Adding upstream version 1.64.0+dfsg1.upstream/1.64.0+dfsg1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

3 files changed, 1557 insertions, 0 deletions

diff --git a/vendor/futures-channel/src/mpsc/mod.rs b/vendor/futures-channel/src/mpsc/mod.rs
new file mode 100644
index 000000000..44834b7c9
--- /dev/null
+++ b/vendor/futures-channel/src/mpsc/mod.rs
@@ -0,0 +1,1308 @@
+//! A multi-producer, single-consumer queue for sending values across
+//! asynchronous tasks.
+//!
+//! Similarly to the `std`, channel creation provides [`Receiver`] and
+//! [`Sender`] handles. [`Receiver`] implements [`Stream`] and allows a task to
+//! read values out of the channel. If there is no message to read from the
+//! channel, the current task will be notified when a new value is sent.
+//! [`Sender`] implements the `Sink` trait and allows a task to send messages into
+//! the channel. If the channel is at capacity, the send will be rejected and
+//! the task will be notified when additional capacity is available. In other
+//! words, the channel provides backpressure.
+//!
+//! Unbounded channels are also available using the `unbounded` constructor.
+//!
+//! # Disconnection
+//!
+//! When all [`Sender`] handles have been dropped, it is no longer
+//! possible to send values into the channel. This is considered the termination
+//! event of the stream. As such, [`Receiver::poll_next`]
+//! will return `Ok(Ready(None))`.
+//!
+//! If the [`Receiver`] handle is dropped, then messages can no longer
+//! be read out of the channel. In this case, all further attempts to send will
+//! result in an error.
+//!
+//! # Clean Shutdown
+//!
+//! If the [`Receiver`] is simply dropped, then it is possible for
+//! there to be messages still in the channel that will not be processed. As
+//! such, it is usually desirable to perform a "clean" shutdown. To do this, the
+//! receiver will first call `close`, which will prevent any further messages to
+//! be sent into the channel. Then, the receiver consumes the channel to
+//! completion, at which point the receiver can be dropped.
+//!
+//! [`Sender`]: struct.Sender.html
+//! [`Receiver`]: struct.Receiver.html
+//! [`Stream`]: ../../futures_core/stream/trait.Stream.html
+//! [`Receiver::poll_next`]:
+//!     ../../futures_core/stream/trait.Stream.html#tymethod.poll_next
+
+// At the core, the channel uses an atomic FIFO queue for message passing. This
+// queue is used as the primary coordination primitive. In order to enforce
+// capacity limits and handle back pressure, a secondary FIFO queue is used to
+// send parked task handles.
+//
+// The general idea is that the channel is created with a `buffer` size of `n`.
+// The channel capacity is `n + num-senders`. Each sender gets one "guaranteed"
+// slot to hold a message. This allows `Sender` to know for a fact that a send
+// will succeed *before* starting to do the actual work of sending the value.
+// Since most of this work is lock-free, once the work starts, it is impossible
+// to safely revert.
+//
+// If the sender is unable to process a send operation, then the current
+// task is parked and the handle is sent on the parked task queue.
+//
+// Note that the implementation guarantees that the channel capacity will never
+// exceed the configured limit, however there is no *strict* guarantee that the
+// receiver will wake up a parked task *immediately* when a slot becomes
+// available. However, it will almost always unpark a task when a slot becomes
+// available and it is *guaranteed* that a sender will be unparked when the
+// message that caused the sender to become parked is read out of the channel.
+//
+// The steps for sending a message are roughly:
+//
+// 1) Increment the channel message count
+// 2) If the channel is at capacity, push the task handle onto the wait queue
+// 3) Push the message onto the message queue.
+//
+// The steps for receiving a message are roughly:
+//
+// 1) Pop a message from the message queue
+// 2) Pop a task handle from the wait queue
+// 3) Decrement the channel message count.
+//
+// It's important for the order of operations on lock-free structures to happen
+// in reverse order between the sender and receiver. This makes the message
+// queue the primary coordination structure and establishes the necessary
+// happens-before semantics required for the acquire / release semantics used
+// by the queue structure.
+
+use futures_core::stream::{FusedStream, Stream};
+use futures_core::task::__internal::AtomicWaker;
+use futures_core::task::{Context, Poll, Waker};
+use std::fmt;
+use std::pin::Pin;
+use std::sync::atomic::AtomicUsize;
+use std::sync::atomic::Ordering::SeqCst;
+use std::sync::{Arc, Mutex};
+use std::thread;
+
+use crate::mpsc::queue::Queue;
+
+mod queue;
+#[cfg(feature = "sink")]
+mod sink_impl;
+
+#[derive(Debug)]
+struct UnboundedSenderInner<T> {
+    // Channel state shared between the sender and receiver.
+    inner: Arc<UnboundedInner<T>>,
+}
+
+#[derive(Debug)]
+struct BoundedSenderInner<T> {
+    // Channel state shared between the sender and receiver.
+    inner: Arc<BoundedInner<T>>,
+
+    // Handle to the task that is blocked on this sender. This handle is sent
+    // to the receiver half in order to be notified when the sender becomes
+    // unblocked.
+    sender_task: Arc<Mutex<SenderTask>>,
+
+    // `true` if the sender might be blocked. This is an optimization to avoid
+    // having to lock the mutex most of the time.
+    maybe_parked: bool,
+}
+
+// We never project Pin<&mut SenderInner> to `Pin<&mut T>`
+impl<T> Unpin for UnboundedSenderInner<T> {}
+impl<T> Unpin for BoundedSenderInner<T> {}
+
+/// The transmission end of a bounded mpsc channel.
+///
+/// This value is created by the [`channel`](channel) function.
+#[derive(Debug)]
+pub struct Sender<T>(Option<BoundedSenderInner<T>>);
+
+/// The transmission end of an unbounded mpsc channel.
+///
+/// This value is created by the [`unbounded`](unbounded) function.
+#[derive(Debug)]
+pub struct UnboundedSender<T>(Option<UnboundedSenderInner<T>>);
+
+trait AssertKinds: Send + Sync + Clone {}
+impl AssertKinds for UnboundedSender<u32> {}
+
+/// The receiving end of a bounded mpsc channel.
+///
+/// This value is created by the [`channel`](channel) function.
+#[derive(Debug)]
+pub struct Receiver<T> {
+    inner: Option<Arc<BoundedInner<T>>>,
+}
+
+/// The receiving end of an unbounded mpsc channel.
+///
+/// This value is created by the [`unbounded`](unbounded) function.
+#[derive(Debug)]
+pub struct UnboundedReceiver<T> {
+    inner: Option<Arc<UnboundedInner<T>>>,
+}
+
+// `Pin<&mut UnboundedReceiver<T>>` is never projected to `Pin<&mut T>`
+impl<T> Unpin for UnboundedReceiver<T> {}
+
+/// The error type for [`Sender`s](Sender) used as `Sink`s.
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub struct SendError {
+    kind: SendErrorKind,
+}
+
+/// The error type returned from [`try_send`](Sender::try_send).
+#[derive(Clone, PartialEq, Eq)]
+pub struct TrySendError<T> {
+    err: SendError,
+    val: T,
+}
+
+#[derive(Clone, Debug, PartialEq, Eq)]
+enum SendErrorKind {
+    Full,
+    Disconnected,
+}
+
+/// The error type returned from [`try_next`](Receiver::try_next).
+pub struct TryRecvError {
+    _priv: (),
+}
+
+impl fmt::Display for SendError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        if self.is_full() {
+            write!(f, "send failed because channel is full")
+        } else {
+            write!(f, "send failed because receiver is gone")
+        }
+    }
+}
+
+impl std::error::Error for SendError {}
+
+impl SendError {
+    /// Returns `true` if this error is a result of the channel being full.
+    pub fn is_full(&self) -> bool {
+        match self.kind {
+            SendErrorKind::Full => true,
+            _ => false,
+        }
+    }
+
+    /// Returns `true` if this error is a result of the receiver being dropped.
+    pub fn is_disconnected(&self) -> bool {
+        match self.kind {
+            SendErrorKind::Disconnected => true,
+            _ => false,
+        }
+    }
+}
+
+impl<T> fmt::Debug for TrySendError<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("TrySendError").field("kind", &self.err.kind).finish()
+    }
+}
+
+impl<T> fmt::Display for TrySendError<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        if self.is_full() {
+            write!(f, "send failed because channel is full")
+        } else {
+            write!(f, "send failed because receiver is gone")
+        }
+    }
+}
+
+impl<T: core::any::Any> std::error::Error for TrySendError<T> {}
+
+impl<T> TrySendError<T> {
+    /// Returns `true` if this error is a result of the channel being full.
+    pub fn is_full(&self) -> bool {
+        self.err.is_full()
+    }
+
+    /// Returns `true` if this error is a result of the receiver being dropped.
+    pub fn is_disconnected(&self) -> bool {
+        self.err.is_disconnected()
+    }
+
+    /// Returns the message that was attempted to be sent but failed.
+    pub fn into_inner(self) -> T {
+        self.val
+    }
+
+    /// Drops the message and converts into a `SendError`.
+    pub fn into_send_error(self) -> SendError {
+        self.err
+    }
+}
+
+impl fmt::Debug for TryRecvError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_tuple("TryRecvError").finish()
+    }
+}
+
+impl fmt::Display for TryRecvError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "receiver channel is empty")
+    }
+}
+
+impl std::error::Error for TryRecvError {}
+
+#[derive(Debug)]
+struct UnboundedInner<T> {
+    // Internal channel state. Consists of the number of messages stored in the
+    // channel as well as a flag signalling that the channel is closed.
+    state: AtomicUsize,
+
+    // Atomic, FIFO queue used to send messages to the receiver
+    message_queue: Queue<T>,
+
+    // Number of senders in existence
+    num_senders: AtomicUsize,
+
+    // Handle to the receiver's task.
+    recv_task: AtomicWaker,
+}
+
+#[derive(Debug)]
+struct BoundedInner<T> {
+    // Max buffer size of the channel. If `None` then the channel is unbounded.
+    buffer: usize,
+
+    // Internal channel state. Consists of the number of messages stored in the
+    // channel as well as a flag signalling that the channel is closed.
+    state: AtomicUsize,
+
+    // Atomic, FIFO queue used to send messages to the receiver
+    message_queue: Queue<T>,
+
+    // Atomic, FIFO queue used to send parked task handles to the receiver.
+    parked_queue: Queue<Arc<Mutex<SenderTask>>>,
+
+    // Number of senders in existence
+    num_senders: AtomicUsize,
+
+    // Handle to the receiver's task.
+    recv_task: AtomicWaker,
+}
+
+// Struct representation of `Inner::state`.
+#[derive(Debug, Clone, Copy)]
+struct State {
+    // `true` when the channel is open
+    is_open: bool,
+
+    // Number of messages in the channel
+    num_messages: usize,
+}
+
+// The `is_open` flag is stored in the left-most bit of `Inner::state`
+const OPEN_MASK: usize = usize::max_value() - (usize::max_value() >> 1);
+
+// When a new channel is created, it is created in the open state with no
+// pending messages.
+const INIT_STATE: usize = OPEN_MASK;
+
+// The maximum number of messages that a channel can track is `usize::max_value() >> 1`
+const MAX_CAPACITY: usize = !(OPEN_MASK);
+
+// The maximum requested buffer size must be less than the maximum capacity of
+// a channel. This is because each sender gets a guaranteed slot.
+const MAX_BUFFER: usize = MAX_CAPACITY >> 1;
+
+// Sent to the consumer to wake up blocked producers
+#[derive(Debug)]
+struct SenderTask {
+    task: Option<Waker>,
+    is_parked: bool,
+}
+
+impl SenderTask {
+    fn new() -> Self {
+        Self { task: None, is_parked: false }
+    }
+
+    fn notify(&mut self) {
+        self.is_parked = false;
+
+        if let Some(task) = self.task.take() {
+            task.wake();
+        }
+    }
+}
+
+/// Creates a bounded mpsc channel for communicating between asynchronous tasks.
+///
+/// Being bounded, this channel provides backpressure to ensure that the sender
+/// outpaces the receiver by only a limited amount. The channel's capacity is
+/// equal to `buffer + num-senders`. In other words, each sender gets a
+/// guaranteed slot in the channel capacity, and on top of that there are
+/// `buffer` "first come, first serve" slots available to all senders.
+///
+/// The [`Receiver`](Receiver) returned implements the
+/// [`Stream`](futures_core::stream::Stream) trait, while [`Sender`](Sender) implements
+/// `Sink`.
+pub fn channel<T>(buffer: usize) -> (Sender<T>, Receiver<T>) {
+    // Check that the requested buffer size does not exceed the maximum buffer
+    // size permitted by the system.
+    assert!(buffer < MAX_BUFFER, "requested buffer size too large");
+
+    let inner = Arc::new(BoundedInner {
+        buffer,
+        state: AtomicUsize::new(INIT_STATE),
+        message_queue: Queue::new(),
+        parked_queue: Queue::new(),
+        num_senders: AtomicUsize::new(1),
+        recv_task: AtomicWaker::new(),
+    });
+
+    let tx = BoundedSenderInner {
+        inner: inner.clone(),
+        sender_task: Arc::new(Mutex::new(SenderTask::new())),
+        maybe_parked: false,
+    };
+
+    let rx = Receiver { inner: Some(inner) };
+
+    (Sender(Some(tx)), rx)
+}
+
+/// Creates an unbounded mpsc channel for communicating between asynchronous
+/// tasks.
+///
+/// A `send` on this channel will always succeed as long as the receive half has
+/// not been closed. If the receiver falls behind, messages will be arbitrarily
+/// buffered.
+///
+/// **Note** that the amount of available system memory is an implicit bound to
+/// the channel. Using an `unbounded` channel has the ability of causing the
+/// process to run out of memory. In this case, the process will be aborted.
+pub fn unbounded<T>() -> (UnboundedSender<T>, UnboundedReceiver<T>) {
+    let inner = Arc::new(UnboundedInner {
+        state: AtomicUsize::new(INIT_STATE),
+        message_queue: Queue::new(),
+        num_senders: AtomicUsize::new(1),
+        recv_task: AtomicWaker::new(),
+    });
+
+    let tx = UnboundedSenderInner { inner: inner.clone() };
+
+    let rx = UnboundedReceiver { inner: Some(inner) };
+
+    (UnboundedSender(Some(tx)), rx)
+}
+
+/*
+ *
+ * ===== impl Sender =====
+ *
+ */
+
+impl<T> UnboundedSenderInner<T> {
+    fn poll_ready_nb(&self) -> Poll<Result<(), SendError>> {
+        let state = decode_state(self.inner.state.load(SeqCst));
+        if state.is_open {
+            Poll::Ready(Ok(()))
+        } else {
+            Poll::Ready(Err(SendError { kind: SendErrorKind::Disconnected }))
+        }
+    }
+
+    // Push message to the queue and signal to the receiver
+    fn queue_push_and_signal(&self, msg: T) {
+        // Push the message onto the message queue
+        self.inner.message_queue.push(msg);
+
+        // Signal to the receiver that a message has been enqueued. If the
+        // receiver is parked, this will unpark the task.
+        self.inner.recv_task.wake();
+    }
+
+    // Increment the number of queued messages. Returns the resulting number.
+    fn inc_num_messages(&self) -> Option<usize> {
+        let mut curr = self.inner.state.load(SeqCst);
+
+        loop {
+            let mut state = decode_state(curr);
+
+            // The receiver end closed the channel.
+            if !state.is_open {
+                return None;
+            }
+
+            // This probably is never hit? Odds are the process will run out of
+            // memory first. It may be worth to return something else in this
+            // case?
+            assert!(
+                state.num_messages < MAX_CAPACITY,
+                "buffer space \
+                    exhausted; sending this messages would overflow the state"
+            );
+
+            state.num_messages += 1;
+
+            let next = encode_state(&state);
+            match self.inner.state.compare_exchange(curr, next, SeqCst, SeqCst) {
+                Ok(_) => return Some(state.num_messages),
+                Err(actual) => curr = actual,
+            }
+        }
+    }
+
+    /// Returns whether the senders send to the same receiver.
+    fn same_receiver(&self, other: &Self) -> bool {
+        Arc::ptr_eq(&self.inner, &other.inner)
+    }
+
+    /// Returns whether the sender send to this receiver.
+    fn is_connected_to(&self, inner: &Arc<UnboundedInner<T>>) -> bool {
+        Arc::ptr_eq(&self.inner, inner)
+    }
+
+    /// Returns pointer to the Arc containing sender
+    ///
+    /// The returned pointer is not referenced and should be only used for hashing!
+    fn ptr(&self) -> *const UnboundedInner<T> {
+        &*self.inner
+    }
+
+    /// Returns whether this channel is closed without needing a context.
+    fn is_closed(&self) -> bool {
+        !decode_state(self.inner.state.load(SeqCst)).is_open
+    }
+
+    /// Closes this channel from the sender side, preventing any new messages.
+    fn close_channel(&self) {
+        // There's no need to park this sender, its dropping,
+        // and we don't want to check for capacity, so skip
+        // that stuff from `do_send`.
+
+        self.inner.set_closed();
+        self.inner.recv_task.wake();
+    }
+}
+
+impl<T> BoundedSenderInner<T> {
+    /// Attempts to send a message on this `Sender`, returning the message
+    /// if there was an error.
+    fn try_send(&mut self, msg: T) -> Result<(), TrySendError<T>> {
+        // If the sender is currently blocked, reject the message
+        if !self.poll_unparked(None).is_ready() {
+            return Err(TrySendError { err: SendError { kind: SendErrorKind::Full }, val: msg });
+        }
+
+        // The channel has capacity to accept the message, so send it
+        self.do_send_b(msg)
+    }
+
+    // Do the send without failing.
+    // Can be called only by bounded sender.
+    fn do_send_b(&mut self, msg: T) -> Result<(), TrySendError<T>> {
+        // Anyone calling do_send *should* make sure there is room first,
+        // but assert here for tests as a sanity check.
+        debug_assert!(self.poll_unparked(None).is_ready());
+
+        // First, increment the number of messages contained by the channel.
+        // This operation will also atomically determine if the sender task
+        // should be parked.
+        //
+        // `None` is returned in the case that the channel has been closed by the
+        // receiver. This happens when `Receiver::close` is called or the
+        // receiver is dropped.
+        let park_self = match self.inc_num_messages() {
+            Some(num_messages) => {
+                // Block if the current number of pending messages has exceeded
+                // the configured buffer size
+                num_messages > self.inner.buffer
+            }
+            None => {
+                return Err(TrySendError {
+                    err: SendError { kind: SendErrorKind::Disconnected },
+                    val: msg,
+                })
+            }
+        };
+
+        // If the channel has reached capacity, then the sender task needs to
+        // be parked. This will send the task handle on the parked task queue.
+        //
+        // However, when `do_send` is called while dropping the `Sender`,
+        // `task::current()` can't be called safely. In this case, in order to
+        // maintain internal consistency, a blank message is pushed onto the
+        // parked task queue.
+        if park_self {
+            self.park();
+        }
+
+        self.queue_push_and_signal(msg);
+
+        Ok(())
+    }
+
+    // Push message to the queue and signal to the receiver
+    fn queue_push_and_signal(&self, msg: T) {
+        // Push the message onto the message queue
+        self.inner.message_queue.push(msg);
+
+        // Signal to the receiver that a message has been enqueued. If the
+        // receiver is parked, this will unpark the task.
+        self.inner.recv_task.wake();
+    }
+
+    // Increment the number of queued messages. Returns the resulting number.
+    fn inc_num_messages(&self) -> Option<usize> {
+        let mut curr = self.inner.state.load(SeqCst);
+
+        loop {
+            let mut state = decode_state(curr);
+
+            // The receiver end closed the channel.
+            if !state.is_open {
+                return None;
+            }
+
+            // This probably is never hit? Odds are the process will run out of
+            // memory first. It may be worth to return something else in this
+            // case?
+            assert!(
+                state.num_messages < MAX_CAPACITY,
+                "buffer space \
+                    exhausted; sending this messages would overflow the state"
+            );
+
+            state.num_messages += 1;
+
+            let next = encode_state(&state);
+            match self.inner.state.compare_exchange(curr, next, SeqCst, SeqCst) {
+                Ok(_) => return Some(state.num_messages),
+                Err(actual) => curr = actual,
+            }
+        }
+    }
+
+    fn park(&mut self) {
+        {
+            let mut sender = self.sender_task.lock().unwrap();
+            sender.task = None;
+            sender.is_parked = true;
+        }
+
+        // Send handle over queue
+        let t = self.sender_task.clone();
+        self.inner.parked_queue.push(t);
+
+        // Check to make sure we weren't closed after we sent our task on the
+        // queue
+        let state = decode_state(self.inner.state.load(SeqCst));
+        self.maybe_parked = state.is_open;
+    }
+
+    /// Polls the channel to determine if there is guaranteed capacity to send
+    /// at least one item without waiting.
+    ///
+    /// # Return value
+    ///
+    /// This method returns:
+    ///
+    /// - `Poll::Ready(Ok(_))` if there is sufficient capacity;
+    /// - `Poll::Pending` if the channel may not have
+    ///   capacity, in which case the current task is queued to be notified once
+    ///   capacity is available;
+    /// - `Poll::Ready(Err(SendError))` if the receiver has been dropped.
+    fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), SendError>> {
+        let state = decode_state(self.inner.state.load(SeqCst));
+        if !state.is_open {
+            return Poll::Ready(Err(SendError { kind: SendErrorKind::Disconnected }));
+        }
+
+        self.poll_unparked(Some(cx)).map(Ok)
+    }
+
+    /// Returns whether the senders send to the same receiver.
+    fn same_receiver(&self, other: &Self) -> bool {
+        Arc::ptr_eq(&self.inner, &other.inner)
+    }
+
+    /// Returns whether the sender send to this receiver.
+    fn is_connected_to(&self, receiver: &Arc<BoundedInner<T>>) -> bool {
+        Arc::ptr_eq(&self.inner, receiver)
+    }
+
+    /// Returns pointer to the Arc containing sender
+    ///
+    /// The returned pointer is not referenced and should be only used for hashing!
+    fn ptr(&self) -> *const BoundedInner<T> {
+        &*self.inner
+    }
+
+    /// Returns whether this channel is closed without needing a context.
+    fn is_closed(&self) -> bool {
+        !decode_state(self.inner.state.load(SeqCst)).is_open
+    }
+
+    /// Closes this channel from the sender side, preventing any new messages.
+    fn close_channel(&self) {
+        // There's no need to park this sender, its dropping,
+        // and we don't want to check for capacity, so skip
+        // that stuff from `do_send`.
+
+        self.inner.set_closed();
+        self.inner.recv_task.wake();
+    }
+
+    fn poll_unparked(&mut self, cx: Option<&mut Context<'_>>) -> Poll<()> {
+        // First check the `maybe_parked` variable. This avoids acquiring the
+        // lock in most cases
+        if self.maybe_parked {
+            // Get a lock on the task handle
+            let mut task = self.sender_task.lock().unwrap();
+
+            if !task.is_parked {
+                self.maybe_parked = false;
+                return Poll::Ready(());
+            }
+
+            // At this point, an unpark request is pending, so there will be an
+            // unpark sometime in the future. We just need to make sure that
+            // the correct task will be notified.
+            //
+            // Update the task in case the `Sender` has been moved to another
+            // task
+            task.task = cx.map(|cx| cx.waker().clone());
+
+            Poll::Pending
+        } else {
+            Poll::Ready(())
+        }
+    }
+}
+
+impl<T> Sender<T> {
+    /// Attempts to send a message on this `Sender`, returning the message
+    /// if there was an error.
+    pub fn try_send(&mut self, msg: T) -> Result<(), TrySendError<T>> {
+        if let Some(inner) = &mut self.0 {
+            inner.try_send(msg)
+        } else {
+            Err(TrySendError { err: SendError { kind: SendErrorKind::Disconnected }, val: msg })
+        }
+    }
+
+    /// Send a message on the channel.
+    ///
+    /// This function should only be called after
+    /// [`poll_ready`](Sender::poll_ready) has reported that the channel is
+    /// ready to receive a message.
+    pub fn start_send(&mut self, msg: T) -> Result<(), SendError> {
+        self.try_send(msg).map_err(|e| e.err)
+    }
+
+    /// Polls the channel to determine if there is guaranteed capacity to send
+    /// at least one item without waiting.
+    ///
+    /// # Return value
+    ///
+    /// This method returns:
+    ///
+    /// - `Poll::Ready(Ok(_))` if there is sufficient capacity;
+    /// - `Poll::Pending` if the channel may not have
+    ///   capacity, in which case the current task is queued to be notified once
+    ///   capacity is available;
+    /// - `Poll::Ready(Err(SendError))` if the receiver has been dropped.
+    pub fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), SendError>> {
+        let inner = self.0.as_mut().ok_or(SendError { kind: SendErrorKind::Disconnected })?;
+        inner.poll_ready(cx)
+    }
+
+    /// Returns whether this channel is closed without needing a context.
+    pub fn is_closed(&self) -> bool {
+        self.0.as_ref().map(BoundedSenderInner::is_closed).unwrap_or(true)
+    }
+
+    /// Closes this channel from the sender side, preventing any new messages.
+    pub fn close_channel(&mut self) {
+        if let Some(inner) = &mut self.0 {
+            inner.close_channel();
+        }
+    }
+
+    /// Disconnects this sender from the channel, closing it if there are no more senders left.
+    pub fn disconnect(&mut self) {
+        self.0 = None;
+    }
+
+    /// Returns whether the senders send to the same receiver.
+    pub fn same_receiver(&self, other: &Self) -> bool {
+        match (&self.0, &other.0) {
+            (Some(inner), Some(other)) => inner.same_receiver(other),
+            _ => false,
+        }
+    }
+
+    /// Returns whether the sender send to this receiver.
+    pub fn is_connected_to(&self, receiver: &Receiver<T>) -> bool {
+        match (&self.0, &receiver.inner) {
+            (Some(inner), Some(receiver)) => inner.is_connected_to(receiver),
+            _ => false,
+        }
+    }
+
+    /// Hashes the receiver into the provided hasher
+    pub fn hash_receiver<H>(&self, hasher: &mut H)
+    where
+        H: std::hash::Hasher,
+    {
+        use std::hash::Hash;
+
+        let ptr = self.0.as_ref().map(|inner| inner.ptr());
+        ptr.hash(hasher);
+    }
+}
+
+impl<T> UnboundedSender<T> {
+    /// Check if the channel is ready to receive a message.
+    pub fn poll_ready(&self, _: &mut Context<'_>) -> Poll<Result<(), SendError>> {
+        let inner = self.0.as_ref().ok_or(SendError { kind: SendErrorKind::Disconnected })?;
+        inner.poll_ready_nb()
+    }
+
+    /// Returns whether this channel is closed without needing a context.
+    pub fn is_closed(&self) -> bool {
+        self.0.as_ref().map(UnboundedSenderInner::is_closed).unwrap_or(true)
+    }
+
+    /// Closes this channel from the sender side, preventing any new messages.
+    pub fn close_channel(&self) {
+        if let Some(inner) = &self.0 {
+            inner.close_channel();
+        }
+    }
+
+    /// Disconnects this sender from the channel, closing it if there are no more senders left.
+    pub fn disconnect(&mut self) {
+        self.0 = None;
+    }
+
+    // Do the send without parking current task.
+    fn do_send_nb(&self, msg: T) -> Result<(), TrySendError<T>> {
+        if let Some(inner) = &self.0 {
+            if inner.inc_num_messages().is_some() {
+                inner.queue_push_and_signal(msg);
+                return Ok(());
+            }
+        }
+
+        Err(TrySendError { err: SendError { kind: SendErrorKind::Disconnected }, val: msg })
+    }
+
+    /// Send a message on the channel.
+    ///
+    /// This method should only be called after `poll_ready` has been used to
+    /// verify that the channel is ready to receive a message.
+    pub fn start_send(&mut self, msg: T) -> Result<(), SendError> {
+        self.do_send_nb(msg).map_err(|e| e.err)
+    }
+
+    /// Sends a message along this channel.
+    ///
+    /// This is an unbounded sender, so this function differs from `Sink::send`
+    /// by ensuring the return type reflects that the channel is always ready to
+    /// receive messages.
+    pub fn unbounded_send(&self, msg: T) -> Result<(), TrySendError<T>> {
+        self.do_send_nb(msg)
+    }
+
+    /// Returns whether the senders send to the same receiver.
+    pub fn same_receiver(&self, other: &Self) -> bool {
+        match (&self.0, &other.0) {
+            (Some(inner), Some(other)) => inner.same_receiver(other),
+            _ => false,
+        }
+    }
+
+    /// Returns whether the sender send to this receiver.
+    pub fn is_connected_to(&self, receiver: &UnboundedReceiver<T>) -> bool {
+        match (&self.0, &receiver.inner) {
+            (Some(inner), Some(receiver)) => inner.is_connected_to(receiver),
+            _ => false,
+        }
+    }
+
+    /// Hashes the receiver into the provided hasher
+    pub fn hash_receiver<H>(&self, hasher: &mut H)
+    where
+        H: std::hash::Hasher,
+    {
+        use std::hash::Hash;
+
+        let ptr = self.0.as_ref().map(|inner| inner.ptr());
+        ptr.hash(hasher);
+    }
+}
+
+impl<T> Clone for Sender<T> {
+    fn clone(&self) -> Self {
+        Self(self.0.clone())
+    }
+}
+
+impl<T> Clone for UnboundedSender<T> {
+    fn clone(&self) -> Self {
+        Self(self.0.clone())
+    }
+}
+
+impl<T> Clone for UnboundedSenderInner<T> {
+    fn clone(&self) -> Self {
+        // Since this atomic op isn't actually guarding any memory and we don't
+        // care about any orderings besides the ordering on the single atomic
+        // variable, a relaxed ordering is acceptable.
+        let mut curr = self.inner.num_senders.load(SeqCst);
+
+        loop {
+            // If the maximum number of senders has been reached, then fail
+            if curr == MAX_BUFFER {
+                panic!("cannot clone `Sender` -- too many outstanding senders");
+            }
+
+            debug_assert!(curr < MAX_BUFFER);
+
+            let next = curr + 1;
+            match self.inner.num_senders.compare_exchange(curr, next, SeqCst, SeqCst) {
+                Ok(_) => {
+                    // The ABA problem doesn't matter here. We only care that the
+                    // number of senders never exceeds the maximum.
+                    return Self { inner: self.inner.clone() };
+                }
+                Err(actual) => curr = actual,
+            }
+        }
+    }
+}
+
+impl<T> Clone for BoundedSenderInner<T> {
+    fn clone(&self) -> Self {
+        // Since this atomic op isn't actually guarding any memory and we don't
+        // care about any orderings besides the ordering on the single atomic
+        // variable, a relaxed ordering is acceptable.
+        let mut curr = self.inner.num_senders.load(SeqCst);
+
+        loop {
+            // If the maximum number of senders has been reached, then fail
+            if curr == self.inner.max_senders() {
+                panic!("cannot clone `Sender` -- too many outstanding senders");
+            }
+
+            debug_assert!(curr < self.inner.max_senders());
+
+            let next = curr + 1;
+            match self.inner.num_senders.compare_exchange(curr, next, SeqCst, SeqCst) {
+                Ok(_) => {
+                    // The ABA problem doesn't matter here. We only care that the
+                    // number of senders never exceeds the maximum.
+                    return Self {
+                        inner: self.inner.clone(),
+                        sender_task: Arc::new(Mutex::new(SenderTask::new())),
+                        maybe_parked: false,
+                    };
+                }
+                Err(actual) => curr = actual,
+            }
+        }
+    }
+}
+
+impl<T> Drop for UnboundedSenderInner<T> {
+    fn drop(&mut self) {
+        // Ordering between variables don't matter here
+        let prev = self.inner.num_senders.fetch_sub(1, SeqCst);
+
+        if prev == 1 {
+            self.close_channel();
+        }
+    }
+}
+
+impl<T> Drop for BoundedSenderInner<T> {
+    fn drop(&mut self) {
+        // Ordering between variables don't matter here
+        let prev = self.inner.num_senders.fetch_sub(1, SeqCst);
+
+        if prev == 1 {
+            self.close_channel();
+        }
+    }
+}
+
+/*
+ *
+ * ===== impl Receiver =====
+ *
+ */
+
+impl<T> Receiver<T> {
+    /// Closes the receiving half of a channel, without dropping it.
+    ///
+    /// This prevents any further messages from being sent on the channel while
+    /// still enabling the receiver to drain messages that are buffered.
+    pub fn close(&mut self) {
+        if let Some(inner) = &mut self.inner {
+            inner.set_closed();
+
+            // Wake up any threads waiting as they'll see that we've closed the
+            // channel and will continue on their merry way.
+            while let Some(task) = unsafe { inner.parked_queue.pop_spin() } {
+                task.lock().unwrap().notify();
+            }
+        }
+    }
+
+    /// Tries to receive the next message without notifying a context if empty.
+    ///
+    /// It is not recommended to call this function from inside of a future,
+    /// only when you've otherwise arranged to be notified when the channel is
+    /// no longer empty.
+    ///
+    /// This function returns:
+    /// * `Ok(Some(t))` when message is fetched
+    /// * `Ok(None)` when channel is closed and no messages left in the queue
+    /// * `Err(e)` when there are no messages available, but channel is not yet closed
+    pub fn try_next(&mut self) -> Result<Option<T>, TryRecvError> {
+        match self.next_message() {
+            Poll::Ready(msg) => Ok(msg),
+            Poll::Pending => Err(TryRecvError { _priv: () }),
+        }
+    }
+
+    fn next_message(&mut self) -> Poll<Option<T>> {
+        let inner = match self.inner.as_mut() {
+            None => return Poll::Ready(None),
+            Some(inner) => inner,
+        };
+        // Pop off a message
+        match unsafe { inner.message_queue.pop_spin() } {
+            Some(msg) => {
+                // If there are any parked task handles in the parked queue,
+                // pop one and unpark it.
+                self.unpark_one();
+
+                // Decrement number of messages
+                self.dec_num_messages();
+
+                Poll::Ready(Some(msg))
+            }
+            None => {
+                let state = decode_state(inner.state.load(SeqCst));
+                if state.is_closed() {
+                    // If closed flag is set AND there are no pending messages
+                    // it means end of stream
+                    self.inner = None;
+                    Poll::Ready(None)
+                } else {
+                    // If queue is open, we need to return Pending
+                    // to be woken up when new messages arrive.
+                    // If queue is closed but num_messages is non-zero,
+                    // it means that senders updated the state,
+                    // but didn't put message to queue yet,
+                    // so we need to park until sender unparks the task
+                    // after queueing the message.
+                    Poll::Pending
+                }
+            }
+        }
+    }
+
+    // Unpark a single task handle if there is one pending in the parked queue
+    fn unpark_one(&mut self) {
+        if let Some(inner) = &mut self.inner {
+            if let Some(task) = unsafe { inner.parked_queue.pop_spin() } {
+                task.lock().unwrap().notify();
+            }
+        }
+    }
+
+    fn dec_num_messages(&self) {
+        if let Some(inner) = &self.inner {
+            // OPEN_MASK is highest bit, so it's unaffected by subtraction
+            // unless there's underflow, and we know there's no underflow
+            // because number of messages at this point is always > 0.
+            inner.state.fetch_sub(1, SeqCst);
+        }
+    }
+}
+
+// The receiver does not ever take a Pin to the inner T
+impl<T> Unpin for Receiver<T> {}
+
+impl<T> FusedStream for Receiver<T> {
+    fn is_terminated(&self) -> bool {
+        self.inner.is_none()
+    }
+}
+
+impl<T> Stream for Receiver<T> {
+    type Item = T;
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<T>> {
+        // Try to read a message off of the message queue.
+        match self.next_message() {
+            Poll::Ready(msg) => {
+                if msg.is_none() {
+                    self.inner = None;
+                }
+                Poll::Ready(msg)
+            }
+            Poll::Pending => {
+                // There are no messages to read, in this case, park.
+                self.inner.as_ref().unwrap().recv_task.register(cx.waker());
+                // Check queue again after parking to prevent race condition:
+                // a message could be added to the queue after previous `next_message`
+                // before `register` call.
+                self.next_message()
+            }
+        }
+    }
+}
+
+impl<T> Drop for Receiver<T> {
+    fn drop(&mut self) {
+        // Drain the channel of all pending messages
+        self.close();
+        if self.inner.is_some() {
+            loop {
+                match self.next_message() {
+                    Poll::Ready(Some(_)) => {}
+                    Poll::Ready(None) => break,
+                    Poll::Pending => {
+                        let state = decode_state(self.inner.as_ref().unwrap().state.load(SeqCst));
+
+                        // If the channel is closed, then there is no need to park.
+                        if state.is_closed() {
+                            break;
+                        }
+
+                        // TODO: Spinning isn't ideal, it might be worth
+                        // investigating using a condvar or some other strategy
+                        // here. That said, if this case is hit, then another thread
+                        // is about to push the value into the queue and this isn't
+                        // the only spinlock in the impl right now.
+                        thread::yield_now();
+                    }
+                }
+            }
+        }
+    }
+}
+
+impl<T> UnboundedReceiver<T> {
+    /// Closes the receiving half of a channel, without dropping it.
+    ///
+    /// This prevents any further messages from being sent on the channel while
+    /// still enabling the receiver to drain messages that are buffered.
+    pub fn close(&mut self) {
+        if let Some(inner) = &mut self.inner {
+            inner.set_closed();
+        }
+    }
+
+    /// Tries to receive the next message without notifying a context if empty.
+    ///
+    /// It is not recommended to call this function from inside of a future,
+    /// only when you've otherwise arranged to be notified when the channel is
+    /// no longer empty.
+    ///
+    /// This function returns:
+    /// * `Ok(Some(t))` when message is fetched
+    /// * `Ok(None)` when channel is closed and no messages left in the queue
+    /// * `Err(e)` when there are no messages available, but channel is not yet closed
+    pub fn try_next(&mut self) -> Result<Option<T>, TryRecvError> {
+        match self.next_message() {
+            Poll::Ready(msg) => Ok(msg),
+            Poll::Pending => Err(TryRecvError { _priv: () }),
+        }
+    }
+
+    fn next_message(&mut self) -> Poll<Option<T>> {
+        let inner = match self.inner.as_mut() {
+            None => return Poll::Ready(None),
+            Some(inner) => inner,
+        };
+        // Pop off a message
+        match unsafe { inner.message_queue.pop_spin() } {
+            Some(msg) => {
+                // Decrement number of messages
+                self.dec_num_messages();
+
+                Poll::Ready(Some(msg))
+            }
+            None => {
+                let state = decode_state(inner.state.load(SeqCst));
+                if state.is_closed() {
+                    // If closed flag is set AND there are no pending messages
+                    // it means end of stream
+                    self.inner = None;
+                    Poll::Ready(None)
+                } else {
+                    // If queue is open, we need to return Pending
+                    // to be woken up when new messages arrive.
+                    // If queue is closed but num_messages is non-zero,
+                    // it means that senders updated the state,
+                    // but didn't put message to queue yet,
+                    // so we need to park until sender unparks the task
+                    // after queueing the message.
+                    Poll::Pending
+                }
+            }
+        }
+    }
+
+    fn dec_num_messages(&self) {
+        if let Some(inner) = &self.inner {
+            // OPEN_MASK is highest bit, so it's unaffected by subtraction
+            // unless there's underflow, and we know there's no underflow
+            // because number of messages at this point is always > 0.
+            inner.state.fetch_sub(1, SeqCst);
+        }
+    }
+}
+
+impl<T> FusedStream for UnboundedReceiver<T> {
+    fn is_terminated(&self) -> bool {
+        self.inner.is_none()
+    }
+}
+
+impl<T> Stream for UnboundedReceiver<T> {
+    type Item = T;
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<T>> {
+        // Try to read a message off of the message queue.
+        match self.next_message() {
+            Poll::Ready(msg) => {
+                if msg.is_none() {
+                    self.inner = None;
+                }
+                Poll::Ready(msg)
+            }
+            Poll::Pending => {
+                // There are no messages to read, in this case, park.
+                self.inner.as_ref().unwrap().recv_task.register(cx.waker());
+                // Check queue again after parking to prevent race condition:
+                // a message could be added to the queue after previous `next_message`
+                // before `register` call.
+                self.next_message()
+            }
+        }
+    }
+}
+
+impl<T> Drop for UnboundedReceiver<T> {
+    fn drop(&mut self) {
+        // Drain the channel of all pending messages
+        self.close();
+        if self.inner.is_some() {
+            loop {
+                match self.next_message() {
+                    Poll::Ready(Some(_)) => {}
+                    Poll::Ready(None) => break,
+                    Poll::Pending => {
+                        let state = decode_state(self.inner.as_ref().unwrap().state.load(SeqCst));
+
+                        // If the channel is closed, then there is no need to park.
+                        if state.is_closed() {
+                            break;
+                        }
+
+                        // TODO: Spinning isn't ideal, it might be worth
+                        // investigating using a condvar or some other strategy
+                        // here. That said, if this case is hit, then another thread
+                        // is about to push the value into the queue and this isn't
+                        // the only spinlock in the impl right now.
+                        thread::yield_now();
+                    }
+                }
+            }
+        }
+    }
+}
+
+/*
+ *
+ * ===== impl Inner =====
+ *
+ */
+
+impl<T> UnboundedInner<T> {
+    // Clear `open` flag in the state, keep `num_messages` intact.
+    fn set_closed(&self) {
+        let curr = self.state.load(SeqCst);
+        if !decode_state(curr).is_open {
+            return;
+        }
+
+        self.state.fetch_and(!OPEN_MASK, SeqCst);
+    }
+}
+
+impl<T> BoundedInner<T> {
+    // The return value is such that the total number of messages that can be
+    // enqueued into the channel will never exceed MAX_CAPACITY
+    fn max_senders(&self) -> usize {
+        MAX_CAPACITY - self.buffer
+    }
+
+    // Clear `open` flag in the state, keep `num_messages` intact.
+    fn set_closed(&self) {
+        let curr = self.state.load(SeqCst);
+        if !decode_state(curr).is_open {
+            return;
+        }
+
+        self.state.fetch_and(!OPEN_MASK, SeqCst);
+    }
+}
+
+unsafe impl<T: Send> Send for UnboundedInner<T> {}
+unsafe impl<T: Send> Sync for UnboundedInner<T> {}
+
+unsafe impl<T: Send> Send for BoundedInner<T> {}
+unsafe impl<T: Send> Sync for BoundedInner<T> {}
+
+impl State {
+    fn is_closed(&self) -> bool {
+        !self.is_open && self.num_messages == 0
+    }
+}
+
+/*
+ *
+ * ===== Helpers =====
+ *
+ */
+
+fn decode_state(num: usize) -> State {
+    State { is_open: num & OPEN_MASK == OPEN_MASK, num_messages: num & MAX_CAPACITY }
+}
+
+fn encode_state(state: &State) -> usize {
+    let mut num = state.num_messages;
+
+    if state.is_open {
+        num |= OPEN_MASK;
+    }
+
+    num
+}
diff --git a/vendor/futures-channel/src/mpsc/queue.rs b/vendor/futures-channel/src/mpsc/queue.rs
new file mode 100644
index 000000000..57dc7f565
--- /dev/null
+++ b/vendor/futures-channel/src/mpsc/queue.rs
@@ -0,0 +1,176 @@
+/* Copyright (c) 2010-2011 Dmitry Vyukov. All rights reserved.
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ *    1. Redistributions of source code must retain the above copyright notice,
+ *       this list of conditions and the following disclaimer.
+ *
+ *    2. Redistributions in binary form must reproduce the above copyright
+ *       notice, this list of conditions and the following disclaimer in the
+ *       documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY DMITRY VYUKOV "AS IS" AND ANY EXPRESS OR IMPLIED
+ * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT
+ * SHALL DMITRY VYUKOV OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+ * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
+ * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE
+ * OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
+ * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ * The views and conclusions contained in the software and documentation are
+ * those of the authors and should not be interpreted as representing official
+ * policies, either expressed or implied, of Dmitry Vyukov.
+ */
+
+//! A mostly lock-free multi-producer, single consumer queue for sending
+//! messages between asynchronous tasks.
+//!
+//! The queue implementation is essentially the same one used for mpsc channels
+//! in the standard library.
+//!
+//! Note that the current implementation of this queue has a caveat of the `pop`
+//! method, and see the method for more information about it. Due to this
+//! caveat, this queue may not be appropriate for all use-cases.
+
+// http://www.1024cores.net/home/lock-free-algorithms
+//                         /queues/non-intrusive-mpsc-node-based-queue
+
+// NOTE: this implementation is lifted from the standard library and only
+//       slightly modified
+
+pub(super) use self::PopResult::*;
+
+use std::cell::UnsafeCell;
+use std::ptr;
+use std::sync::atomic::{AtomicPtr, Ordering};
+use std::thread;
+
+/// A result of the `pop` function.
+pub(super) enum PopResult<T> {
+    /// Some data has been popped
+    Data(T),
+    /// The queue is empty
+    Empty,
+    /// The queue is in an inconsistent state. Popping data should succeed, but
+    /// some pushers have yet to make enough progress in order allow a pop to
+    /// succeed. It is recommended that a pop() occur "in the near future" in
+    /// order to see if the sender has made progress or not
+    Inconsistent,
+}
+
+#[derive(Debug)]
+struct Node<T> {
+    next: AtomicPtr<Self>,
+    value: Option<T>,
+}
+
+/// The multi-producer single-consumer structure. This is not cloneable, but it
+/// may be safely shared so long as it is guaranteed that there is only one
+/// popper at a time (many pushers are allowed).
+#[derive(Debug)]
+pub(super) struct Queue<T> {
+    head: AtomicPtr<Node<T>>,
+    tail: UnsafeCell<*mut Node<T>>,
+}
+
+unsafe impl<T: Send> Send for Queue<T> {}
+unsafe impl<T: Send> Sync for Queue<T> {}
+
+impl<T> Node<T> {
+    unsafe fn new(v: Option<T>) -> *mut Self {
+        Box::into_raw(Box::new(Self { next: AtomicPtr::new(ptr::null_mut()), value: v }))
+    }
+}
+
+impl<T> Queue<T> {
+    /// Creates a new queue that is safe to share among multiple producers and
+    /// one consumer.
+    pub(super) fn new() -> Self {
+        let stub = unsafe { Node::new(None) };
+        Self { head: AtomicPtr::new(stub), tail: UnsafeCell::new(stub) }
+    }
+
+    /// Pushes a new value onto this queue.
+    pub(super) fn push(&self, t: T) {
+        unsafe {
+            let n = Node::new(Some(t));
+            let prev = self.head.swap(n, Ordering::AcqRel);
+            (*prev).next.store(n, Ordering::Release);
+        }
+    }
+
+    /// Pops some data from this queue.
+    ///
+    /// Note that the current implementation means that this function cannot
+    /// return `Option<T>`. It is possible for this queue to be in an
+    /// inconsistent state where many pushes have succeeded and completely
+    /// finished, but pops cannot return `Some(t)`. This inconsistent state
+    /// happens when a pusher is preempted at an inopportune moment.
+    ///
+    /// This inconsistent state means that this queue does indeed have data, but
+    /// it does not currently have access to it at this time.
+    ///
+    /// This function is unsafe because only one thread can call it at a time.
+    pub(super) unsafe fn pop(&self) -> PopResult<T> {
+        let tail = *self.tail.get();
+        let next = (*tail).next.load(Ordering::Acquire);
+
+        if !next.is_null() {
+            *self.tail.get() = next;
+            assert!((*tail).value.is_none());
+            assert!((*next).value.is_some());
+            let ret = (*next).value.take().unwrap();
+            drop(Box::from_raw(tail));
+            return Data(ret);
+        }
+
+        if self.head.load(Ordering::Acquire) == tail {
+            Empty
+        } else {
+            Inconsistent
+        }
+    }
+
+    /// Pop an element similarly to `pop` function, but spin-wait on inconsistent
+    /// queue state instead of returning `Inconsistent`.
+    ///
+    /// This function is unsafe because only one thread can call it at a time.
+    pub(super) unsafe fn pop_spin(&self) -> Option<T> {
+        loop {
+            match self.pop() {
+                Empty => return None,
+                Data(t) => return Some(t),
+                // Inconsistent means that there will be a message to pop
+                // in a short time. This branch can only be reached if
+                // values are being produced from another thread, so there
+                // are a few ways that we can deal with this:
+                //
+                // 1) Spin
+                // 2) thread::yield_now()
+                // 3) task::current().unwrap() & return Pending
+                //
+                // For now, thread::yield_now() is used, but it would
+                // probably be better to spin a few times then yield.
+                Inconsistent => {
+                    thread::yield_now();
+                }
+            }
+        }
+    }
+}
+
+impl<T> Drop for Queue<T> {
+    fn drop(&mut self) {
+        unsafe {
+            let mut cur = *self.tail.get();
+            while !cur.is_null() {
+                let next = (*cur).next.load(Ordering::Relaxed);
+                drop(Box::from_raw(cur));
+                cur = next;
+            }
+        }
+    }
+}
diff --git a/vendor/futures-channel/src/mpsc/sink_impl.rs b/vendor/futures-channel/src/mpsc/sink_impl.rs
new file mode 100644
index 000000000..1be20162c
--- /dev/null
+++ b/vendor/futures-channel/src/mpsc/sink_impl.rs
@@ -0,0 +1,73 @@
+use super::{SendError, Sender, TrySendError, UnboundedSender};
+use futures_core::task::{Context, Poll};
+use futures_sink::Sink;
+use std::pin::Pin;
+
+impl<T> Sink<T> for Sender<T> {
+    type Error = SendError;
+
+    fn poll_ready(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
+        (*self).poll_ready(cx)
+    }
+
+    fn start_send(mut self: Pin<&mut Self>, msg: T) -> Result<(), Self::Error> {
+        (*self).start_send(msg)
+    }
+
+    fn poll_flush(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
+        match (*self).poll_ready(cx) {
+            Poll::Ready(Err(ref e)) if e.is_disconnected() => {
+                // If the receiver disconnected, we consider the sink to be flushed.
+                Poll::Ready(Ok(()))
+            }
+            x => x,
+        }
+    }
+
+    fn poll_close(mut self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
+        self.disconnect();
+        Poll::Ready(Ok(()))
+    }
+}
+
+impl<T> Sink<T> for UnboundedSender<T> {
+    type Error = SendError;
+
+    fn poll_ready(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
+        Self::poll_ready(&*self, cx)
+    }
+
+    fn start_send(mut self: Pin<&mut Self>, msg: T) -> Result<(), Self::Error> {
+        Self::start_send(&mut *self, msg)
+    }
+
+    fn poll_flush(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
+        Poll::Ready(Ok(()))
+    }
+
+    fn poll_close(mut self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
+        self.disconnect();
+        Poll::Ready(Ok(()))
+    }
+}
+
+impl<T> Sink<T> for &UnboundedSender<T> {
+    type Error = SendError;
+
+    fn poll_ready(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
+        UnboundedSender::poll_ready(*self, cx)
+    }
+
+    fn start_send(self: Pin<&mut Self>, msg: T) -> Result<(), Self::Error> {
+        self.unbounded_send(msg).map_err(TrySendError::into_send_error)
+    }
+
+    fn poll_flush(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
+        Poll::Ready(Ok(()))
+    }
+
+    fn poll_close(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
+        self.close_channel();
+        Poll::Ready(Ok(()))
+    }
+}