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    /// Synchronous channels/ports
    ///
    /// This channel implementation differs significantly from the asynchronous
    /// implementations found next to it (oneshot/stream/share). This is an
    /// implementation of a synchronous, bounded buffer channel.
    ///
    /// Each channel is created with some amount of backing buffer, and sends will
    /// *block* until buffer space becomes available. A buffer size of 0 is valid,
    /// which means that every successful send is paired with a successful recv.
    ///
    /// This flavor of channels defines a new `send_opt` method for channels which
    /// is the method by which a message is sent but the thread does not panic if it
    /// cannot be delivered.
    ///
    /// Another major difference is that send() will *always* return back the data
    /// if it couldn't be sent. This is because it is deterministically known when
    /// the data is received and when it is not received.
    ///
    /// Implementation-wise, it can all be summed up with "use a mutex plus some
    /// logic". The mutex used here is an OS native mutex, meaning that no user code
    /// is run inside of the mutex (to prevent context switching). This
    /// implementation shares almost all code for the buffered and unbuffered cases
    /// of a synchronous channel. There are a few branches for the unbuffered case,
    /// but they're mostly just relevant to blocking senders.

    pub use self::Failure::*;
    use self::Blocker::*;

    use core::intrinsics::abort;
    use core::isize;
    use core::mem;
    use core::ptr;

    use crate::sync::atomic::{Ordering, AtomicUsize};
    use crate::sync::mpsc::blocking::{self, WaitToken, SignalToken};
    use crate::sync::mpsc::select::StartResult::{self, Installed, Abort};
    use crate::sync::{Mutex, MutexGuard};
    use crate::time::Instant;

    const MAX_REFCOUNT: usize = (isize::MAX) as usize;

    pub struct Packet<T> {
        /// Only field outside of the mutex. Just done for kicks, but mainly because
        /// the other shared channel already had the code implemented
        channels: AtomicUsize,

        lock: Mutex<State<T>>,
    }

    unsafe impl<T: Send> Send for Packet<T> { }

    unsafe impl<T: Send> Sync for Packet<T> { }

    struct State<T> {
        disconnected: bool, // Is the channel disconnected yet?
        queue: Queue,       // queue of senders waiting to send data
        blocker: Blocker,   // currently blocked thread on this channel
        buf: Buffer<T>,     // storage for buffered messages
        cap: usize,         // capacity of this channel

        /// A curious flag used to indicate whether a sender failed or succeeded in
        /// blocking. This is used to transmit information back to the thread that it
        /// must dequeue its message from the buffer because it was not received.
        /// This is only relevant in the 0-buffer case. This obviously cannot be
        /// safely constructed, but it's guaranteed to always have a valid pointer
        /// value.
        canceled: Option<&'static mut bool>,
    }

    unsafe impl<T: Send> Send for State<T> {}

    /// Possible flavors of threads who can be blocked on this channel.
    enum Blocker {
        BlockedSender(SignalToken),
        BlockedReceiver(SignalToken),
        NoneBlocked
    }

    /// Simple queue for threading threads together. Nodes are stack-allocated, so
    /// this structure is not safe at all
    struct Queue {
        head: *mut Node,
        tail: *mut Node,
    }

    struct Node {
        token: Option<SignalToken>,
        next: *mut Node,
    }

    unsafe impl Send for Node {}

    /// A simple ring-buffer
    struct Buffer<T> {
        buf: Vec<Option<T>>,
        start: usize,
        size: usize,
    }

    #[derive(Debug)]
    pub enum Failure {
        Empty,
        Disconnected,
    }

    /// Atomically blocks the current thread, placing it into `slot`, unlocking `lock`
    /// in the meantime. This re-locks the mutex upon returning.
    fn wait<'a, 'b, T>(lock: &'a Mutex<State<T>>,
                       mut guard: MutexGuard<'b, State<T>>,
                       f: fn(SignalToken) -> Blocker)
                       -> MutexGuard<'a, State<T>>
    {
        let (wait_token, signal_token) = blocking::tokens();
        match mem::replace(&mut guard.blocker, f(signal_token)) {
            NoneBlocked => {}
            _ => unreachable!(),
        }
        drop(guard);         // unlock
        wait_token.wait();   // block
        lock.lock().unwrap() // relock
    }

    /// Same as wait, but waiting at most until `deadline`.
    fn wait_timeout_receiver<'a, 'b, T>(lock: &'a Mutex<State<T>>,
                                        deadline: Instant,
                                        mut guard: MutexGuard<'b, State<T>>,
                                        success: &mut bool)
                                        -> MutexGuard<'a, State<T>>
    {
        let (wait_token, signal_token) = blocking::tokens();
        match mem::replace(&mut guard.blocker, BlockedReceiver(signal_token)) {
            NoneBlocked => {}
            _ => unreachable!(),
        }
        drop(guard);         // unlock
        *success = wait_token.wait_max_until(deadline);   // block
        let mut new_guard = lock.lock().unwrap(); // relock
        if !*success {
            abort_selection(&mut new_guard);
        }
        new_guard
    }

    fn abort_selection<'a, T>(guard: &mut MutexGuard<'a , State<T>>) -> bool {
        match mem::replace(&mut guard.blocker, NoneBlocked) {
            NoneBlocked => true,
            BlockedSender(token) => {
                guard.blocker = BlockedSender(token);
                true
            }
            BlockedReceiver(token) => { drop(token); false }
        }
    }

    /// Wakes up a thread, dropping the lock at the correct time
    fn wakeup<T>(token: SignalToken, guard: MutexGuard<'_, State<T>>) {
        // We need to be careful to wake up the waiting thread *outside* of the mutex
        // in case it incurs a context switch.
        drop(guard);
        token.signal();
    }

    impl<T> Packet<T> {
        pub fn new(cap: usize) -> Packet<T> {
            Packet {
                channels: AtomicUsize::new(1),
                lock: Mutex::new(State {
                    disconnected: false,
                    blocker: NoneBlocked,
                    cap,
                    canceled: None,
                    queue: Queue {
                        head: ptr::null_mut(),
                        tail: ptr::null_mut(),
                    },
                    buf: Buffer {
                        buf: (0..cap + if cap == 0 {1} else {0}).map(|_| None).collect(),
                        start: 0,
                        size: 0,
                    },
                }),
            }
        }

        // wait until a send slot is available, returning locked access to
        // the channel state.
        fn acquire_send_slot(&self) -> MutexGuard<'_, State<T>> {
            let mut node = Node { token: None, next: ptr::null_mut() };
            loop {
                let mut guard = self.lock.lock().unwrap();
                // are we ready to go?
                if guard.disconnected || guard.buf.size() < guard.buf.cap() {
                    return guard;
                }
                // no room; actually block
                let wait_token = guard.queue.enqueue(&mut node);
                drop(guard);
                wait_token.wait();
            }
        }

        pub fn send(&self, t: T) -> Result<(), T> {
            let mut guard = self.acquire_send_slot();
            if guard.disconnected { return Err(t) }
            guard.buf.enqueue(t);

            match mem::replace(&mut guard.blocker, NoneBlocked) {
                // if our capacity is 0, then we need to wait for a receiver to be
                // available to take our data. After waiting, we check again to make
                // sure the port didn't go away in the meantime. If it did, we need
                // to hand back our data.
                NoneBlocked if guard.cap == 0 => {
                    let mut canceled = false;
                    assert!(guard.canceled.is_none());
                    guard.canceled = Some(unsafe { mem::transmute(&mut canceled) });
                    let mut guard = wait(&self.lock, guard, BlockedSender);
                    if canceled {Err(guard.buf.dequeue())} else {Ok(())}
                }

                // success, we buffered some data
                NoneBlocked => Ok(()),

                // success, someone's about to receive our buffered data.
                BlockedReceiver(token) => { wakeup(token, guard); Ok(()) }

                BlockedSender(..) => panic!("lolwut"),
            }
        }

        pub fn try_send(&self, t: T) -> Result<(), super::TrySendError<T>> {
            let mut guard = self.lock.lock().unwrap();
            if guard.disconnected {
                Err(super::TrySendError::Disconnected(t))
            } else if guard.buf.size() == guard.buf.cap() {
                Err(super::TrySendError::Full(t))
            } else if guard.cap == 0 {
                // With capacity 0, even though we have buffer space we can't
                // transfer the data unless there's a receiver waiting.
                match mem::replace(&mut guard.blocker, NoneBlocked) {
                    NoneBlocked => Err(super::TrySendError::Full(t)),
                    BlockedSender(..) => unreachable!(),
                    BlockedReceiver(token) => {
                        guard.buf.enqueue(t);
                        wakeup(token, guard);
                        Ok(())
                    }
                }
            } else {
                // If the buffer has some space and the capacity isn't 0, then we
                // just enqueue the data for later retrieval, ensuring to wake up
                // any blocked receiver if there is one.
                assert!(guard.buf.size() < guard.buf.cap());
                guard.buf.enqueue(t);
                match mem::replace(&mut guard.blocker, NoneBlocked) {
                    BlockedReceiver(token) => wakeup(token, guard),
                    NoneBlocked => {}
                    BlockedSender(..) => unreachable!(),
                }
                Ok(())
            }
        }

        // Receives a message from this channel
        //
        // When reading this, remember that there can only ever be one receiver at
        // time.
        pub fn recv(&self, deadline: Option<Instant>) -> Result<T, Failure> {
            let mut guard = self.lock.lock().unwrap();

            let mut woke_up_after_waiting = false;
            // Wait for the buffer to have something in it. No need for a
            // while loop because we're the only receiver.
            if !guard.disconnected && guard.buf.size() == 0 {
                if let Some(deadline) = deadline {
                    guard = wait_timeout_receiver(&self.lock,
                                                  deadline,
                                                  guard,
                                                  &mut woke_up_after_waiting);
                } else {
                    guard = wait(&self.lock, guard, BlockedReceiver);
                    woke_up_after_waiting = true;
                }
            }

            // N.B., channel could be disconnected while waiting, so the order of
            // these conditionals is important.
            if guard.disconnected && guard.buf.size() == 0 {
                return Err(Disconnected);
            }

            // Pick up the data, wake up our neighbors, and carry on
            assert!(guard.buf.size() > 0 || (deadline.is_some() && !woke_up_after_waiting));

            if guard.buf.size() == 0 { return Err(Empty); }

            let ret = guard.buf.dequeue();
            self.wakeup_senders(woke_up_after_waiting, guard);
            Ok(ret)
        }

        pub fn try_recv(&self) -> Result<T, Failure> {
            let mut guard = self.lock.lock().unwrap();

            // Easy cases first
            if guard.disconnected && guard.buf.size() == 0 { return Err(Disconnected) }
            if guard.buf.size() == 0 { return Err(Empty) }

            // Be sure to wake up neighbors
            let ret = Ok(guard.buf.dequeue());
            self.wakeup_senders(false, guard);
            ret
        }

        // Wake up pending senders after some data has been received
        //
        // * `waited` - flag if the receiver blocked to receive some data, or if it
        //              just picked up some data on the way out
        // * `guard` - the lock guard that is held over this channel's lock
        fn wakeup_senders(&self, waited: bool, mut guard: MutexGuard<'_, State<T>>) {
            let pending_sender1: Option<SignalToken> = guard.queue.dequeue();

            // If this is a no-buffer channel (cap == 0), then if we didn't wait we
            // need to ACK the sender. If we waited, then the sender waking us up
            // was already the ACK.
            let pending_sender2 = if guard.cap == 0 && !waited {
                match mem::replace(&mut guard.blocker, NoneBlocked) {
                    NoneBlocked => None,
                    BlockedReceiver(..) => unreachable!(),
                    BlockedSender(token) => {
                        guard.canceled.take();
                        Some(token)
                    }
                }
            } else {
                None
            };
            mem::drop(guard);

            // only outside of the lock do we wake up the pending threads
            pending_sender1.map(|t| t.signal());
            pending_sender2.map(|t| t.signal());
        }

        // Prepares this shared packet for a channel clone, essentially just bumping
        // a refcount.
        pub fn clone_chan(&self) {
            let old_count = self.channels.fetch_add(1, Ordering::SeqCst);

            // See comments on Arc::clone() on why we do this (for `mem::forget`).
            if old_count > MAX_REFCOUNT {
                unsafe {
                    abort();
                }
            }
        }

        pub fn drop_chan(&self) {
            // Only flag the channel as disconnected if we're the last channel
            match self.channels.fetch_sub(1, Ordering::SeqCst) {
                1 => {}
                _ => return
            }

            // Not much to do other than wake up a receiver if one's there
            let mut guard = self.lock.lock().unwrap();
            if guard.disconnected { return }
            guard.disconnected = true;
            match mem::replace(&mut guard.blocker, NoneBlocked) {
                NoneBlocked => {}
                BlockedSender(..) => unreachable!(),
                BlockedReceiver(token) => wakeup(token, guard),
            }
        }

        pub fn drop_port(&self) {
            let mut guard = self.lock.lock().unwrap();

            if guard.disconnected { return }
            guard.disconnected = true;

            // If the capacity is 0, then the sender may want its data back after
            // we're disconnected. Otherwise it's now our responsibility to destroy
            // the buffered data. As with many other portions of this code, this
            // needs to be careful to destroy the data *outside* of the lock to
            // prevent deadlock.
            let _data = if guard.cap != 0 {
                mem::replace(&mut guard.buf.buf, Vec::new())
            } else {
                Vec::new()
            };
            let mut queue = mem::replace(&mut guard.queue, Queue {
                head: ptr::null_mut(),
                tail: ptr::null_mut(),
            });

            let waiter = match mem::replace(&mut guard.blocker, NoneBlocked) {
                NoneBlocked => None,
                BlockedSender(token) => {
                    *guard.canceled.take().unwrap() = true;
                    Some(token)
                }
                BlockedReceiver(..) => unreachable!(),
            };
            mem::drop(guard);

            while let Some(token) = queue.dequeue() { token.signal(); }
            waiter.map(|t| t.signal());
        }

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

        // If Ok, the value is whether this port has data, if Err, then the upgraded
        // port needs to be checked instead of this one.
        pub fn can_recv(&self) -> bool {
            let guard = self.lock.lock().unwrap();
            guard.disconnected || guard.buf.size() > 0
        }

        // Attempts to start selection on this port. This can either succeed or fail
        // because there is data waiting.
        pub fn start_selection(&self, token: SignalToken) -> StartResult {
            let mut guard = self.lock.lock().unwrap();
            if guard.disconnected || guard.buf.size() > 0 {
                Abort
            } else {
                match mem::replace(&mut guard.blocker, BlockedReceiver(token)) {
                    NoneBlocked => {}
                    BlockedSender(..) => unreachable!(),
                    BlockedReceiver(..) => unreachable!(),
                }
                Installed
            }
        }

        // 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) -> bool {
            let mut guard = self.lock.lock().unwrap();
            abort_selection(&mut guard)
        }
    }

    impl<T> Drop for Packet<T> {
        fn drop(&mut self) {
            assert_eq!(self.channels.load(Ordering::SeqCst), 0);
            let mut guard = self.lock.lock().unwrap();
            assert!(guard.queue.dequeue().is_none());
            assert!(guard.canceled.is_none());
        }
    }

    ////////////////////////////////////////////////////////////////////////////////
    // Buffer, a simple ring buffer backed by Vec<T>
    ////////////////////////////////////////////////////////////////////////////////

    impl<T> Buffer<T> {
        fn enqueue(&mut self, t: T) {
            let pos = (self.start + self.size) % self.buf.len();
            self.size += 1;
            let prev = mem::replace(&mut self.buf[pos], Some(t));
            assert!(prev.is_none());
        }

        fn dequeue(&mut self) -> T {
            let start = self.start;
            self.size -= 1;
            self.start = (self.start + 1) % self.buf.len();
            let result = &mut self.buf[start];
            result.take().unwrap()
        }

        fn size(&self) -> usize { self.size }
        fn cap(&self) -> usize { self.buf.len() }
    }

    ////////////////////////////////////////////////////////////////////////////////
    // Queue, a simple queue to enqueue threads with (stack-allocated nodes)
    ////////////////////////////////////////////////////////////////////////////////

    impl Queue {
        fn enqueue(&mut self, node: &mut Node) -> WaitToken {
            let (wait_token, signal_token) = blocking::tokens();
            node.token = Some(signal_token);
            node.next = ptr::null_mut();

            if self.tail.is_null() {
                self.head = node as *mut Node;
                self.tail = node as *mut Node;
            } else {
                unsafe {
                    (*self.tail).next = node as *mut Node;
                    self.tail = node as *mut Node;
                }
            }

            wait_token
        }

        fn dequeue(&mut self) -> Option<SignalToken> {
            if self.head.is_null() {
                return None
            }
            let node = self.head;
            self.head = unsafe { (*node).next };
            if self.head.is_null() {
                self.tail = ptr::null_mut();
            }
            unsafe {
                (*node).next = ptr::null_mut();
                Some((*node).token.take().unwrap())
            }
        }
    }

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