ops::Deref for Sender<C> {
+    type Target = C;
+
+    fn deref(&self) -> &C {
+        &self.counter().chan
+    }
+}
+
+impl<C> PartialEq for Sender<C> {
+    fn eq(&self, other: &Sender<C>) -> bool {
+        self.counter == other.counter
+    }
+}
+
+/// The receiving side.
+pub(crate) struct Receiver<C> {
+    counter: *mut Counter<C>,
+}
+
+impl<C> Receiver<C> {
+    /// Returns the internal `Counter`.
+    fn counter(&self) -> &Counter<C> {
+        unsafe { &*self.counter }
+    }
+
+    /// Acquires another receiver reference.
+    pub(crate) fn acquire(&self) -> Receiver<C> {
+        let count = self.counter().receivers.fetch_add(1, Ordering::Relaxed);
+
+        // Cloning receivers and calling `mem::forget` on the clones could potentially overflow the
+        // counter. It's very difficult to recover sensibly from such degenerate scenarios so we
+        // just abort when the count becomes very large.
+        if count > isize::MAX as usize {
+            process::abort();
+        }
+
+        Receiver {
+            counter: self.counter,
+        }
+    }
+
+    /// Releases the receiver reference.
+    ///
+    /// Function `disconnect` will be called if this is the last receiver reference.
+    pub(crate) unsafe fn release<F: FnOnce(&C) -> bool>(&self, disconnect: F) {
+        if self.counter().receivers.fetch_sub(1, Ordering::AcqRel) == 1 {
+            disconnect(&self.counter().chan);
+
+            if self.counter().destroy.swap(true, Ordering::AcqRel) {
+                drop(Box::from_raw(self.counter));
+            }
+        }
+    }
+}
+
+impl<C> ops::Deref for Receiver<C> {
+    type Target = C;
+
+    fn deref(&self) -> &C {
+        &self.counter().chan
+    }
+}
+
+impl<C> PartialEq for Receiver<C> {
+    fn eq(&self, other: &Receiver<C>) -> bool {
+        self.counter == other.counter
+    }
+}

vendor/crossbeam-channel/src/err.rs

@@ -0,0 +1,378 @@
+use std::error;
+use std::fmt;
+
+/// An error returned from the [`send`] method.
+///
+/// The message could not be sent because the channel is disconnected.
+///
+/// The error contains the message so it can be recovered.
+///
+/// [`send`]: super::Sender::send
+#[derive(PartialEq, Eq, Clone, Copy)]
+pub struct SendError<T>(pub T);
+
+/// An error returned from the [`try_send`] method.
+///
+/// The error contains the message being sent so it can be recovered.
+///
+/// [`try_send`]: super::Sender::try_send
+#[derive(PartialEq, Eq, Clone, Copy)]
+pub enum TrySendError<T> {
+    /// The message could not be sent because the channel is full.
+    ///
+    /// If this is a zero-capacity channel, then the error indicates that there was no receiver
+    /// available to receive the message at the time.
+    Full(T),
+
+    /// The message could not be sent because the channel is disconnected.
+    Disconnected(T),
+}
+
+/// An error returned from the [`send_timeout`] method.
+///
+/// The error contains the message being sent so it can be recovered.
+///
+/// [`send_timeout`]: super::Sender::send_timeout
+#[derive(PartialEq, Eq, Clone, Copy)]
+pub enum SendTimeoutError<T> {
+    /// The message could not be sent because the channel is full and the operation timed out.
+    ///
+    /// If this is a zero-capacity channel, then the error indicates that there was no receiver
+    /// available to receive the message and the operation timed out.
+    Timeout(T),
+
+    /// The message could not be sent because the channel is disconnected.
+    Disconnected(T),
+}
+
+/// An error returned from the [`recv`] method.
+///
+/// A message could not be received because the channel is empty and disconnected.
+///
+/// [`recv`]: super::Receiver::recv
+#[derive(PartialEq, Eq, Clone, Copy, Debug)]
+pub struct RecvError;
+
+/// An error returned from the [`try_recv`] method.
+///
+/// [`try_recv`]: super::Receiver::try_recv
+#[derive(PartialEq, Eq, Clone, Copy, Debug)]
+pub enum TryRecvError {
+    /// A message could not be received because the channel is empty.
+    ///
+    /// If this is a zero-capacity channel, then the error indicates that there was no sender
+    /// available to send a message at the time.
+    Empty,
+
+    /// The message could not be received because the channel is empty and disconnected.
+    Disconnected,
+}
+
+/// An error returned from the [`recv_timeout`] method.
+///
+/// [`recv_timeout`]: super::Receiver::recv_timeout
+#[derive(PartialEq, Eq, Clone, Copy, Debug)]
+pub enum RecvTimeoutError {
+    /// A message could not be received because the channel is empty and the operation timed out.
+    ///
+    /// If this is a zero-capacity channel, then the error indicates that there was no sender
+    /// available to send a message and the operation timed out.
+    Timeout,
+
+    /// The message could not be received because the channel is empty and disconnected.
+    Disconnected,
+}
+
+/// An error returned from the [`try_select`] method.
+///
+/// Failed because none of the channel operations were ready.
+///
+/// [`try_select`]: super::Select::try_select
+#[derive(PartialEq, Eq, Clone, Copy, Debug)]
+pub struct TrySelectError;
+
+/// An error returned from the [`select_timeout`] method.
+///
+/// Failed because none of the channel operations became ready before the timeout.
+///
+/// [`select_timeout`]: super::Select::select_timeout
+#[derive(PartialEq, Eq, Clone, Copy, Debug)]
+pub struct SelectTimeoutError;
+
+/// An error returned from the [`try_ready`] method.
+///
+/// Failed because none of the channel operations were ready.
+///
+/// [`try_ready`]: super::Select::try_ready
+#[derive(PartialEq, Eq, Clone, Copy, Debug)]
+pub struct TryReadyError;
+
+/// An error returned from the [`ready_timeout`] method.
+///
+/// Failed because none of the channel operations became ready before the timeout.
+///
+/// [`ready_timeout`]: super::Select::ready_timeout
+#[derive(PartialEq, Eq, Clone, Copy, Debug)]
+pub struct ReadyTimeoutError;
+
+impl<T> fmt::Debug for SendError<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        "SendError(..)".fmt(f)
+    }
+}
+
+impl<T> fmt::Display for SendError<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        "sending on a disconnected channel".fmt(f)
+    }
+}
+
+impl<T: Send> error::Error for SendError<T> {}
+
+impl<T> SendError<T> {
+    /// Unwraps the message.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_channel::unbounded;
+    ///
+    /// let (s, r) = unbounded();
+    /// drop(r);
+    ///
+    /// if let Err(err) = s.send("foo") {
+    ///     assert_eq!(err.into_inner(), "foo");
+    /// }
+    /// ```
+    pub fn into_inner(self) -> T {
+        self.0
+    }
+}
+
+impl<T> fmt::Debug for TrySendError<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match *self {
+            TrySendError::Full(..) => "Full(..)".fmt(f),
+            TrySendError::Disconnected(..) => "Disconnected(..)".fmt(f),
+        }
+    }
+}
+
+impl<T> fmt::Display for TrySendError<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match *self {
+            TrySendError::Full(..) => "sending on a full channel".fmt(f),
+            TrySendError::Disconnected(..) => "sending on a disconnected channel".fmt(f),
+        }
+    }
+}
+
+impl<T: Send> error::Error for TrySendError<T> {}
+
+impl<T> From<SendError<T>> for TrySendError<T> {
+    fn from(err: SendError<T>) -> TrySendError<T> {
+        match err {
+            SendError(t) => TrySendError::Disconnected(t),
+        }
+    }
+}
+
+impl<T> TrySendError<T> {
+    /// Unwraps the message.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_channel::bounded;
+    ///
+    /// let (s, r) = bounded(0);
+    ///
+    /// if let Err(err) = s.try_send("foo") {
+    ///     assert_eq!(err.into_inner(), "foo");
+    /// }
+    /// ```
+    pub fn into_inner(self) -> T {
+        match self {
+            TrySendError::Full(v) => v,
+            TrySendError::Disconnected(v) => v,
+        }
+    }
+
+    /// Returns `true` if the send operation failed because the channel is full.
+    pub fn is_full(&self) -> bool {
+        match self {
+            TrySendError::Full(_) => true,
+            _ => false,
+        }
+    }
+
+    /// Returns `true` if the send operation failed because the channel is disconnected.
+    pub fn is_disconnected(&self) -> bool {
+        match self {
+            TrySendError::Disconnected(_) => true,
+            _ => false,
+        }
+    }
+}
+
+impl<T> fmt::Debug for SendTimeoutError<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        "SendTimeoutError(..)".fmt(f)
+    }
+}
+
+impl<T> fmt::Display for SendTimeoutError<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match *self {
+            SendTimeoutError::Timeout(..) => "timed out waiting on send operation".fmt(f),
+            SendTimeoutError::Disconnected(..) => "sending on a disconnected channel".fmt(f),
+        }
+    }
+}
+
+impl<T: Send> error::Error for SendTimeoutError<T> {}
+
+impl<T> From<SendError<T>> for SendTimeoutError<T> {
+    fn from(err: SendError<T>) -> SendTimeoutError<T> {
+        match err {
+            SendError(e) => SendTimeoutError::Disconnected(e),
+        }
+    }
+}
+
+impl<T> SendTimeoutError<T> {
+    /// Unwraps the message.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::time::Duration;
+    /// use crossbeam_channel::unbounded;
+    ///
+    /// let (s, r) = unbounded();
+    ///
+    /// if let Err(err) = s.send_timeout("foo", Duration::from_secs(1)) {
+    ///     assert_eq!(err.into_inner(), "foo");
+    /// }
+    /// ```
+    pub fn into_inner(self) -> T {
+        match self {
+            SendTimeoutError::Timeout(v) => v,
+            SendTimeoutError::Disconnected(v) => v,
+        }
+    }
+
+    /// Returns `true` if the send operation timed out.
+    pub fn is_timeout(&self) -> bool {
+        match self {
+            SendTimeoutError::Timeout(_) => true,
+            _ => false,
+        }
+    }
+
+    /// Returns `true` if the send operation failed because the channel is disconnected.
+    pub fn is_disconnected(&self) -> bool {
+        match self {
+            SendTimeoutError::Disconnected(_) => true,
+            _ => false,
+        }
+    }
+}
+
+impl fmt::Display for RecvError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        "receiving on an empty and disconnected channel".fmt(f)
+    }
+}
+
+impl error::Error for RecvError {}
+
+impl fmt::Display for TryRecvError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match *self {
+            TryRecvError::Empty => "receiving on an empty channel".fmt(f),
+            TryRecvError::Disconnected => "receiving on an empty and disconnected channel".fmt(f),
+        }
+    }
+}
+
+impl error::Error for TryRecvError {}
+
+impl From<RecvError> for TryRecvError {
+    fn from(err: RecvError) -> TryRecvError {
+        match err {
+            RecvError => TryRecvError::Disconnected,
+        }
+    }
+}
+
+impl TryRecvError {
+    /// Returns `true` if the receive operation failed because the channel is empty.
+    pub fn is_empty(&self) -> bool {
+        match self {
+            TryRecvError::Empty => true,
+            _ => false,
+        }
+    }
+
+    /// Returns `true` if the receive operation failed because the channel is disconnected.
+    pub fn is_disconnected(&self) -> bool {
+        match self {
+            TryRecvError::Disconnected => true,
+            _ => false,
+        }
+    }
+}
+
+impl fmt::Display for RecvTimeoutError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match *self {
+            RecvTimeoutError::Timeout => "timed out waiting on receive operation".fmt(f),
+            RecvTimeoutError::Disconnected => "channel is empty and disconnected".fmt(f),
+        }
+    }
+}
+
+impl error::Error for RecvTimeoutError {}
+
+impl From<RecvError> for RecvTimeoutError {
+    fn from(err: RecvError) -> RecvTimeoutError {
+        match err {
+            RecvError => RecvTimeoutError::Disconnected,
+        }
+    }
+}
+
+impl RecvTimeoutError {
+    /// Returns `true` if the receive operation timed out.
+    pub fn is_timeout(&self) -> bool {
+        match self {
+            RecvTimeoutError::Timeout => true,
+            _ => false,
+        }
+    }
+
+    /// Returns `true` if the receive operation failed because the channel is disconnected.
+    pub fn is_disconnected(&self) -> bool {
+        match self {
+            RecvTimeoutError::Disconnected => true,
+            _ => false,
+        }
+    }
+}
+
+impl fmt::Display for TrySelectError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        "all operations in select would block".fmt(f)
+    }
+}
+
+impl error::Error for TrySelectError {}
+
+impl fmt::Display for SelectTimeoutError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        "timed out waiting on select".fmt(f)
+    }
+}
+
+impl error::Error for SelectTimeoutError {}

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 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);
+    }
+}

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) {}
+}

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

@@ -0,0 +1,773 @@
+//! Unbounded channel implemented as a linked list.
+
+use std::alloc::{alloc_zeroed, handle_alloc_error, Layout};
+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> {
+    /// 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> {
+    const LAYOUT: Layout = {
+        let layout = Layout::new::<Self>();
+        assert!(
+            layout.size() != 0,
+            "Block should never be zero-sized, as it has an AtomicPtr field"
+        );
+        layout
+    };
+
+    /// Creates an empty block.
+    fn new() -> Box<Self> {
+        // SAFETY: layout is not zero-sized
+        let ptr = unsafe { alloc_zeroed(Self::LAYOUT) };
+        // Handle allocation failure
+        if ptr.is_null() {
+            handle_alloc_error(Self::LAYOUT)
+        }
+        // SAFETY: This is safe because:
+        //  [1] `Block::next` (AtomicPtr) may be safely zero initialized.
+        //  [2] `Block::slots` (Array) may be safely zero initialized because of [3, 4].
+        //  [3] `Slot::msg` (UnsafeCell) may be safely zero initialized because it
+        //       holds a MaybeUninit.
+        //  [4] `Slot::state` (AtomicUsize) may be safely zero initialized.
+        // TODO: unsafe { Box::new_zeroed().assume_init() }
+        unsafe { Box::from_raw(ptr.cast()) }
+    }
+
+    /// 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(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(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);
+        // The channel may be uninitialized, so we have to swap to avoid overwriting any sender's attempts
+        // to initialize the first block before noticing that the receivers disconnected. Late allocations
+        // will be deallocated by the sender in Drop
+        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.swap(ptr::null_mut(), Ordering::AcqRel);
+            }
+        }
+
+        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) {}
+}

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;

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) {}
+}

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) {}
+}

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);
+    }
+}

vendor/crossbeam-channel/src/lib.rs

@@ -0,0 +1,378 @@
+//! Multi-producer multi-consumer channels for message passing.
+//!
+//! This crate is an alternative to [`std::sync::mpsc`] with more features and better performance.
+//!
+//! # Hello, world!
+//!
+//! ```
+//! use crossbeam_channel::unbounded;
+//!
+//! // Create a channel of unbounded capacity.
+//! let (s, r) = unbounded();
+//!
+//! // Send a message into the channel.
+//! s.send("Hello, world!").unwrap();
+//!
+//! // Receive the message from the channel.
+//! assert_eq!(r.recv(), Ok("Hello, world!"));
+//! ```
+//!
+//! # Channel types
+//!
+//! Channels can be created using two functions:
+//!
+//! * [`bounded`] creates a channel of bounded capacity, i.e. there is a limit to how many messages
+//!   it can hold at a time.
+//!
+//! * [`unbounded`] creates a channel of unbounded capacity, i.e. it can hold any number of
+//!   messages at a time.
+//!
+//! Both functions return a [`Sender`] and a [`Receiver`], which represent the two opposite sides
+//! of a channel.
+//!
+//! Creating a bounded channel:
+//!
+//! ```
+//! use crossbeam_channel::bounded;
+//!
+//! // Create a channel that can hold at most 5 messages at a time.
+//! let (s, r) = bounded(5);
+//!
+//! // Can send only 5 messages without blocking.
+//! for i in 0..5 {
+//!     s.send(i).unwrap();
+//! }
+//!
+//! // Another call to `send` would block because the channel is full.
+//! // s.send(5).unwrap();
+//! ```
+//!
+//! Creating an unbounded channel:
+//!
+//! ```
+//! use crossbeam_channel::unbounded;
+//!
+//! // Create an unbounded channel.
+//! let (s, r) = unbounded();
+//!
+//! // Can send any number of messages into the channel without blocking.
+//! for i in 0..1000 {
+//!     s.send(i).unwrap();
+//! }
+//! ```
+//!
+//! A special case is zero-capacity channel, which cannot hold any messages. Instead, send and
+//! receive operations must appear at the same time in order to pair up and pass the message over:
+//!
+//! ```
+//! use std::thread;
+//! use crossbeam_channel::bounded;
+//!
+//! // Create a zero-capacity channel.
+//! let (s, r) = bounded(0);
+//!
+//! // Sending blocks until a receive operation appears on the other side.
+//! thread::spawn(move || s.send("Hi!").unwrap());
+//!
+//! // Receiving blocks until a send operation appears on the other side.
+//! assert_eq!(r.recv(), Ok("Hi!"));
+//! ```
+//!
+//! # Sharing channels
+//!
+//! Senders and receivers can be cloned and sent to other threads:
+//!
+//! ```
+//! use std::thread;
+//! use crossbeam_channel::bounded;
+//!
+//! let (s1, r1) = bounded(0);
+//! let (s2, r2) = (s1.clone(), r1.clone());
+//!
+//! // Spawn a thread that receives a message and then sends one.
+//! thread::spawn(move || {
+//!     r2.recv().unwrap();
+//!     s2.send(2).unwrap();
+//! });
+//!
+//! // Send a message and then receive one.
+//! s1.send(1).unwrap();
+//! r1.recv().unwrap();
+//! ```
+//!
+//! Note that cloning only creates a new handle to the same sending or receiving side. It does not
+//! create a separate stream of messages in any way:
+//!
+//! ```
+//! use crossbeam_channel::unbounded;
+//!
+//! let (s1, r1) = unbounded();
+//! let (s2, r2) = (s1.clone(), r1.clone());
+//! let (s3, r3) = (s2.clone(), r2.clone());
+//!
+//! s1.send(10).unwrap();
+//! s2.send(20).unwrap();
+//! s3.send(30).unwrap();
+//!
+//! assert_eq!(r3.recv(), Ok(10));
+//! assert_eq!(r1.recv(), Ok(20));
+//! assert_eq!(r2.recv(), Ok(30));
+//! ```
+//!
+//! It's also possible to share senders and receivers by reference:
+//!
+//! ```
+//! use crossbeam_channel::bounded;
+//! use crossbeam_utils::thread::scope;
+//!
+//! let (s, r) = bounded(0);
+//!
+//! scope(|scope| {
+//!     // Spawn a thread that receives a message and then sends one.
+//!     scope.spawn(|_| {
+//!         r.recv().unwrap();
+//!         s.send(2).unwrap();
+//!     });
+//!
+//!     // Send a message and then receive one.
+//!     s.send(1).unwrap();
+//!     r.recv().unwrap();
+//! }).unwrap();
+//! ```
+//!
+//! # Disconnection
+//!
+//! When all senders or all receivers associated with a channel get dropped, the channel becomes
+//! disconnected. No more messages can be sent, but any remaining messages can still be received.
+//! Send and receive operations on a disconnected channel never block.
+//!
+//! ```
+//! use crossbeam_channel::{unbounded, RecvError};
+//!
+//! let (s, r) = unbounded();
+//! s.send(1).unwrap();
+//! s.send(2).unwrap();
+//! s.send(3).unwrap();
+//!
+//! // The only sender is dropped, disconnecting the channel.
+//! drop(s);
+//!
+//! // The remaining messages can be received.
+//! assert_eq!(r.recv(), Ok(1));
+//! assert_eq!(r.recv(), Ok(2));
+//! assert_eq!(r.recv(), Ok(3));
+//!
+//! // There are no more messages in the channel.
+//! assert!(r.is_empty());
+//!
+//! // Note that calling `r.recv()` does not block.
+//! // Instead, `Err(RecvError)` is returned immediately.
+//! assert_eq!(r.recv(), Err(RecvError));
+//! ```
+//!
+//! # Blocking operations
+//!
+//! Send and receive operations come in three flavors:
+//!
+//! * Non-blocking (returns immediately with success or failure).
+//! * Blocking (waits until the operation succeeds or the channel becomes disconnected).
+//! * Blocking with a timeout (blocks only for a certain duration of time).
+//!
+//! A simple example showing the difference between non-blocking and blocking operations:
+//!
+//! ```
+//! use crossbeam_channel::{bounded, RecvError, TryRecvError};
+//!
+//! let (s, r) = bounded(1);
+//!
+//! // Send a message into the channel.
+//! s.send("foo").unwrap();
+//!
+//! // This call would block because the channel is full.
+//! // s.send("bar").unwrap();
+//!
+//! // Receive the message.
+//! assert_eq!(r.recv(), Ok("foo"));
+//!
+//! // This call would block because the channel is empty.
+//! // r.recv();
+//!
+//! // Try receiving a message without blocking.
+//! assert_eq!(r.try_recv(), Err(TryRecvError::Empty));
+//!
+//! // Disconnect the channel.
+//! drop(s);
+//!
+//! // This call doesn't block because the channel is now disconnected.
+//! assert_eq!(r.recv(), Err(RecvError));
+//! ```
+//!
+//! # Iteration
+//!
+//! Receivers can be used as iterators. For example, method [`iter`] creates an iterator that
+//! receives messages until the channel becomes empty and disconnected. Note that iteration may
+//! block waiting for next message to arrive.
+//!
+//! ```
+//! use std::thread;
+//! use crossbeam_channel::unbounded;
+//!
+//! let (s, r) = unbounded();
+//!
+//! thread::spawn(move || {
+//!     s.send(1).unwrap();
+//!     s.send(2).unwrap();
+//!     s.send(3).unwrap();
+//!     drop(s); // Disconnect the channel.
+//! });
+//!
+//! // Collect all messages from the channel.
+//! // Note that the call to `collect` blocks until the sender is dropped.
+//! let v: Vec<_> = r.iter().collect();
+//!
+//! assert_eq!(v, [1, 2, 3]);
+//! ```
+//!
+//! A non-blocking iterator can be created using [`try_iter`], which receives all available
+//! messages without blocking:
+//!
+//! ```
+//! use crossbeam_channel::unbounded;
+//!
+//! let (s, r) = unbounded();
+//! s.send(1).unwrap();
+//! s.send(2).unwrap();
+//! s.send(3).unwrap();
+//! // No need to drop the sender.
+//!
+//! // Receive all messages currently in the channel.
+//! let v: Vec<_> = r.try_iter().collect();
+//!
+//! assert_eq!(v, [1, 2, 3]);
+//! ```
+//!
+//! # Selection
+//!
+//! The [`select!`] macro allows you to define a set of channel operations, wait until any one of
+//! them becomes ready, and finally execute it. If multiple operations are ready at the same time,
+//! a random one among them is selected.
+//!
+//! It is also possible to define a `default` case that gets executed if none of the operations are
+//! ready, either right away or for a certain duration of time.
+//!
+//! An operation is considered to be ready if it doesn't have to block. Note that it is ready even
+//! when it will simply return an error because the channel is disconnected.
+//!
+//! An example of receiving a message from two channels:
+//!
+//! ```
+//! use std::thread;
+//! use std::time::Duration;
+//! use crossbeam_channel::{select, unbounded};
+//!
+//! let (s1, r1) = unbounded();
+//! let (s2, r2) = unbounded();
+//!
+//! thread::spawn(move || s1.send(10).unwrap());
+//! thread::spawn(move || s2.send(20).unwrap());
+//!
+//! // At most one of these two receive operations will be executed.
+//! select! {
+//!     recv(r1) -> msg => assert_eq!(msg, Ok(10)),
+//!     recv(r2) -> msg => assert_eq!(msg, Ok(20)),
+//!     default(Duration::from_secs(1)) => println!("timed out"),
+//! }
+//! ```
+//!
+//! If you need to select over a dynamically created list of channel operations, use [`Select`]
+//! instead. The [`select!`] macro is just a convenience wrapper around [`Select`].
+//!
+//! # Extra channels
+//!
+//! Three functions can create special kinds of channels, all of which return just a [`Receiver`]
+//! handle:
+//!
+//! * [`after`] creates a channel that delivers a single message after a certain duration of time.
+//! * [`tick`] creates a channel that delivers messages periodically.
+//! * [`never`](never()) creates a channel that never delivers messages.
+//!
+//! These channels are very efficient because messages get lazily generated on receive operations.
+//!
+//! An example that prints elapsed time every 50 milliseconds for the duration of 1 second:
+//!
+//! ```
+//! use std::time::{Duration, Instant};
+//! use crossbeam_channel::{after, select, tick};
+//!
+//! let start = Instant::now();
+//! let ticker = tick(Duration::from_millis(50));
+//! let timeout = after(Duration::from_secs(1));
+//!
+//! loop {
+//!     select! {
+//!         recv(ticker) -> _ => println!("elapsed: {:?}", start.elapsed()),
+//!         recv(timeout) -> _ => break,
+//!     }
+//! }
+//! ```
+//!
+//! [`send`]: Sender::send
+//! [`recv`]: Receiver::recv
+//! [`iter`]: Receiver::iter
+//! [`try_iter`]: Receiver::try_iter
+
+#![no_std]
+#![doc(test(
+    no_crate_inject,
+    attr(
+        deny(warnings, rust_2018_idioms),
+        allow(dead_code, unused_assignments, unused_variables)
+    )
+))]
+#![warn(
+    missing_docs,
+    missing_debug_implementations,
+    rust_2018_idioms,
+    unreachable_pub
+)]
+
+#[cfg(feature = "std")]
+extern crate std;
+
+#[cfg(feature = "std")]
+mod channel;
+#[cfg(feature = "std")]
+mod context;
+#[cfg(feature = "std")]
+mod counter;
+#[cfg(feature = "std")]
+mod err;
+#[cfg(feature = "std")]
+mod flavors;
+#[cfg(feature = "std")]
+mod select;
+#[cfg(feature = "std")]
+mod select_macro;
+#[cfg(feature = "std")]
+mod utils;
+#[cfg(feature = "std")]
+mod waker;
+
+/// Crate internals used by the `select!` macro.
+#[doc(hidden)]
+#[cfg(feature = "std")]
+pub mod internal {
+    pub use crate::select::{select, select_timeout, try_select, SelectHandle};
+}
+
+#[cfg(feature = "std")]
+pub use crate::{
+    channel::{
+        after, at, bounded, never, tick, unbounded, IntoIter, Iter, Receiver, Sender, TryIter,
+    },
+    err::{
+        ReadyTimeoutError, RecvError, RecvTimeoutError, SelectTimeoutError, SendError,
+        SendTimeoutError, TryReadyError, TryRecvError, TrySelectError, TrySendError,
+    },
+    select::{Select, SelectedOperation},
+};

vendor/crossbeam-channel/src/utils.rs

@@ -0,0 +1,58 @@
+//! Miscellaneous utilities.
+
+use std::cell::Cell;
+use std::num::Wrapping;
+use std::thread;
+use std::time::{Duration, Instant};
+
+/// Randomly shuffles a slice.
+pub(crate) fn shuffle<T>(v: &mut [T]) {
+    let len = v.len();
+    if len <= 1 {
+        return;
+    }
+
+    std::thread_local! {
+        static RNG: Cell<Wrapping<u32>> = const { Cell::new(Wrapping(1_406_868_647)) };
+    }
+
+    let _ = RNG.try_with(|rng| {
+        for i in 1..len {
+            // This is the 32-bit variant of Xorshift.
+            //
+            // Source: https://en.wikipedia.org/wiki/Xorshift
+            let mut x = rng.get();
+            x ^= x << 13;
+            x ^= x >> 17;
+            x ^= x << 5;
+            rng.set(x);
+
+            let x = x.0;
+            let n = i + 1;
+
+            // This is a fast alternative to `let j = x % n`.
+            //
+            // Author: Daniel Lemire
+            // Source: https://lemire.me/blog/2016/06/27/a-fast-alternative-to-the-modulo-reduction/
+            let j = ((x as u64).wrapping_mul(n as u64) >> 32) as u32 as usize;
+
+            v.swap(i, j);
+        }
+    });
+}
+
+/// Sleeps until the deadline, or forever if the deadline isn't specified.
+pub(crate) fn sleep_until(deadline: Option<Instant>) {
+    loop {
+        match deadline {
+            None => thread::sleep(Duration::from_secs(1000)),
+            Some(d) => {
+                let now = Instant::now();
+                if now >= d {
+                    break;
+                }
+                thread::sleep(d - now);
+            }
+        }
+    }
+}

vendor/crossbeam-channel/src/waker.rs

@@ -0,0 +1,287 @@
+//! Waking mechanism for threads blocked on channel operations.
+
+use std::ptr;
+use std::sync::atomic::{AtomicBool, Ordering};
+use std::sync::Mutex;
+use std::thread::{self, ThreadId};
+use std::vec::Vec;
+
+use crate::context::Context;
+use crate::select::{Operation, Selected};
+
+/// Represents a thread blocked on a specific channel operation.
+pub(crate) struct Entry {
+    /// The operation.
+    pub(crate) oper: Operation,
+
+    /// Optional packet.
+    pub(crate) packet: *mut (),
+
+    /// Context associated with the thread owning this operation.
+    pub(crate) cx: Context,
+}
+
+/// A queue of threads blocked on channel operations.
+///
+/// This data structure is used by threads to register blocking operations and get woken up once
+/// an operation becomes ready.
+pub(crate) struct Waker {
+    /// A list of select operations.
+    selectors: Vec<Entry>,
+
+    /// A list of operations waiting to be ready.
+    observers: Vec<Entry>,
+}
+
+impl Waker {
+    /// Creates a new `Waker`.
+    #[inline]
+    pub(crate) fn new() -> Self {
+        Waker {
+            selectors: Vec::new(),
+            observers: Vec::new(),
+        }
+    }
+
+    /// Registers a select operation.
+    #[inline]
+    pub(crate) fn register(&mut self, oper: Operation, cx: &Context) {
+        self.register_with_packet(oper, ptr::null_mut(), cx);
+    }
+
+    /// Registers a select operation and a packet.
+    #[inline]
+    pub(crate) fn register_with_packet(&mut self, oper: Operation, packet: *mut (), cx: &Context) {
+        self.selectors.push(Entry {
+            oper,
+            packet,
+            cx: cx.clone(),
+        });
+    }
+
+    /// Unregisters a select operation.
+    #[inline]
+    pub(crate) fn unregister(&mut self, oper: Operation) -> Option<Entry> {
+        if let Some((i, _)) = self
+            .selectors
+            .iter()
+            .enumerate()
+            .find(|&(_, entry)| entry.oper == oper)
+        {
+            let entry = self.selectors.remove(i);
+            Some(entry)
+        } else {
+            None
+        }
+    }
+
+    /// Attempts to find another thread's entry, select the operation, and wake it up.
+    #[inline]
+    pub(crate) fn try_select(&mut self) -> Option<Entry> {
+        if self.selectors.is_empty() {
+            None
+        } else {
+            let thread_id = current_thread_id();
+
+            self.selectors
+                .iter()
+                .position(|selector| {
+                    // Does the entry belong to a different thread?
+                    selector.cx.thread_id() != thread_id
+                        && selector // Try selecting this operation.
+                            .cx
+                            .try_select(Selected::Operation(selector.oper))
+                            .is_ok()
+                        && {
+                            // Provide the packet.
+                            selector.cx.store_packet(selector.packet);
+                            // Wake the thread up.
+                            selector.cx.unpark();
+                            true
+                        }
+                })
+                // Remove the entry from the queue to keep it clean and improve
+                // performance.
+                .map(|pos| self.selectors.remove(pos))
+        }
+    }
+
+    /// Returns `true` if there is an entry which can be selected by the current thread.
+    #[inline]
+    pub(crate) fn can_select(&self) -> bool {
+        if self.selectors.is_empty() {
+            false
+        } else {
+            let thread_id = current_thread_id();
+
+            self.selectors.iter().any(|entry| {
+                entry.cx.thread_id() != thread_id && entry.cx.selected() == Selected::Waiting
+            })
+        }
+    }
+
+    /// Registers an operation waiting to be ready.
+    #[inline]
+    pub(crate) fn watch(&mut self, oper: Operation, cx: &Context) {
+        self.observers.push(Entry {
+            oper,
+            packet: ptr::null_mut(),
+            cx: cx.clone(),
+        });
+    }
+
+    /// Unregisters an operation waiting to be ready.
+    #[inline]
+    pub(crate) fn unwatch(&mut self, oper: Operation) {
+        self.observers.retain(|e| e.oper != oper);
+    }
+
+    /// Notifies all operations waiting to be ready.
+    #[inline]
+    pub(crate) fn notify(&mut self) {
+        for entry in self.observers.drain(..) {
+            if entry.cx.try_select(Selected::Operation(entry.oper)).is_ok() {
+                entry.cx.unpark();
+            }
+        }
+    }
+
+    /// Notifies all registered operations that the channel is disconnected.
+    #[inline]
+    pub(crate) fn disconnect(&mut self) {
+        for entry in self.selectors.iter() {
+            if entry.cx.try_select(Selected::Disconnected).is_ok() {
+                // Wake the thread up.
+                //
+                // Here we don't remove the entry from the queue. Registered threads must
+                // unregister from the waker by themselves. They might also want to recover the
+                // packet value and destroy it, if necessary.
+                entry.cx.unpark();
+            }
+        }
+
+        self.notify();
+    }
+}
+
+impl Drop for Waker {
+    #[inline]
+    fn drop(&mut self) {
+        debug_assert_eq!(self.selectors.len(), 0);
+        debug_assert_eq!(self.observers.len(), 0);
+    }
+}
+
+/// A waker that can be shared among threads without locking.
+///
+/// This is a simple wrapper around `Waker` that internally uses a mutex for synchronization.
+pub(crate) struct SyncWaker {
+    /// The inner `Waker`.
+    inner: Mutex<Waker>,
+
+    /// `true` if the waker is empty.
+    is_empty: AtomicBool,
+}
+
+impl SyncWaker {
+    /// Creates a new `SyncWaker`.
+    #[inline]
+    pub(crate) fn new() -> Self {
+        SyncWaker {
+            inner: Mutex::new(Waker::new()),
+            is_empty: AtomicBool::new(true),
+        }
+    }
+
+    /// Registers the current thread with an operation.
+    #[inline]
+    pub(crate) fn register(&self, oper: Operation, cx: &Context) {
+        let mut inner = self.inner.lock().unwrap();
+        inner.register(oper, cx);
+        self.is_empty.store(
+            inner.selectors.is_empty() && inner.observers.is_empty(),
+            Ordering::SeqCst,
+        );
+    }
+
+    /// Unregisters an operation previously registered by the current thread.
+    #[inline]
+    pub(crate) fn unregister(&self, oper: Operation) -> Option<Entry> {
+        let mut inner = self.inner.lock().unwrap();
+        let entry = inner.unregister(oper);
+        self.is_empty.store(
+            inner.selectors.is_empty() && inner.observers.is_empty(),
+            Ordering::SeqCst,
+        );
+        entry
+    }
+
+    /// Attempts to find one thread (not the current one), select its operation, and wake it up.
+    #[inline]
+    pub(crate) fn notify(&self) {
+        if !self.is_empty.load(Ordering::SeqCst) {
+            let mut inner = self.inner.lock().unwrap();
+            if !self.is_empty.load(Ordering::SeqCst) {
+                inner.try_select();
+                inner.notify();
+                self.is_empty.store(
+                    inner.selectors.is_empty() && inner.observers.is_empty(),
+                    Ordering::SeqCst,
+                );
+            }
+        }
+    }
+
+    /// Registers an operation waiting to be ready.
+    #[inline]
+    pub(crate) fn watch(&self, oper: Operation, cx: &Context) {
+        let mut inner = self.inner.lock().unwrap();
+        inner.watch(oper, cx);
+        self.is_empty.store(
+            inner.selectors.is_empty() && inner.observers.is_empty(),
+            Ordering::SeqCst,
+        );
+    }
+
+    /// Unregisters an operation waiting to be ready.
+    #[inline]
+    pub(crate) fn unwatch(&self, oper: Operation) {
+        let mut inner = self.inner.lock().unwrap();
+        inner.unwatch(oper);
+        self.is_empty.store(
+            inner.selectors.is_empty() && inner.observers.is_empty(),
+            Ordering::SeqCst,
+        );
+    }
+
+    /// Notifies all threads that the channel is disconnected.
+    #[inline]
+    pub(crate) fn disconnect(&self) {
+        let mut inner = self.inner.lock().unwrap();
+        inner.disconnect();
+        self.is_empty.store(
+            inner.selectors.is_empty() && inner.observers.is_empty(),
+            Ordering::SeqCst,
+        );
+    }
+}
+
+impl Drop for SyncWaker {
+    #[inline]
+    fn drop(&mut self) {
+        debug_assert!(self.is_empty.load(Ordering::SeqCst));
+    }
+}
+
+/// Returns the id of the current thread.
+#[inline]
+fn current_thread_id() -> ThreadId {
+    std::thread_local! {
+        /// Cached thread-local id.
+        static THREAD_ID: ThreadId = thread::current().id();
+    }
+
+    THREAD_ID
+        .try_with(|id| *id)
+        .unwrap_or_else(|_| thread::current().id())
+}