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    /// Oneshot channels/ports
    ///
    /// This is the initial flavor of channels/ports used for comm module. This is
    /// an optimization for the one-use case of a channel. The major optimization of
    /// this type is to have one and exactly one allocation when the chan/port pair
    /// is created.
    ///
    /// Another possible optimization would be to not use an Arc box because
    /// in theory we know when the shared packet can be deallocated (no real need
    /// for the atomic reference counting), but I was having trouble how to destroy
    /// the data early in a drop of a Port.
    ///
    /// # Implementation
    ///
    /// Oneshots are implemented around one atomic usize variable. This variable
    /// indicates both the state of the port/chan but also contains any threads
    /// blocked on the port. All atomic operations happen on this one word.
    ///
    /// In order to upgrade a oneshot channel, an upgrade is considered a disconnect
    /// on behalf of the channel side of things (it can be mentally thought of as
    /// consuming the port). This upgrade is then also stored in the shared packet.
    /// The one caveat to consider is that when a port sees a disconnected channel
    /// it must check for data because there is no "data plus upgrade" state.
    pub use self::Failure::*;
    use self::MyUpgrade::*;
    pub use self::UpgradeResult::*;

    use crate::cell::UnsafeCell;
    use crate::ptr;
    use crate::sync::atomic::{AtomicUsize, Ordering};
    use crate::sync::mpsc::blocking::{self, SignalToken};
    use crate::sync::mpsc::Receiver;
    use crate::time::Instant;

    // Various states you can find a port in.
    const EMPTY: usize = 0; // initial state: no data, no blocked receiver
    const DATA: usize = 1; // data ready for receiver to take
    const DISCONNECTED: usize = 2; // channel is disconnected OR upgraded
    // Any other value represents a pointer to a SignalToken value. The
    // protocol ensures that when the state moves *to* a pointer,
    // ownership of the token is given to the packet, and when the state
    // moves *from* a pointer, ownership of the token is transferred to
    // whoever changed the state.

    pub struct Packet<T> {
        // Internal state of the chan/port pair (stores the blocked thread as well)
        state: AtomicUsize,
        // One-shot data slot location
        data: UnsafeCell<Option<T>>,
        // when used for the second time, a oneshot channel must be upgraded, and
        // this contains the slot for the upgrade
        upgrade: UnsafeCell<MyUpgrade<T>>,
    }

    pub enum Failure<T> {
        Empty,
        Disconnected,
        Upgraded(Receiver<T>),
    }

    pub enum UpgradeResult {
        UpSuccess,
        UpDisconnected,
        UpWoke(SignalToken),
    }

    enum MyUpgrade<T> {
        NothingSent,
        SendUsed,
        GoUp(Receiver<T>),
    }

    impl<T> Packet<T> {
        pub fn new() -> Packet<T> {
            Packet {
                data: UnsafeCell::new(None),
                upgrade: UnsafeCell::new(NothingSent),
                state: AtomicUsize::new(EMPTY),
            }
        }

        pub fn send(&self, t: T) -> Result<(), T> {
            unsafe {
                // Sanity check
                match *self.upgrade.get() {
                    NothingSent => {}
                    _ => panic!("sending on a oneshot that's already sent on "),
                }
                assert!((*self.data.get()).is_none());
                ptr::write(self.data.get(), Some(t));
                ptr::write(self.upgrade.get(), SendUsed);

                match self.state.swap(DATA, Ordering::SeqCst) {
                    // Sent the data, no one was waiting
                    EMPTY => Ok(()),

                    // Couldn't send the data, the port hung up first. Return the data
                    // back up the stack.
                    DISCONNECTED => {
                        self.state.swap(DISCONNECTED, Ordering::SeqCst);
                        ptr::write(self.upgrade.get(), NothingSent);
                        Err((&mut *self.data.get()).take().unwrap())
                    }

                    // Not possible, these are one-use channels
                    DATA => unreachable!(),

                    // There is a thread waiting on the other end. We leave the 'DATA'
                    // state inside so it'll pick it up on the other end.
                    ptr => {
                        SignalToken::cast_from_usize(ptr).signal();
                        Ok(())
                    }
                }
            }
        }

        // Just tests whether this channel has been sent on or not, this is only
        // safe to use from the sender.
        pub fn sent(&self) -> bool {
            unsafe { !matches!(*self.upgrade.get(), NothingSent) }
        }

        pub fn recv(&self, deadline: Option<Instant>) -> Result<T, Failure<T>> {
            // Attempt to not block the thread (it's a little expensive). If it looks
            // like we're not empty, then immediately go through to `try_recv`.
            if self.state.load(Ordering::SeqCst) == EMPTY {
                let (wait_token, signal_token) = blocking::tokens();
                let ptr = unsafe { signal_token.cast_to_usize() };

                // race with senders to enter the blocking state
                if self.state.compare_exchange(EMPTY, ptr, Ordering::SeqCst, Ordering::SeqCst).is_ok() {
                    if let Some(deadline) = deadline {
                        let timed_out = !wait_token.wait_max_until(deadline);
                        // Try to reset the state
                        if timed_out {
                            self.abort_selection().map_err(Upgraded)?;
                        }
                    } else {
                        wait_token.wait();
                        debug_assert!(self.state.load(Ordering::SeqCst) != EMPTY);
                    }
                } else {
                    // drop the signal token, since we never blocked
                    drop(unsafe { SignalToken::cast_from_usize(ptr) });
                }
            }

            self.try_recv()
        }

        pub fn try_recv(&self) -> Result<T, Failure<T>> {
            unsafe {
                match self.state.load(Ordering::SeqCst) {
                    EMPTY => Err(Empty),

                    // We saw some data on the channel, but the channel can be used
                    // again to send us an upgrade. As a result, we need to re-insert
                    // into the channel that there's no data available (otherwise we'll
                    // just see DATA next time). This is done as a cmpxchg because if
                    // the state changes under our feet we'd rather just see that state
                    // change.
                    DATA => {
                        let _ = self.state.compare_exchange(
                            DATA,
                            EMPTY,
                            Ordering::SeqCst,
                            Ordering::SeqCst,
                        );
                        match (&mut *self.data.get()).take() {
                            Some(data) => Ok(data),
                            None => unreachable!(),
                        }
                    }

                    // There's no guarantee that we receive before an upgrade happens,
                    // and an upgrade flags the channel as disconnected, so when we see
                    // this we first need to check if there's data available and *then*
                    // we go through and process the upgrade.
                    DISCONNECTED => match (&mut *self.data.get()).take() {
                        Some(data) => Ok(data),
                        None => match ptr::replace(self.upgrade.get(), SendUsed) {
                            SendUsed | NothingSent => Err(Disconnected),
                            GoUp(upgrade) => Err(Upgraded(upgrade)),
                        },
                    },

                    // We are the sole receiver; there cannot be a blocking
                    // receiver already.
                    _ => unreachable!(),
                }
            }
        }

        // Returns whether the upgrade was completed. If the upgrade wasn't
        // completed, then the port couldn't get sent to the other half (it will
        // never receive it).
        pub fn upgrade(&self, up: Receiver<T>) -> UpgradeResult {
            unsafe {
                let prev = match *self.upgrade.get() {
                    NothingSent => NothingSent,
                    SendUsed => SendUsed,
                    _ => panic!("upgrading again"),
                };
                ptr::write(self.upgrade.get(), GoUp(up));

                match self.state.swap(DISCONNECTED, Ordering::SeqCst) {
                    // If the channel is empty or has data on it, then we're good to go.
                    // Senders will check the data before the upgrade (in case we
                    // plastered over the DATA state).
                    DATA | EMPTY => UpSuccess,

                    // If the other end is already disconnected, then we failed the
                    // upgrade. Be sure to trash the port we were given.
                    DISCONNECTED => {
                        ptr::replace(self.upgrade.get(), prev);
                        UpDisconnected
                    }

                    // If someone's waiting, we gotta wake them up
                    ptr => UpWoke(SignalToken::cast_from_usize(ptr)),
                }
            }
        }

        pub fn drop_chan(&self) {
            match self.state.swap(DISCONNECTED, Ordering::SeqCst) {
                DATA | DISCONNECTED | EMPTY => {}

                // If someone's waiting, we gotta wake them up
                ptr => unsafe {
                    SignalToken::cast_from_usize(ptr).signal();
                },
            }
        }

        pub fn drop_port(&self) {
            match self.state.swap(DISCONNECTED, Ordering::SeqCst) {
                // An empty channel has nothing to do, and a remotely disconnected
                // channel also has nothing to do b/c we're about to run the drop
                // glue
                DISCONNECTED | EMPTY => {}

                // There's data on the channel, so make sure we destroy it promptly.
                // This is why not using an arc is a little difficult (need the box
                // to stay valid while we take the data).
                DATA => unsafe {
                    (&mut *self.data.get()).take().unwrap();
                },

                // We're the only ones that can block on this port
                _ => unreachable!(),
            }
        }

        ////////////////////////////////////////////////////////////////////////////
        // select implementation
        ////////////////////////////////////////////////////////////////////////////

        // Remove a previous selecting thread from this port. This ensures that the
        // blocked thread will no longer be visible to any other threads.
        //
        // The return value indicates whether there's data on this port.
        pub fn abort_selection(&self) -> Result<bool, Receiver<T>> {
            let state = match self.state.load(Ordering::SeqCst) {
                // Each of these states means that no further activity will happen
                // with regard to abortion selection
                s @ (EMPTY | DATA | DISCONNECTED) => s,

                // If we've got a blocked thread, then use an atomic to gain ownership
                // of it (may fail)
                ptr => self
                    .state
                    .compare_exchange(ptr, EMPTY, Ordering::SeqCst, Ordering::SeqCst)
                    .unwrap_or_else(|x| x),
            };

            // Now that we've got ownership of our state, figure out what to do
            // about it.
            match state {
                EMPTY => unreachable!(),
                // our thread used for select was stolen
                DATA => Ok(true),

                // If the other end has hung up, then we have complete ownership
                // of the port. First, check if there was data waiting for us. This
                // is possible if the other end sent something and then hung up.
                //
                // We then need to check to see if there was an upgrade requested,
                // and if so, the upgraded port needs to have its selection aborted.
                DISCONNECTED => unsafe {
                    if (*self.data.get()).is_some() {
                        Ok(true)
                    } else {
                        match ptr::replace(self.upgrade.get(), SendUsed) {
                            GoUp(port) => Err(port),
                            _ => Ok(true),
                        }
                    }
                },

                // We woke ourselves up from select.
                ptr => unsafe {
                    drop(SignalToken::cast_from_usize(ptr));
                    Ok(false)
                },
            }
        }
    }

    impl<T> Drop for Packet<T> {
        fn drop(&mut self) {
            assert_eq!(self.state.load(Ordering::SeqCst), DISCONNECTED);
        }
    }