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    use {sys, Token};
    use event_imp::{self as event, Ready, Event, Evented, PollOpt};
    use std::{fmt, io, ptr, usize};
    use std::cell::UnsafeCell;
    use std::{mem, ops, isize};
    #[cfg(all(unix, not(target_os = "fuchsia")))]
    use std::os::unix::io::AsRawFd;
    #[cfg(all(unix, not(target_os = "fuchsia")))]
    use std::os::unix::io::RawFd;
    use std::process;
    use std::sync::{Arc, Mutex, Condvar};
    use std::sync::atomic::{AtomicUsize, AtomicPtr, AtomicBool};
    use std::sync::atomic::Ordering::{self, Acquire, Release, AcqRel, Relaxed, SeqCst};
    use std::time::{Duration, Instant};

    // Poll is backed by two readiness queues. The first is a system readiness queue
    // represented by `sys::Selector`. The system readiness queue handles events
    // provided by the system, such as TCP and UDP. The second readiness queue is
    // implemented in user space by `ReadinessQueue`. It provides a way to implement
    // purely user space `Evented` types.
    //
    // `ReadinessQueue` is backed by a MPSC queue that supports reuse of linked
    // list nodes. This significantly reduces the number of required allocations.
    // Each `Registration` / `SetReadiness` pair allocates a single readiness node
    // that is used for the lifetime of the registration.
    //
    // The readiness node also includes a single atomic variable, `state` that
    // tracks most of the state associated with the registration. This includes the
    // current readiness, interest, poll options, and internal state. When the node
    // state is mutated, it is queued in the MPSC channel. A call to
    // `ReadinessQueue::poll` will dequeue and process nodes. The node state can
    // still be mutated while it is queued in the channel for processing.
    // Intermediate state values do not matter as long as the final state is
    // included in the call to `poll`. This is the eventually consistent nature of
    // the readiness queue.
    //
    // The readiness node is ref counted using the `ref_count` field. On creation,
    // the ref_count is initialized to 3: one `Registration` handle, one
    // `SetReadiness` handle, and one for the readiness queue. Since the readiness queue
    // doesn't *always* hold a handle to the node, we don't use the Arc type for
    // managing ref counts (this is to avoid constantly incrementing and
    // decrementing the ref count when pushing & popping from the queue). When the
    // `Registration` handle is dropped, the `dropped` flag is set on the node, then
    // the node is pushed into the registration queue. When Poll::poll pops the
    // node, it sees the drop flag is set, and decrements it's ref count.
    //
    // The MPSC queue is a modified version of the intrusive MPSC node based queue
    // described by 1024cores [1].
    //
    // The first modification is that two markers are used instead of a single
    // `stub`. The second marker is a `sleep_marker` which is used to signal to
    // producers that the consumer is going to sleep. This sleep_marker is only used
    // when the queue is empty, implying that the only node in the queue is
    // `end_marker`.
    //
    // The second modification is an `until` argument passed to the dequeue
    // function. When `poll` encounters a level-triggered node, the node will be
    // immediately pushed back into the queue. In order to avoid an infinite loop,
    // `poll` before pushing the node, the pointer is saved off and then passed
    // again as the `until` argument. If the next node to pop is `until`, then
    // `Dequeue::Empty` is returned.
    //
    // [1] http://www.1024cores.net/home/lock-free-algorithms/queues/intrusive-mpsc-node-based-queue

    /// Polls for readiness events on all registered values.
    ///
    /// `Poll` allows a program to monitor a large number of `Evented` types,
    /// waiting until one or more become "ready" for some class of operations; e.g.
    /// reading and writing. An `Evented` type is considered ready if it is possible
    /// to immediately perform a corresponding operation; e.g. [`read`] or
    /// [`write`].
    ///
    /// To use `Poll`, an `Evented` type must first be registered with the `Poll`
    /// instance using the [`register`] method, supplying readiness interest. The
    /// readiness interest tells `Poll` which specific operations on the handle to
    /// monitor for readiness. A `Token` is also passed to the [`register`]
    /// function. When `Poll` returns a readiness event, it will include this token.
    /// This associates the event with the `Evented` handle that generated the
    /// event.
    ///
    /// [`read`]: tcp/struct.TcpStream.html#method.read
    /// [`write`]: tcp/struct.TcpStream.html#method.write
    /// [`register`]: #method.register
    ///
    /// # Examples
    ///
    /// A basic example -- establishing a `TcpStream` connection.
    ///
    /// ```
    /// # use std::error::Error;
    /// # fn try_main() -> Result<(), Box<Error>> {
    /// use mio::{Events, Poll, Ready, PollOpt, Token};
    /// use mio::net::TcpStream;
    ///
    /// use std::net::{TcpListener, SocketAddr};
    ///
    /// // Bind a server socket to connect to.
    /// let addr: SocketAddr = "127.0.0.1:0".parse()?;
    /// let server = TcpListener::bind(&addr)?;
    ///
    /// // Construct a new `Poll` handle as well as the `Events` we'll store into
    /// let poll = Poll::new()?;
    /// let mut events = Events::with_capacity(1024);
    ///
    /// // Connect the stream
    /// let stream = TcpStream::connect(&server.local_addr()?)?;
    ///
    /// // Register the stream with `Poll`
    /// poll.register(&stream, Token(0), Ready::readable() | Ready::writable(), PollOpt::edge())?;
    ///
    /// // Wait for the socket to become ready. This has to happens in a loop to
    /// // handle spurious wakeups.
    /// loop {
    ///     poll.poll(&mut events, None)?;
    ///
    ///     for event in &events {
    ///         if event.token() == Token(0) && event.readiness().is_writable() {
    ///             // The socket connected (probably, it could still be a spurious
    ///             // wakeup)
    ///             return Ok(());
    ///         }
    ///     }
    /// }
    /// #     Ok(())
    /// # }
    /// #
    /// # fn main() {
    /// #     try_main().unwrap();
    /// # }
    /// ```
    ///
    /// # Edge-triggered and level-triggered
    ///
    /// An [`Evented`] registration may request edge-triggered events or
    /// level-triggered events. This is done by setting `register`'s
    /// [`PollOpt`] argument to either [`edge`] or [`level`].
    ///
    /// The difference between the two can be described as follows. Supposed that
    /// this scenario happens:
    ///
    /// 1. A [`TcpStream`] is registered with `Poll`.
    /// 2. The socket receives 2kb of data.
    /// 3. A call to [`Poll::poll`] returns the token associated with the socket
    ///    indicating readable readiness.
    /// 4. 1kb is read from the socket.
    /// 5. Another call to [`Poll::poll`] is made.
    ///
    /// If when the socket was registered with `Poll`, edge triggered events were
    /// requested, then the call to [`Poll::poll`] done in step **5** will
    /// (probably) hang despite there being another 1kb still present in the socket
    /// read buffer. The reason for this is that edge-triggered mode delivers events
    /// only when changes occur on the monitored [`Evented`]. So, in step *5* the
    /// caller might end up waiting for some data that is already present inside the
    /// socket buffer.
    ///
    /// With edge-triggered events, operations **must** be performed on the
    /// `Evented` type until [`WouldBlock`] is returned. In other words, after
    /// receiving an event indicating readiness for a certain operation, one should
    /// assume that [`Poll::poll`] may never return another event for the same token
    /// and readiness until the operation returns [`WouldBlock`].
    ///
    /// By contrast, when level-triggered notifications was requested, each call to
    /// [`Poll::poll`] will return an event for the socket as long as data remains
    /// in the socket buffer. Generally, level-triggered events should be avoided if
    /// high performance is a concern.
    ///
    /// Since even with edge-triggered events, multiple events can be generated upon
    /// receipt of multiple chunks of data, the caller has the option to set the
    /// [`oneshot`] flag. This tells `Poll` to disable the associated [`Evented`]
    /// after the event is returned from [`Poll::poll`]. The subsequent calls to
    /// [`Poll::poll`] will no longer include events for [`Evented`] handles that
    /// are disabled even if the readiness state changes. The handle can be
    /// re-enabled by calling [`reregister`]. When handles are disabled, internal
    /// resources used to monitor the handle are maintained until the handle is
    /// dropped or deregistered. This makes re-registering the handle a fast
    /// operation.
    ///
    /// For example, in the following scenario:
    ///
    /// 1. A [`TcpStream`] is registered with `Poll`.
    /// 2. The socket receives 2kb of data.
    /// 3. A call to [`Poll::poll`] returns the token associated with the socket
    ///    indicating readable readiness.
    /// 4. 2kb is read from the socket.
    /// 5. Another call to read is issued and [`WouldBlock`] is returned
    /// 6. The socket receives another 2kb of data.
    /// 7. Another call to [`Poll::poll`] is made.
    ///
    /// Assuming the socket was registered with `Poll` with the [`edge`] and
    /// [`oneshot`] options, then the call to [`Poll::poll`] in step 7 would block. This
    /// is because, [`oneshot`] tells `Poll` to disable events for the socket after
    /// returning an event.
    ///
    /// In order to receive the event for the data received in step 6, the socket
    /// would need to be reregistered using [`reregister`].
    ///
    /// [`PollOpt`]: struct.PollOpt.html
    /// [`edge`]: struct.PollOpt.html#method.edge
    /// [`level`]: struct.PollOpt.html#method.level
    /// [`Poll::poll`]: struct.Poll.html#method.poll
    /// [`WouldBlock`]: https://doc.rust-lang.org/std/io/enum.ErrorKind.html#variant.WouldBlock
    /// [`Evented`]: event/trait.Evented.html
    /// [`TcpStream`]: tcp/struct.TcpStream.html
    /// [`reregister`]: #method.reregister
    /// [`oneshot`]: struct.PollOpt.html#method.oneshot
    ///
    /// # Portability
    ///
    /// Using `Poll` provides a portable interface across supported platforms as
    /// long as the caller takes the following into consideration:
    ///
    /// ### Spurious events
    ///
    /// [`Poll::poll`] may return readiness events even if the associated
    /// [`Evented`] handle is not actually ready. Given the same code, this may
    /// happen more on some platforms than others. It is important to never assume
    /// that, just because a readiness notification was received, that the
    /// associated operation will succeed as well.
    ///
    /// If operation fails with [`WouldBlock`], then the caller should not treat
    /// this as an error, but instead should wait until another readiness event is
    /// received.
    ///
    /// ### Draining readiness
    ///
    /// When using edge-triggered mode, once a readiness event is received, the
    /// corresponding operation must be performed repeatedly until it returns
    /// [`WouldBlock`]. Unless this is done, there is no guarantee that another
    /// readiness event will be delivered, even if further data is received for the
    /// [`Evented`] handle.
    ///
    /// For example, in the first scenario described above, after step 5, even if
    /// the socket receives more data there is no guarantee that another readiness
    /// event will be delivered.
    ///
    /// ### Readiness operations
    ///
    /// The only readiness operations that are guaranteed to be present on all
    /// supported platforms are [`readable`] and [`writable`]. All other readiness
    /// operations may have false negatives and as such should be considered
    /// **hints**. This means that if a socket is registered with [`readable`],
    /// [`error`], and [`hup`] interest, and either an error or hup is received, a
    /// readiness event will be generated for the socket, but it **may** only
    /// include `readable` readiness. Also note that, given the potential for
    /// spurious events, receiving a readiness event with `hup` or `error` doesn't
    /// actually mean that a `read` on the socket will return a result matching the
    /// readiness event.
    ///
    /// In other words, portable programs that explicitly check for [`hup`] or
    /// [`error`] readiness should be doing so as an **optimization** and always be
    /// able to handle an error or HUP situation when performing the actual read
    /// operation.
    ///
    /// [`readable`]: struct.Ready.html#method.readable
    /// [`writable`]: struct.Ready.html#method.writable
    /// [`error`]: unix/struct.UnixReady.html#method.error
    /// [`hup`]: unix/struct.UnixReady.html#method.hup
    ///
    /// ### Registering handles
    ///
    /// Unless otherwise noted, it should be assumed that types implementing
    /// [`Evented`] will never become ready unless they are registered with `Poll`.
    ///
    /// For example:
    ///
    /// ```
    /// # use std::error::Error;
    /// # fn try_main() -> Result<(), Box<Error>> {
    /// use mio::{Poll, Ready, PollOpt, Token};
    /// use mio::net::TcpStream;
    /// use std::time::Duration;
    /// use std::thread;
    ///
    /// let sock = TcpStream::connect(&"216.58.193.100:80".parse()?)?;
    ///
    /// thread::sleep(Duration::from_secs(1));
    ///
    /// let poll = Poll::new()?;
    ///
    /// // The connect is not guaranteed to have started until it is registered at
    /// // this point
    /// poll.register(&sock, Token(0), Ready::readable() | Ready::writable(), PollOpt::edge())?;
    /// #     Ok(())
    /// # }
    /// #
    /// # fn main() {
    /// #     try_main().unwrap();
    /// # }
    /// ```
    ///
    /// # Implementation notes
    ///
    /// `Poll` is backed by the selector provided by the operating system.
    ///
    /// |      OS    |  Selector |
    /// |------------|-----------|
    /// | Linux      | [epoll]   |
    /// | OS X, iOS  | [kqueue]  |
    /// | Windows    | [IOCP]    |
    /// | FreeBSD    | [kqueue]  |
    /// | Android    | [epoll]   |
    ///
    /// On all supported platforms, socket operations are handled by using the
    /// system selector. Platform specific extensions (e.g. [`EventedFd`]) allow
    /// accessing other features provided by individual system selectors. For
    /// example, Linux's [`signalfd`] feature can be used by registering the FD with
    /// `Poll` via [`EventedFd`].
    ///
    /// On all platforms except windows, a call to [`Poll::poll`] is mostly just a
    /// direct call to the system selector. However, [IOCP] uses a completion model
    /// instead of a readiness model. In this case, `Poll` must adapt the completion
    /// model Mio's API. While non-trivial, the bridge layer is still quite
    /// efficient. The most expensive part being calls to `read` and `write` require
    /// data to be copied into an intermediate buffer before it is passed to the
    /// kernel.
    ///
    /// Notifications generated by [`SetReadiness`] are handled by an internal
    /// readiness queue. A single call to [`Poll::poll`] will collect events from
    /// both from the system selector and the internal readiness queue.
    ///
    /// [epoll]: http://man7.org/linux/man-pages/man7/epoll.7.html
    /// [kqueue]: https://www.freebsd.org/cgi/man.cgi?query=kqueue&sektion=2
    /// [IOCP]: https://msdn.microsoft.com/en-us/library/windows/desktop/aa365198(v=vs.85).aspx
    /// [`signalfd`]: http://man7.org/linux/man-pages/man2/signalfd.2.html
    /// [`EventedFd`]: unix/struct.EventedFd.html
    /// [`SetReadiness`]: struct.SetReadiness.html
    /// [`Poll::poll`]: struct.Poll.html#method.poll
    pub struct Poll {
        // Platform specific IO selector
        selector: sys::Selector,

        // Custom readiness queue
        readiness_queue: ReadinessQueue,

        // Use an atomic to first check if a full lock will be required. This is a
        // fast-path check for single threaded cases avoiding the extra syscall
        lock_state: AtomicUsize,

        // Sequences concurrent calls to `Poll::poll`
        lock: Mutex<()>,

        // Wakeup the next waiter
        condvar: Condvar,
    }

    /// Handle to a user space `Poll` registration.
    ///
    /// `Registration` allows implementing [`Evented`] for types that cannot work
    /// with the [system selector]. A `Registration` is always paired with a
    /// `SetReadiness`, which allows updating the registration's readiness state.
    /// When [`set_readiness`] is called and the `Registration` is associated with a
    /// [`Poll`] instance, a readiness event will be created and eventually returned
    /// by [`poll`].
    ///
    /// A `Registration` / `SetReadiness` pair is created by calling
    /// [`Registration::new2`]. At this point, the registration is not being
    /// monitored by a [`Poll`] instance, so calls to `set_readiness` will not
    /// result in any readiness notifications.
    ///
    /// `Registration` implements [`Evented`], so it can be used with [`Poll`] using
    /// the same [`register`], [`reregister`], and [`deregister`] functions used
    /// with TCP, UDP, etc... types. Once registered with [`Poll`], readiness state
    /// changes result in readiness events being dispatched to the [`Poll`] instance
    /// with which `Registration` is registered.
    ///
    /// **Note**, before using `Registration` be sure to read the
    /// [`set_readiness`] documentation and the [portability] notes. The
    /// guarantees offered by `Registration` may be weaker than expected.
    ///
    /// For high level documentation, see [`Poll`].
    ///
    /// # Examples
    ///
    /// ```
    /// use mio::{Ready, Registration, Poll, PollOpt, Token};
    /// use mio::event::Evented;
    ///
    /// use std::io;
    /// use std::time::Instant;
    /// use std::thread;
    ///
    /// pub struct Deadline {
    ///     when: Instant,
    ///     registration: Registration,
    /// }
    ///
    /// impl Deadline {
    ///     pub fn new(when: Instant) -> Deadline {
    ///         let (registration, set_readiness) = Registration::new2();
    ///
    ///         thread::spawn(move || {
    ///             let now = Instant::now();
    ///
    ///             if now < when {
    ///                 thread::sleep(when - now);
    ///             }
    ///
    ///             set_readiness.set_readiness(Ready::readable());
    ///         });
    ///
    ///         Deadline {
    ///             when: when,
    ///             registration: registration,
    ///         }
    ///     }
    ///
    ///     pub fn is_elapsed(&self) -> bool {
    ///         Instant::now() >= self.when
    ///     }
    /// }
    ///
    /// impl Evented for Deadline {
    ///     fn register(&self, poll: &Poll, token: Token, interest: Ready, opts: PollOpt)
    ///         -> io::Result<()>
    ///     {
    ///         self.registration.register(poll, token, interest, opts)
    ///     }
    ///
    ///     fn reregister(&self, poll: &Poll, token: Token, interest: Ready, opts: PollOpt)
    ///         -> io::Result<()>
    ///     {
    ///         self.registration.reregister(poll, token, interest, opts)
    ///     }
    ///
    ///     fn deregister(&self, poll: &Poll) -> io::Result<()> {
    ///         poll.deregister(&self.registration)
    ///     }
    /// }
    /// ```
    ///
    /// [system selector]: struct.Poll.html#implementation-notes
    /// [`Poll`]: struct.Poll.html
    /// [`Registration::new2`]: struct.Registration.html#method.new2
    /// [`Evented`]: event/trait.Evented.html
    /// [`set_readiness`]: struct.SetReadiness.html#method.set_readiness
    /// [`register`]: struct.Poll.html#method.register
    /// [`reregister`]: struct.Poll.html#method.reregister
    /// [`deregister`]: struct.Poll.html#method.deregister
    /// [portability]: struct.Poll.html#portability
    pub struct Registration {
        inner: RegistrationInner,
    }

    unsafe impl Send for Registration {}
    unsafe impl Sync for Registration {}

    /// Updates the readiness state of the associated `Registration`.
    ///
    /// See [`Registration`] for more documentation on using `SetReadiness` and
    /// [`Poll`] for high level polling documentation.
    ///
    /// [`Poll`]: struct.Poll.html
    /// [`Registration`]: struct.Registration.html
    #[derive(Clone)]
    pub struct SetReadiness {
        inner: RegistrationInner,
    }

    unsafe impl Send for SetReadiness {}
    unsafe impl Sync for SetReadiness {}

    /// Used to associate an IO type with a Selector
    #[derive(Debug)]
    pub struct SelectorId {
        id: AtomicUsize,
    }

    struct RegistrationInner {
        // Unsafe pointer to the registration's node. The node is ref counted. This
        // cannot "simply" be tracked by an Arc because `Poll::poll` has an implicit
        // handle though it isn't stored anywhere. In other words, `Poll::poll`
        // needs to decrement the ref count before the node is freed.
        node: *mut ReadinessNode,
    }

    #[derive(Clone)]
    struct ReadinessQueue {
        inner: Arc<ReadinessQueueInner>,
    }

    unsafe impl Send for ReadinessQueue {}
    unsafe impl Sync for ReadinessQueue {}

    struct ReadinessQueueInner {
        // Used to wake up `Poll` when readiness is set in another thread.
        awakener: sys::Awakener,

        // Head of the MPSC queue used to signal readiness to `Poll::poll`.
        head_readiness: AtomicPtr<ReadinessNode>,

        // Tail of the readiness queue.
        //
        // Only accessed by Poll::poll. Coordination will be handled by the poll fn
        tail_readiness: UnsafeCell<*mut ReadinessNode>,

        // Fake readiness node used to punctuate the end of the readiness queue.
        // Before attempting to read from the queue, this node is inserted in order
        // to partition the queue between nodes that are "owned" by the dequeue end
        // and nodes that will be pushed on by producers.
        end_marker: Box<ReadinessNode>,

        // Similar to `end_marker`, but this node signals to producers that `Poll`
        // has gone to sleep and must be woken up.
        sleep_marker: Box<ReadinessNode>,

        // Similar to `end_marker`, but the node signals that the queue is closed.
        // This happens when `ReadyQueue` is dropped and signals to producers that
        // the nodes should no longer be pushed into the queue.
        closed_marker: Box<ReadinessNode>,
    }

    /// Node shared by a `Registration` / `SetReadiness` pair as well as the node
    /// queued into the MPSC channel.
    struct ReadinessNode {
        // Node state, see struct docs for `ReadinessState`
        //
        // This variable is the primary point of coordination between all the
        // various threads concurrently accessing the node.
        state: AtomicState,

        // The registration token cannot fit into the `state` variable, so it is
        // broken out here. In order to atomically update both the state and token
        // we have to jump through a few hoops.
        //
        // First, `state` includes `token_read_pos` and `token_write_pos`. These can
        // either be 0, 1, or 2 which represent a token slot. `token_write_pos` is
        // the token slot that contains the most up to date registration token.
        // `token_read_pos` is the token slot that `poll` is currently reading from.
        //
        // When a call to `update` includes a different token than the one currently
        // associated with the registration (token_write_pos), first an unused token
        // slot is found. The unused slot is the one not represented by
        // `token_read_pos` OR `token_write_pos`. The new token is written to this
        // slot, then `state` is updated with the new `token_write_pos` value. This
        // requires that there is only a *single* concurrent call to `update`.
        //
        // When `poll` reads a node state, it checks that `token_read_pos` matches
        // `token_write_pos`. If they do not match, then it atomically updates
        // `state` such that `token_read_pos` is set to `token_write_pos`. It will
        // then read the token at the newly updated `token_read_pos`.
        token_0: UnsafeCell<Token>,
        token_1: UnsafeCell<Token>,
        token_2: UnsafeCell<Token>,

        // Used when the node is queued in the readiness linked list. Accessing
        // this field requires winning the "queue" lock
        next_readiness: AtomicPtr<ReadinessNode>,

        // Ensures that there is only one concurrent call to `update`.
        //
        // Each call to `update` will attempt to swap `update_lock` from `false` to
        // `true`. If the CAS succeeds, the thread has obtained the update lock. If
        // the CAS fails, then the `update` call returns immediately and the update
        // is discarded.
        update_lock: AtomicBool,

        // Pointer to Arc<ReadinessQueueInner>
        readiness_queue: AtomicPtr<()>,

        // Tracks the number of `ReadyRef` pointers
        ref_count: AtomicUsize,
    }

    /// Stores the ReadinessNode state in an AtomicUsize. This wrapper around the
    /// atomic variable handles encoding / decoding `ReadinessState` values.
    struct AtomicState {
        inner: AtomicUsize,
    }

    const MASK_2: usize = 4 - 1;
    const MASK_4: usize = 16 - 1;
    const QUEUED_MASK: usize = 1 << QUEUED_SHIFT;
    const DROPPED_MASK: usize = 1 << DROPPED_SHIFT;

    const READINESS_SHIFT: usize = 0;
    const INTEREST_SHIFT: usize = 4;
    const POLL_OPT_SHIFT: usize = 8;
    const TOKEN_RD_SHIFT: usize = 12;
    const TOKEN_WR_SHIFT: usize = 14;
    const QUEUED_SHIFT: usize = 16;
    const DROPPED_SHIFT: usize = 17;

    /// Tracks all state for a single `ReadinessNode`. The state is packed into a
    /// `usize` variable from low to high bit as follows:
    ///
    /// 4 bits: Registration current readiness
    /// 4 bits: Registration interest
    /// 4 bits: Poll options
    /// 2 bits: Token position currently being read from by `poll`
    /// 2 bits: Token position last written to by `update`
    /// 1 bit:  Queued flag, set when node is being pushed into MPSC queue.
    /// 1 bit:  Dropped flag, set when all `Registration` handles have been dropped.
    #[derive(Debug, Copy, Clone, Eq, PartialEq)]
    struct ReadinessState(usize);

    /// Returned by `dequeue_node`. Represents the different states as described by
    /// the queue documentation on 1024cores.net.
    enum Dequeue {
        Data(*mut ReadinessNode),
        Empty,
        Inconsistent,
    }

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

    /*
     *
     * ===== Poll =====
     *
     */

    impl Poll {
        /// Return a new `Poll` handle.
        ///
        /// This function will make a syscall to the operating system to create the
        /// system selector. If this syscall fails, `Poll::new` will return with the
        /// error.
        ///
        /// See [struct] level docs for more details.
        ///
        /// [struct]: struct.Poll.html
        ///
        /// # Examples
        ///
        /// ```
        /// # use std::error::Error;
        /// # fn try_main() -> Result<(), Box<Error>> {
        /// use mio::{Poll, Events};
        /// use std::time::Duration;
        ///
        /// let poll = match Poll::new() {
        ///     Ok(poll) => poll,
        ///     Err(e) => panic!("failed to create Poll instance; err={:?}", e),
        /// };
        ///
        /// // Create a structure to receive polled events
        /// let mut events = Events::with_capacity(1024);
        ///
        /// // Wait for events, but none will be received because no `Evented`
        /// // handles have been registered with this `Poll` instance.
        /// let n = poll.poll(&mut events, Some(Duration::from_millis(500)))?;
        /// assert_eq!(n, 0);
        /// #     Ok(())
        /// # }
        /// #
        /// # fn main() {
        /// #     try_main().unwrap();
        /// # }
        /// ```
        pub fn new() -> io::Result<Poll> {
            is_send::<Poll>();
            is_sync::<Poll>();

            let poll = Poll {
                selector: sys::Selector::new()?,
                readiness_queue: ReadinessQueue::new()?,
                lock_state: AtomicUsize::new(0),
                lock: Mutex::new(()),
                condvar: Condvar::new(),
            };

            // Register the notification wakeup FD with the IO poller
            poll.readiness_queue.inner.awakener.register(&poll, AWAKEN, Ready::readable(), PollOpt::edge())?;

            Ok(poll)
        }

        /// Register an `Evented` handle with the `Poll` instance.
        ///
        /// Once registered, the `Poll` instance will monitor the `Evented` handle
        /// for readiness state changes. When it notices a state change, it will
        /// return a readiness event for the handle the next time [`poll`] is
        /// called.
        ///
        /// See the [`struct`] docs for a high level overview.
        ///
        /// # Arguments
        ///
        /// `handle: &E: Evented`: This is the handle that the `Poll` instance
        /// should monitor for readiness state changes.
        ///
        /// `token: Token`: The caller picks a token to associate with the socket.
        /// When [`poll`] returns an event for the handle, this token is included.
        /// This allows the caller to map the event to its handle. The token
        /// associated with the `Evented` handle can be changed at any time by
        /// calling [`reregister`].
        ///
        /// `token` cannot be `Token(usize::MAX)` as it is reserved for internal
        /// usage.
        ///
        /// See documentation on [`Token`] for an example showing how to pick
        /// [`Token`] values.
        ///
        /// `interest: Ready`: Specifies which operations `Poll` should monitor for
        /// readiness. `Poll` will only return readiness events for operations
        /// specified by this argument.
        ///
        /// If a socket is registered with readable interest and the socket becomes
        /// writable, no event will be returned from [`poll`].
        ///
        /// The readiness interest for an `Evented` handle can be changed at any
        /// time by calling [`reregister`].
        ///
        /// `opts: PollOpt`: Specifies the registration options. The most common
        /// options being [`level`] for level-triggered events, [`edge`] for
        /// edge-triggered events, and [`oneshot`].
        ///
        /// The registration options for an `Evented` handle can be changed at any
        /// time by calling [`reregister`].
        ///
        /// # Notes
        ///
        /// Unless otherwise specified, the caller should assume that once an
        /// `Evented` handle is registered with a `Poll` instance, it is bound to
        /// that `Poll` instance for the lifetime of the `Evented` handle. This
        /// remains true even if the `Evented` handle is deregistered from the poll
        /// instance using [`deregister`].
        ///
        /// This function is **thread safe**. It can be called concurrently from
        /// multiple threads.
        ///
        /// [`struct`]: #
        /// [`reregister`]: #method.reregister
        /// [`deregister`]: #method.deregister
        /// [`poll`]: #method.poll
        /// [`level`]: struct.PollOpt.html#method.level
        /// [`edge`]: struct.PollOpt.html#method.edge
        /// [`oneshot`]: struct.PollOpt.html#method.oneshot
        /// [`Token`]: struct.Token.html
        ///
        /// # Examples
        ///
        /// ```
        /// # use std::error::Error;
        /// # fn try_main() -> Result<(), Box<Error>> {
        /// use mio::{Events, Poll, Ready, PollOpt, Token};
        /// use mio::net::TcpStream;
        /// use std::time::{Duration, Instant};
        ///
        /// let poll = Poll::new()?;
        /// let socket = TcpStream::connect(&"216.58.193.100:80".parse()?)?;
        ///
        /// // Register the socket with `poll`
        /// poll.register(&socket, Token(0), Ready::readable() | Ready::writable(), PollOpt::edge())?;
        ///
        /// let mut events = Events::with_capacity(1024);
        /// let start = Instant::now();
        /// let timeout = Duration::from_millis(500);
        ///
        /// loop {
        ///     let elapsed = start.elapsed();
        ///
        ///     if elapsed >= timeout {
        ///         // Connection timed out
        ///         return Ok(());
        ///     }
        ///
        ///     let remaining = timeout - elapsed;
        ///     poll.poll(&mut events, Some(remaining))?;
        ///
        ///     for event in &events {
        ///         if event.token() == Token(0) {
        ///             // Something (probably) happened on the socket.
        ///             return Ok(());
        ///         }
        ///     }
        /// }
        /// #     Ok(())
        /// # }
        /// #
        /// # fn main() {
        /// #     try_main().unwrap();
        /// # }
        /// ```
        pub fn register<E: ?Sized>(&self, handle: &E, token: Token, interest: Ready, opts: PollOpt) -> io::Result<()>
            where E: Evented
        {
            validate_args(token)?;

            /*
             * Undefined behavior:
             * - Reusing a token with a different `Evented` without deregistering
             * (or closing) the original `Evented`.
             */
            trace!("registering with poller");

            // Register interests for this socket
            handle.register(self, token, interest, opts)?;

            Ok(())
        }

        /// Re-register an `Evented` handle with the `Poll` instance.
        ///
        /// Re-registering an `Evented` handle allows changing the details of the
        /// registration. Specifically, it allows updating the associated `token`,
        /// `interest`, and `opts` specified in previous `register` and `reregister`
        /// calls.
        ///
        /// The `reregister` arguments fully override the previous values. In other
        /// words, if a socket is registered with [`readable`] interest and the call
        /// to `reregister` specifies [`writable`], then read interest is no longer
        /// requested for the handle.
        ///
        /// The `Evented` handle must have previously been registered with this
        /// instance of `Poll` otherwise the call to `reregister` will return with
        /// an error.
        ///
        /// `token` cannot be `Token(usize::MAX)` as it is reserved for internal
        /// usage.
        ///
        /// See the [`register`] documentation for details about the function
        /// arguments and see the [`struct`] docs for a high level overview of
        /// polling.
        ///
        /// # Examples
        ///
        /// ```
        /// # use std::error::Error;
        /// # fn try_main() -> Result<(), Box<Error>> {
        /// use mio::{Poll, Ready, PollOpt, Token};
        /// use mio::net::TcpStream;
        ///
        /// let poll = Poll::new()?;
        /// let socket = TcpStream::connect(&"216.58.193.100:80".parse()?)?;
        ///
        /// // Register the socket with `poll`, requesting readable
        /// poll.register(&socket, Token(0), Ready::readable(), PollOpt::edge())?;
        ///
        /// // Reregister the socket specifying a different token and write interest
        /// // instead. `PollOpt::edge()` must be specified even though that value
        /// // is not being changed.
        /// poll.reregister(&socket, Token(2), Ready::writable(), PollOpt::edge())?;
        /// #     Ok(())
        /// # }
        /// #
        /// # fn main() {
        /// #     try_main().unwrap();
        /// # }
        /// ```
        ///
        /// [`struct`]: #
        /// [`register`]: #method.register
        /// [`readable`]: struct.Ready.html#method.readable
        /// [`writable`]: struct.Ready.html#method.writable
        pub fn reregister<E: ?Sized>(&self, handle: &E, token: Token, interest: Ready, opts: PollOpt) -> io::Result<()>
            where E: Evented
        {
            validate_args(token)?;

            trace!("registering with poller");

            // Register interests for this socket
            handle.reregister(self, token, interest, opts)?;

            Ok(())
        }

        /// Deregister an `Evented` handle with the `Poll` instance.
        ///
        /// When an `Evented` handle is deregistered, the `Poll` instance will
        /// no longer monitor it for readiness state changes. Unlike disabling
        /// handles with oneshot, deregistering clears up any internal resources
        /// needed to track the handle.
        ///
        /// A handle can be passed back to `register` after it has been
        /// deregistered; however, it must be passed back to the **same** `Poll`
        /// instance.
        ///
        /// `Evented` handles are automatically deregistered when they are dropped.
        /// It is common to never need to explicitly call `deregister`.
        ///
        /// # Examples
        ///
        /// ```
        /// # use std::error::Error;
        /// # fn try_main() -> Result<(), Box<Error>> {
        /// use mio::{Events, Poll, Ready, PollOpt, Token};
        /// use mio::net::TcpStream;
        /// use std::time::Duration;
        ///
        /// let poll = Poll::new()?;
        /// let socket = TcpStream::connect(&"216.58.193.100:80".parse()?)?;
        ///
        /// // Register the socket with `poll`
        /// poll.register(&socket, Token(0), Ready::readable(), PollOpt::edge())?;
        ///
        /// poll.deregister(&socket)?;
        ///
        /// let mut events = Events::with_capacity(1024);
        ///
        /// // Set a timeout because this poll should never receive any events.
        /// let n = poll.poll(&mut events, Some(Duration::from_secs(1)))?;
        /// assert_eq!(0, n);
        /// #     Ok(())
        /// # }
        /// #
        /// # fn main() {
        /// #     try_main().unwrap();
        /// # }
        /// ```
        pub fn deregister<E: ?Sized>(&self, handle: &E) -> io::Result<()>
            where E: Evented
        {
            trace!("deregistering handle with poller");

            // Deregister interests for this socket
            handle.deregister(self)?;

            Ok(())
        }

        /// Wait for readiness events
        ///
        /// Blocks the current thread and waits for readiness events for any of the
        /// `Evented` handles that have been registered with this `Poll` instance.
        /// The function will block until either at least one readiness event has
        /// been received or `timeout` has elapsed. A `timeout` of `None` means that
        /// `poll` will block until a readiness event has been received.
        ///
        /// The supplied `events` will be cleared and newly received readiness events
        /// will be pushed onto the end. At most `events.capacity()` events will be
        /// returned. If there are further pending readiness events, they will be
        /// returned on the next call to `poll`.
        ///
        /// A single call to `poll` may result in multiple readiness events being
        /// returned for a single `Evented` handle. For example, if a TCP socket
        /// becomes both readable and writable, it may be possible for a single
        /// readiness event to be returned with both [`readable`] and [`writable`]
        /// readiness **OR** two separate events may be returned, one with
        /// [`readable`] set and one with [`writable`] set.
        ///
        /// Note that the `timeout` will be rounded up to the system clock
        /// granularity (usually 1ms), and kernel scheduling delays mean that
        /// the blocking interval may be overrun by a small amount.
        ///
        /// `poll` returns the number of readiness events that have been pushed into
        /// `events` or `Err` when an error has been encountered with the system
        /// selector.  The value returned is deprecated and will be removed in 0.7.0.
        /// Accessing the events by index is also deprecated.  Events can be
        /// inserted by other events triggering, thus making sequential access
        /// problematic.  Use the iterator API instead.  See [`iter`].
        ///
        /// See the [struct] level documentation for a higher level discussion of
        /// polling.
        ///
        /// [`readable`]: struct.Ready.html#method.readable
        /// [`writable`]: struct.Ready.html#method.writable
        /// [struct]: #
        /// [`iter`]: struct.Events.html#method.iter
        ///
        /// # Examples
        ///
        /// A basic example -- establishing a `TcpStream` connection.
        ///
        /// ```
        /// # use std::error::Error;
        /// # fn try_main() -> Result<(), Box<Error>> {
        /// use mio::{Events, Poll, Ready, PollOpt, Token};
        /// use mio::net::TcpStream;
        ///
        /// use std::net::{TcpListener, SocketAddr};
        /// use std::thread;
        ///
        /// // Bind a server socket to connect to.
        /// let addr: SocketAddr = "127.0.0.1:0".parse()?;
        /// let server = TcpListener::bind(&addr)?;
        /// let addr = server.local_addr()?.clone();
        ///
        /// // Spawn a thread to accept the socket
        /// thread::spawn(move || {
        ///     let _ = server.accept();
        /// });
        ///
        /// // Construct a new `Poll` handle as well as the `Events` we'll store into
        /// let poll = Poll::new()?;
        /// let mut events = Events::with_capacity(1024);
        ///
        /// // Connect the stream
        /// let stream = TcpStream::connect(&addr)?;
        ///
        /// // Register the stream with `Poll`
        /// poll.register(&stream, Token(0), Ready::readable() | Ready::writable(), PollOpt::edge())?;
        ///
        /// // Wait for the socket to become ready. This has to happens in a loop to
        /// // handle spurious wakeups.
        /// loop {
        ///     poll.poll(&mut events, None)?;
        ///
        ///     for event in &events {
        ///         if event.token() == Token(0) && event.readiness().is_writable() {
        ///             // The socket connected (probably, it could still be a spurious
        ///             // wakeup)
        ///             return Ok(());
        ///         }
        ///     }
        /// }
        /// #     Ok(())
        /// # }
        /// #
        /// # fn main() {
        /// #     try_main().unwrap();
        /// # }
        /// ```
        ///
        /// [struct]: #
        pub fn poll(&self, events: &mut Events, timeout: Option<Duration>) -> io::Result<usize> {
            self.poll1(events, timeout, false)
        }

        /// Like `poll`, but may be interrupted by a signal
        ///
        /// If `poll` is inturrupted while blocking, it will transparently retry the syscall.  If you
        /// want to handle signals yourself, however, use `poll_interruptible`.
        pub fn poll_interruptible(&self, events: &mut Events, timeout: Option<Duration>) -> io::Result<usize> {
            self.poll1(events, timeout, true)
        }

        fn poll1(&self, events: &mut Events, mut timeout: Option<Duration>, interruptible: bool) -> io::Result<usize> {
            let zero = Some(Duration::from_millis(0));

            // At a high level, the synchronization strategy is to acquire access to
            // the critical section by transitioning the atomic from unlocked ->
            // locked. If the attempt fails, the thread will wait on the condition
            // variable.
            //
            // # Some more detail
            //
            // The `lock_state` atomic usize combines:
            //
            // - locked flag, stored in the least significant bit
            // - number of waiting threads, stored in the rest of the bits.
            //
            // When a thread transitions the locked flag from 0 -> 1, it has
            // obtained access to the critical section.
            //
            // When entering `poll`, a compare-and-swap from 0 -> 1 is attempted.
            // This is a fast path for the case when there are no concurrent calls
            // to poll, which is very common.
            //
            // On failure, the mutex is locked, and the thread attempts to increment
            // the number of waiting threads component of `lock_state`. If this is
            // successfully done while the locked flag is set, then the thread can
            // wait on the condition variable.
            //
            // When a thread exits the critical section, it unsets the locked flag.
            // If there are any waiters, which is atomically determined while
            // unsetting the locked flag, then the condvar is notified.

            let mut curr = self.lock_state.compare_and_swap(0, 1, SeqCst);

            if 0 != curr {
                // Enter slower path
                let mut lock = self.lock.lock().unwrap();
                let mut inc = false;

                loop {
                    if curr & 1 == 0 {
                        // The lock is currently free, attempt to grab it
                        let mut next = curr | 1;

                        if inc {
                            // The waiter count has previously been incremented, so
                            // decrement it here
                            next -= 2;
                        }

                        let actual = self.lock_state.compare_and_swap(curr, next, SeqCst);

                        if actual != curr {
                            curr = actual;
                            continue;
                        }

                        // Lock acquired, break from the loop
                        break;
                    }

                    if timeout == zero {
                        if inc {
                            self.lock_state.fetch_sub(2, SeqCst);
                        }

                        return Ok(0);
                    }

                    // The lock is currently held, so wait for it to become
                    // free. If the waiter count hasn't been incremented yet, do
                    // so now
                    if !inc {
                        let next = curr.checked_add(2).expect("overflow");
                        let actual = self.lock_state.compare_and_swap(curr, next, SeqCst);

                        if actual != curr {
                            curr = actual;
                            continue;
                        }

                        // Track that the waiter count has been incremented for
                        // this thread and fall through to the condvar waiting
                        inc = true;
                    }

                    lock = match timeout {
                        Some(to) => {
                            let now = Instant::now();

                            // Wait to be notified
                            let (l, _) = self.condvar.wait_timeout(lock, to).unwrap();

                            // See how much time was elapsed in the wait
                            let elapsed = now.elapsed();

                            // Update `timeout` to reflect how much time is left to
                            // wait.
                            if elapsed >= to {
                                timeout = zero;
                            } else {
                                // Update the timeout
                                timeout = Some(to - elapsed);
                            }

                            l
                        }
                        None => {
                            self.condvar.wait(lock).unwrap()
                        }
                    };

                    // Reload the state
                    curr = self.lock_state.load(SeqCst);

                    // Try to lock again...
                }
            }

            let ret = self.poll2(events, timeout, interruptible);

            // Release the lock
            if 1 != self.lock_state.fetch_and(!1, Release) {
                // Acquire the mutex
                let _lock = self.lock.lock().unwrap();

                // There is at least one waiting thread, so notify one
                self.condvar.notify_one();
            }

            ret
        }

        #[inline]
        #[cfg_attr(feature = "cargo-clippy", allow(clippy::if_same_then_else))]
        fn poll2(&self, events: &mut Events, mut timeout: Option<Duration>, interruptible: bool) -> io::Result<usize> {
            // Compute the timeout value passed to the system selector. If the
            // readiness queue has pending nodes, we still want to poll the system
            // selector for new events, but we don't want to block the thread to
            // wait for new events.
            if timeout == Some(Duration::from_millis(0)) {
                // If blocking is not requested, then there is no need to prepare
                // the queue for sleep
                //
                // The sleep_marker should be removed by readiness_queue.poll().
            } else if self.readiness_queue.prepare_for_sleep() {
                // The readiness queue is empty. The call to `prepare_for_sleep`
                // inserts `sleep_marker` into the queue. This signals to any
                // threads setting readiness that the `Poll::poll` is going to
                // sleep, so the awakener should be used.
            } else {
                // The readiness queue is not empty, so do not block the thread.
                timeout = Some(Duration::from_millis(0));
            }

            loop {
                let now = Instant::now();
                // First get selector events
                let res = self.selector.select(&mut events.inner, AWAKEN, timeout);
                match res {
                    Ok(true) => {
                        // Some awakeners require reading from a FD.
                        self.readiness_queue.inner.awakener.cleanup();
                        break;
                    }
                    Ok(false) => break,
                    Err(ref e) if e.kind() == io::ErrorKind::Interrupted && !interruptible => {
                        // Interrupted by a signal; update timeout if necessary and retry
                        if let Some(to) = timeout {
                            let elapsed = now.elapsed();
                            if elapsed >= to {
                                break;
                            } else {
                                timeout = Some(to - elapsed);
                            }
                        }
                    }
                    Err(e) => return Err(e),
                }
            }

            // Poll custom event queue
            self.readiness_queue.poll(&mut events.inner);

            // Return number of polled events
            Ok(events.inner.len())
        }
    }

    fn validate_args(token: Token) -> io::Result<()> {
        if token == AWAKEN {
            return Err(io::Error::new(io::ErrorKind::Other, "invalid token"));
        }

        Ok(())
    }

    impl fmt::Debug for Poll {
        fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
            fmt.debug_struct("Poll")
                .finish()
        }
    }

    #[cfg(all(unix, not(target_os = "fuchsia")))]
    impl AsRawFd for Poll {
        fn as_raw_fd(&self) -> RawFd {
            self.selector.as_raw_fd()
        }
    }

    /// A collection of readiness events.
    ///
    /// `Events` is passed as an argument to [`Poll::poll`] and will be used to
    /// receive any new readiness events received since the last poll. Usually, a
    /// single `Events` instance is created at the same time as a [`Poll`] and
    /// reused on each call to [`Poll::poll`].
    ///
    /// See [`Poll`] for more documentation on polling.
    ///
    /// # Examples
    ///
    /// ```
    /// # use std::error::Error;
    /// # fn try_main() -> Result<(), Box<Error>> {
    /// use mio::{Events, Poll};
    /// use std::time::Duration;
    ///
    /// let mut events = Events::with_capacity(1024);
    /// let poll = Poll::new()?;
    ///
    /// assert_eq!(0, events.len());
    ///
    /// // Register `Evented` handles with `poll`
    ///
    /// poll.poll(&mut events, Some(Duration::from_millis(100)))?;
    ///
    /// for event in &events {
    ///     println!("event={:?}", event);
    /// }
    /// #     Ok(())
    /// # }
    /// #
    /// # fn main() {
    /// #     try_main().unwrap();
    /// # }
    /// ```
    ///
    /// [`Poll::poll`]: struct.Poll.html#method.poll
    /// [`Poll`]: struct.Poll.html
    pub struct Events {
        inner: sys::Events,
    }

    /// [`Events`] iterator.
    ///
    /// This struct is created by the [`iter`] method on [`Events`].
    ///
    /// # Examples
    ///
    /// ```
    /// # use std::error::Error;
    /// # fn try_main() -> Result<(), Box<Error>> {
    /// use mio::{Events, Poll};
    /// use std::time::Duration;
    ///
    /// let mut events = Events::with_capacity(1024);
    /// let poll = Poll::new()?;
    ///
    /// // Register handles with `poll`
    ///
    /// poll.poll(&mut events, Some(Duration::from_millis(100)))?;
    ///
    /// for event in events.iter() {
    ///     println!("event={:?}", event);
    /// }
    /// #     Ok(())
    /// # }
    /// #
    /// # fn main() {
    /// #     try_main().unwrap();
    /// # }
    /// ```
    ///
    /// [`Events`]: struct.Events.html
    /// [`iter`]: struct.Events.html#method.iter
    #[derive(Debug, Clone)]
    pub struct Iter<'a> {
        inner: &'a Events,
        pos: usize,
    }

    /// Owned [`Events`] iterator.
    ///
    /// This struct is created by the `into_iter` method on [`Events`].
    ///
    /// # Examples
    ///
    /// ```
    /// # use std::error::Error;
    /// # fn try_main() -> Result<(), Box<Error>> {
    /// use mio::{Events, Poll};
    /// use std::time::Duration;
    ///
    /// let mut events = Events::with_capacity(1024);
    /// let poll = Poll::new()?;
    ///
    /// // Register handles with `poll`
    ///
    /// poll.poll(&mut events, Some(Duration::from_millis(100)))?;
    ///
    /// for event in events {
    ///     println!("event={:?}", event);
    /// }
    /// #     Ok(())
    /// # }
    /// #
    /// # fn main() {
    /// #     try_main().unwrap();
    /// # }
    /// ```
    /// [`Events`]: struct.Events.html
    #[derive(Debug)]
    pub struct IntoIter {
        inner: Events,
        pos: usize,
    }

    impl Events {
        /// Return a new `Events` capable of holding up to `capacity` events.
        ///
        /// # Examples
        ///
        /// ```
        /// use mio::Events;
        ///
        /// let events = Events::with_capacity(1024);
        ///
        /// assert_eq!(1024, events.capacity());
        /// ```
        pub fn with_capacity(capacity: usize) -> Events {
            Events {
                inner: sys::Events::with_capacity(capacity),
            }
        }

        #[deprecated(since="0.6.10", note="Index access removed in favor of iterator only API.")]
        #[doc(hidden)]
        pub fn get(&self, idx: usize) -> Option<Event> {
            self.inner.get(idx)
        }

        #[doc(hidden)]
        #[deprecated(since="0.6.10", note="Index access removed in favor of iterator only API.")]
        pub fn len(&self) -> usize {
            self.inner.len()
        }

        /// Returns the number of `Event` values that `self` can hold.
        ///
        /// ```
        /// use mio::Events;
        ///
        /// let events = Events::with_capacity(1024);
        ///
        /// assert_eq!(1024, events.capacity());
        /// ```
        pub fn capacity(&self) -> usize {
            self.inner.capacity()
        }

        /// Returns `true` if `self` contains no `Event` values.
        ///
        /// # Examples
        ///
        /// ```
        /// use mio::Events;
        ///
        /// let events = Events::with_capacity(1024);
        ///
        /// assert!(events.is_empty());
        /// ```
        pub fn is_empty(&self) -> bool {
            self.inner.is_empty()
        }

        /// Returns an iterator over the `Event` values.
        ///
        /// # Examples
        ///
        /// ```
        /// # use std::error::Error;
        /// # fn try_main() -> Result<(), Box<Error>> {
        /// use mio::{Events, Poll};
        /// use std::time::Duration;
        ///
        /// let mut events = Events::with_capacity(1024);
        /// let poll = Poll::new()?;
        ///
        /// // Register handles with `poll`
        ///
        /// poll.poll(&mut events, Some(Duration::from_millis(100)))?;
        ///
        /// for event in events.iter() {
        ///     println!("event={:?}", event);
        /// }
        /// #     Ok(())
        /// # }
        /// #
        /// # fn main() {
        /// #     try_main().unwrap();
        /// # }
        /// ```
        pub fn iter(&self) -> Iter {
            Iter {
                inner: self,
                pos: 0
            }
        }

        /// Clearing all `Event` values from container explicitly.
        ///
        /// # Examples
        ///
        /// ```
        /// # use std::error::Error;
        /// # fn try_main() -> Result<(), Box<Error>> {
        /// use mio::{Events, Poll};
        /// use std::time::Duration;
        ///
        /// let mut events = Events::with_capacity(1024);
        /// let poll = Poll::new()?;
        ///
        /// // Register handles with `poll`
        /// for _ in 0..2 {
        ///     events.clear();
        ///     poll.poll(&mut events, Some(Duration::from_millis(100)))?;
        ///
        ///     for event in events.iter() {
        ///         println!("event={:?}", event);
        ///     }
        /// }
        /// #     Ok(())
        /// # }
        /// #
        /// # fn main() {
        /// #     try_main().unwrap();
        /// # }
        /// ```
        pub fn clear(&mut self) {
            self.inner.clear();
        }
    }

    impl<'a> IntoIterator for &'a Events {
        type Item = Event;
        type IntoIter = Iter<'a>;

        fn into_iter(self) -> Self::IntoIter {
            self.iter()
        }
    }

    impl<'a> Iterator for Iter<'a> {
        type Item = Event;

        fn next(&mut self) -> Option<Event> {
            let ret = self.inner.inner.get(self.pos);
            self.pos += 1;
            ret
        }
    }

    impl IntoIterator for Events {
        type Item = Event;
        type IntoIter = IntoIter;

        fn into_iter(self) -> Self::IntoIter {
            IntoIter {
                inner: self,
                pos: 0,
            }
        }
    }

    impl Iterator for IntoIter {
        type Item = Event;

        fn next(&mut self) -> Option<Event> {
            let ret = self.inner.inner.get(self.pos);
            self.pos += 1;
            ret
        }
    }

    impl fmt::Debug for Events {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_struct("Events")
                .field("capacity", &self.capacity())
                .finish()
        }
    }

    // ===== Accessors for internal usage =====

    pub fn selector(poll: &Poll) -> &sys::Selector {
        &poll.selector
    }

    /*
     *
     * ===== Registration =====
     *
     */

    // TODO: get rid of this, windows depends on it for now
    #[allow(dead_code)]
    pub fn new_registration(poll: &Poll, token: Token, ready: Ready, opt: PollOpt)
            -> (Registration, SetReadiness)
    {
        Registration::new_priv(poll, token, ready, opt)
    }

    impl Registration {
        /// Create and return a new `Registration` and the associated
        /// `SetReadiness`.
        ///
        /// See [struct] documentation for more detail and [`Poll`]
        /// for high level documentation on polling.
        ///
        /// # Examples
        ///
        /// ```
        /// # use std::error::Error;
        /// # fn try_main() -> Result<(), Box<Error>> {
        /// use mio::{Events, Ready, Registration, Poll, PollOpt, Token};
        /// use std::thread;
        ///
        /// let (registration, set_readiness) = Registration::new2();
        ///
        /// thread::spawn(move || {
        ///     use std::time::Duration;
        ///     thread::sleep(Duration::from_millis(500));
        ///
        ///     set_readiness.set_readiness(Ready::readable());
        /// });
        ///
        /// let poll = Poll::new()?;
        /// poll.register(&registration, Token(0), Ready::readable() | Ready::writable(), PollOpt::edge())?;
        ///
        /// let mut events = Events::with_capacity(256);
        ///
        /// loop {
        ///     poll.poll(&mut events, None);
        ///
        ///     for event in &events {
        ///         if event.token() == Token(0) && event.readiness().is_readable() {
        ///             return Ok(());
        ///         }
        ///     }
        /// }
        /// #     Ok(())
        /// # }
        /// #
        /// # fn main() {
        /// #     try_main().unwrap();
        /// # }
        /// ```
        /// [struct]: #
        /// [`Poll`]: struct.Poll.html
        pub fn new2() -> (Registration, SetReadiness) {
            // Allocate the registration node. The new node will have `ref_count`
            // set to 2: one SetReadiness, one Registration.
            let node = Box::into_raw(Box::new(ReadinessNode::new(
                        ptr::null_mut(), Token(0), Ready::empty(), PollOpt::empty(), 2)));

            let registration = Registration {
                inner: RegistrationInner {
                    node,
                },
            };

            let set_readiness = SetReadiness {
                inner: RegistrationInner {
                    node,
                },
            };

            (registration, set_readiness)
        }

        #[deprecated(since = "0.6.5", note = "use `new2` instead")]
        #[cfg(feature = "with-deprecated")]
        #[doc(hidden)]
        pub fn new(poll: &Poll, token: Token, interest: Ready, opt: PollOpt)
            -> (Registration, SetReadiness)
        {
            Registration::new_priv(poll, token, interest, opt)
        }

        // TODO: Get rid of this (windows depends on it for now)
        fn new_priv(poll: &Poll, token: Token, interest: Ready, opt: PollOpt)
            -> (Registration, SetReadiness)
        {
            is_send::<Registration>();
            is_sync::<Registration>();
            is_send::<SetReadiness>();
            is_sync::<SetReadiness>();

            // Clone handle to the readiness queue, this bumps the ref count
            let queue = poll.readiness_queue.inner.clone();

            // Convert to a *mut () pointer
            let queue: *mut () = unsafe { mem::transmute(queue) };

            // Allocate the registration node. The new node will have `ref_count`
            // set to 3: one SetReadiness, one Registration, and one Poll handle.
            let node = Box::into_raw(Box::new(ReadinessNode::new(
                        queue, token, interest, opt, 3)));

            let registration = Registration {
                inner: RegistrationInner {
                    node,
                },
            };

            let set_readiness = SetReadiness {
                inner: RegistrationInner {
                    node,
                },
            };

            (registration, set_readiness)
        }

        #[deprecated(since = "0.6.5", note = "use `Evented` impl")]
        #[cfg(feature = "with-deprecated")]
        #[doc(hidden)]
        pub fn update(&self, poll: &Poll, token: Token, interest: Ready, opts: PollOpt) -> io::Result<()> {
            self.inner.update(poll, token, interest, opts)
        }

        #[deprecated(since = "0.6.5", note = "use `Poll::deregister` instead")]
        #[cfg(feature = "with-deprecated")]
        #[doc(hidden)]
        pub fn deregister(&self, poll: &Poll) -> io::Result<()> {
            self.inner.update(poll, Token(0), Ready::empty(), PollOpt::empty())
        }
    }

    impl Evented for Registration {
        fn register(&self, poll: &Poll, token: Token, interest: Ready, opts: PollOpt) -> io::Result<()> {
            self.inner.update(poll, token, interest, opts)
        }

        fn reregister(&self, poll: &Poll, token: Token, interest: Ready, opts: PollOpt) -> io::Result<()> {
            self.inner.update(poll, token, interest, opts)
        }

        fn deregister(&self, poll: &Poll) -> io::Result<()> {
            self.inner.update(poll, Token(0), Ready::empty(), PollOpt::empty())
        }
    }

    impl Drop for Registration {
        fn drop(&mut self) {
            // `flag_as_dropped` toggles the `dropped` flag and notifies
            // `Poll::poll` to release its handle (which is just decrementing
            // the ref count).
            if self.inner.state.flag_as_dropped() {
                // Can't do anything if the queuing fails
                let _ = self.inner.enqueue_with_wakeup();
            }
        }
    }

    impl fmt::Debug for Registration {
        fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
            fmt.debug_struct("Registration")
                .finish()
        }
    }

    impl SetReadiness {
        /// Returns the registration's current readiness.
        ///
        /// # Note
        ///
        /// There is no guarantee that `readiness` establishes any sort of memory
        /// ordering. Any concurrent data access must be synchronized using another
        /// strategy.
        ///
        /// # Examples
        ///
        /// ```
        /// # use std::error::Error;
        /// # fn try_main() -> Result<(), Box<Error>> {
        /// use mio::{Registration, Ready};
        ///
        /// let (registration, set_readiness) = Registration::new2();
        ///
        /// assert!(set_readiness.readiness().is_empty());
        ///
        /// set_readiness.set_readiness(Ready::readable())?;
        /// assert!(set_readiness.readiness().is_readable());
        /// #     Ok(())
        /// # }
        /// #
        /// # fn main() {
        /// #     try_main().unwrap();
        /// # }
        /// ```
        pub fn readiness(&self) -> Ready {
            self.inner.readiness()
        }

        /// Set the registration's readiness
        ///
        /// If the associated `Registration` is registered with a [`Poll`] instance
        /// and has requested readiness events that include `ready`, then a future
        /// call to [`Poll::poll`] will receive a readiness event representing the
        /// readiness state change.
        ///
        /// # Note
        ///
        /// There is no guarantee that `readiness` establishes any sort of memory
        /// ordering. Any concurrent data access must be synchronized using another
        /// strategy.
        ///
        /// There is also no guarantee as to when the readiness event will be
        /// delivered to poll. A best attempt will be made to make the delivery in a
        /// "timely" fashion. For example, the following is **not** guaranteed to
        /// work:
        ///
        /// ```
        /// # use std::error::Error;
        /// # fn try_main() -> Result<(), Box<Error>> {
        /// use mio::{Events, Registration, Ready, Poll, PollOpt, Token};
        ///
        /// let poll = Poll::new()?;
        /// let (registration, set_readiness) = Registration::new2();
        ///
        /// poll.register(&registration,
        ///               Token(0),
        ///               Ready::readable(),
        ///               PollOpt::edge())?;
        ///
        /// // Set the readiness, then immediately poll to try to get the readiness
        /// // event
        /// set_readiness.set_readiness(Ready::readable())?;
        ///
        /// let mut events = Events::with_capacity(1024);
        /// poll.poll(&mut events, None)?;
        ///
        /// // There is NO guarantee that the following will work. It is possible
        /// // that the readiness event will be delivered at a later time.
        /// let event = events.get(0).unwrap();
        /// assert_eq!(event.token(), Token(0));
        /// assert!(event.readiness().is_readable());
        /// #     Ok(())
        /// # }
        /// #
        /// # fn main() {
        /// #     try_main().unwrap();
        /// # }
        /// ```
        ///
        /// # Examples
        ///
        /// A simple example, for a more elaborate example, see the [`Evented`]
        /// documentation.
        ///
        /// ```
        /// # use std::error::Error;
        /// # fn try_main() -> Result<(), Box<Error>> {
        /// use mio::{Registration, Ready};
        ///
        /// let (registration, set_readiness) = Registration::new2();
        ///
        /// assert!(set_readiness.readiness().is_empty());
        ///
        /// set_readiness.set_readiness(Ready::readable())?;
        /// assert!(set_readiness.readiness().is_readable());
        /// #     Ok(())
        /// # }
        /// #
        /// # fn main() {
        /// #     try_main().unwrap();
        /// # }
        /// ```
        ///
        /// [`Registration`]: struct.Registration.html
        /// [`Evented`]: event/trait.Evented.html#examples
        /// [`Poll`]: struct.Poll.html
        /// [`Poll::poll`]: struct.Poll.html#method.poll
        pub fn set_readiness(&self, ready: Ready) -> io::Result<()> {
            self.inner.set_readiness(ready)
        }
    }

    impl fmt::Debug for SetReadiness {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_struct("SetReadiness")
                .finish()
        }
    }

    impl RegistrationInner {
        /// Get the registration's readiness.
        fn readiness(&self) -> Ready {
            self.state.load(Relaxed).readiness()
        }

        /// Set the registration's readiness.
        ///
        /// This function can be called concurrently by an arbitrary number of
        /// SetReadiness handles.
        fn set_readiness(&self, ready: Ready) -> io::Result<()> {
            // Load the current atomic state.
            let mut state = self.state.load(Acquire);
            let mut next;

            loop {
                next = state;

                if state.is_dropped() {
                    // Node is dropped, no more notifications
                    return Ok(());
                }

                // Update the readiness
                next.set_readiness(ready);

                // If the readiness is not blank, try to obtain permission to
                // push the node into the readiness queue.
                if !next.effective_readiness().is_empty() {
                    next.set_queued();
                }

                let actual = self.state.compare_and_swap(state, next, AcqRel);

                if state == actual {
                    break;
                }

                state = actual;
            }

            if !state.is_queued() && next.is_queued() {
                // We toggled the queued flag, making us responsible for queuing the
                // node in the MPSC readiness queue.
                self.enqueue_with_wakeup()?;
            }

            Ok(())
        }

        /// Update the registration details associated with the node
        fn update(&self, poll: &Poll, token: Token, interest: Ready, opt: PollOpt) -> io::Result<()> {
            // First, ensure poll instances match
            //
            // Load the queue pointer, `Relaxed` is sufficient here as only the
            // pointer is being operated on. The actual memory is guaranteed to be
            // visible the `poll: &Poll` ref passed as an argument to the function.
            let mut queue = self.readiness_queue.load(Relaxed);
            let other: &*mut () = unsafe {
                &*(&poll.readiness_queue.inner as *const _ as *const *mut ())
            };
            let other = *other;

            debug_assert!(mem::size_of::<Arc<ReadinessQueueInner>>() == mem::size_of::<*mut ()>());

            if queue.is_null() {
                // Attempt to set the queue pointer. `Release` ordering synchronizes
                // with `Acquire` in `ensure_with_wakeup`.
                let actual = self.readiness_queue.compare_and_swap(
                    queue, other, Release);

                if actual.is_null() {
                    // The CAS succeeded, this means that the node's ref count
                    // should be incremented to reflect that the `poll` function
                    // effectively owns the node as well.
                    //
                    // `Relaxed` ordering used for the same reason as in
                    // RegistrationInner::clone
                    self.ref_count.fetch_add(1, Relaxed);

                    // Note that the `queue` reference stored in our
                    // `readiness_queue` field is intended to be a strong reference,
                    // so now that we've successfully claimed the reference we bump
                    // the refcount here.
                    //
                    // Down below in `release_node` when we deallocate this
                    // `RegistrationInner` is where we'll transmute this back to an
                    // arc and decrement the reference count.
                    mem::forget(poll.readiness_queue.clone());
                } else {
                    // The CAS failed, another thread set the queue pointer, so ensure
                    // that the pointer and `other` match
                    if actual != other {
                        return Err(io::Error::new(io::ErrorKind::Other, "registration handle associated with another `Poll` instance"));
                    }
                }

                queue = other;
            } else if queue != other {
                return Err(io::Error::new(io::ErrorKind::Other, "registration handle associated with another `Poll` instance"));
            }

            unsafe {
                let actual = &poll.readiness_queue.inner as *const _ as *const usize;
                debug_assert_eq!(queue as usize, *actual);
            }

            // The `update_lock` atomic is used as a flag ensuring only a single
            // thread concurrently enters the `update` critical section. Any
            // concurrent calls to update are discarded. If coordinated updates are
            // required, the Mio user is responsible for handling that.
            //
            // Acquire / Release ordering is used on `update_lock` to ensure that
            // data access to the `token_*` variables are scoped to the critical
            // section.

            // Acquire the update lock.
            if self.update_lock.compare_and_swap(false, true, Acquire) {
                // The lock is already held. Discard the update
                return Ok(());
            }

            // Relaxed ordering is acceptable here as the only memory that needs to
            // be visible as part of the update are the `token_*` variables, and
            // ordering has already been handled by the `update_lock` access.
            let mut state = self.state.load(Relaxed);
            let mut next;

            // Read the current token, again this memory has been ordered by the
            // acquire on `update_lock`.
            let curr_token_pos = state.token_write_pos();
            let curr_token = unsafe { self::token(self, curr_token_pos) };

            let mut next_token_pos = curr_token_pos;

            // If the `update` call is changing the token, then compute the next
            // available token slot and write the token there.
            //
            // Note that this computation is happening *outside* of the
            // compare-and-swap loop. The update lock ensures that only a single
            // thread could be mutating the write_token_position, so the
            // `next_token_pos` will never need to be recomputed even if
            // `token_read_pos` concurrently changes. This is because
            // `token_read_pos` can ONLY concurrently change to the current value of
            // `token_write_pos`, so `next_token_pos` will always remain valid.
            if token != curr_token {
                next_token_pos = state.next_token_pos();

                // Update the token
                match next_token_pos {
                    0 => unsafe { *self.token_0.get() = token },
                    1 => unsafe { *self.token_1.get() = token },
                    2 => unsafe { *self.token_2.get() = token },
                    _ => unreachable!(),
                }
            }

            // Now enter the compare-and-swap loop
            loop {
                next = state;

                // The node is only dropped once all `Registration` handles are
                // dropped. Only `Registration` can call `update`.
                debug_assert!(!state.is_dropped());

                // Update the write token position, this will also release the token
                // to Poll::poll.
                next.set_token_write_pos(next_token_pos);

                // Update readiness and poll opts
                next.set_interest(interest);
                next.set_poll_opt(opt);

                // If there is effective readiness, the node will need to be queued
                // for processing. This exact behavior is still TBD, so we are
                // conservative for now and always fire.
                //
                // See https://github.com/carllerche/mio/issues/535.
                if !next.effective_readiness().is_empty() {
                    next.set_queued();
                }

                // compare-and-swap the state values. Only `Release` is needed here.
                // The `Release` ensures that `Poll::poll` will see the token
                // update and the update function doesn't care about any other
                // memory visibility.
                let actual = self.state.compare_and_swap(state, next, Release);

                if actual == state {
                    break;
                }

                // CAS failed, but `curr_token_pos` should not have changed given
                // that we still hold the update lock.
                debug_assert_eq!(curr_token_pos, actual.token_write_pos());

                state = actual;
            }

            // Release the lock
            self.update_lock.store(false, Release);

            if !state.is_queued() && next.is_queued() {
                // We are responsible for enqueing the node.
                enqueue_with_wakeup(queue, self)?;
            }

            Ok(())
        }
    }

    impl ops::Deref for RegistrationInner {
        type Target = ReadinessNode;

        fn deref(&self) -> &ReadinessNode {
            unsafe { &*self.node }
        }
    }

    impl Clone for RegistrationInner {
        fn clone(&self) -> RegistrationInner {
            // Using a relaxed ordering is alright here, as knowledge of the
            // original reference prevents other threads from erroneously deleting
            // the object.
            //
            // As explained in the [Boost documentation][1], Increasing the
            // reference counter can always be done with memory_order_relaxed: New
            // references to an object can only be formed from an existing
            // reference, and passing an existing reference from one thread to
            // another must already provide any required synchronization.
            //
            // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
            let old_size = self.ref_count.fetch_add(1, Relaxed);

            // However we need to guard against massive refcounts in case someone
            // is `mem::forget`ing Arcs. If we don't do this the count can overflow
            // and users will use-after free. We racily saturate to `isize::MAX` on
            // the assumption that there aren't ~2 billion threads incrementing
            // the reference count at once. This branch will never be taken in
            // any realistic program.
            //
            // We abort because such a program is incredibly degenerate, and we
            // don't care to support it.
            if old_size & !MAX_REFCOUNT != 0 {
                process::abort();
            }

            RegistrationInner {
                node: self.node,
            }
        }
    }

    impl Drop for RegistrationInner {
        fn drop(&mut self) {
            // Only handles releasing from `Registration` and `SetReadiness`
            // handles. Poll has to call this itself.
            release_node(self.node);
        }
    }

    /*
     *
     * ===== ReadinessQueue =====
     *
     */

    impl ReadinessQueue {
        /// Create a new `ReadinessQueue`.
        fn new() -> io::Result<ReadinessQueue> {
            is_send::<Self>();
            is_sync::<Self>();

            let end_marker = Box::new(ReadinessNode::marker());
            let sleep_marker = Box::new(ReadinessNode::marker());
            let closed_marker = Box::new(ReadinessNode::marker());

            let ptr = &*end_marker as *const _ as *mut _;

            Ok(ReadinessQueue {
                inner: Arc::new(ReadinessQueueInner {
                    awakener: sys::Awakener::new()?,
                    head_readiness: AtomicPtr::new(ptr),
                    tail_readiness: UnsafeCell::new(ptr),
                    end_marker,
                    sleep_marker,
                    closed_marker,
                })
            })
        }

        /// Poll the queue for new events
        fn poll(&self, dst: &mut sys::Events) {
            // `until` is set with the first node that gets re-enqueued due to being
            // set to have level-triggered notifications. This prevents an infinite
            // loop where `Poll::poll` will keep dequeuing nodes it enqueues.
            let mut until = ptr::null_mut();

            if dst.len() == dst.capacity() {
                // If `dst` is already full, the readiness queue won't be drained.
                // This might result in `sleep_marker` staying in the queue and
                // unecessary pipe writes occuring.
                self.inner.clear_sleep_marker();
            }

            'outer:
            while dst.len() < dst.capacity() {
                // Dequeue a node. If the queue is in an inconsistent state, then
                // stop polling. `Poll::poll` will be called again shortly and enter
                // a syscall, which should be enough to enable the other thread to
                // finish the queuing process.
                let ptr = match unsafe { self.inner.dequeue_node(until) } {
                    Dequeue::Empty | Dequeue::Inconsistent => break,
                    Dequeue::Data(ptr) => ptr,
                };

                let node = unsafe { &*ptr };

                // Read the node state with Acquire ordering. This allows reading
                // the token variables.
                let mut state = node.state.load(Acquire);
                let mut next;
                let mut readiness;
                let mut opt;

                loop {
                    // Build up any changes to the readiness node's state and
                    // attempt the CAS at the end
                    next = state;

                    // Given that the node was just read from the queue, the
                    // `queued` flag should still be set.
                    debug_assert!(state.is_queued());

                    // The dropped flag means we need to release the node and
                    // perform no further processing on it.
                    if state.is_dropped() {
                        // Release the node and continue
                        release_node(ptr);
                        continue 'outer;
                    }

                    // Process the node
                    readiness = state.effective_readiness();
                    opt = state.poll_opt();

                    if opt.is_edge() {
                        // Mark the node as dequeued
                        next.set_dequeued();

                        if opt.is_oneshot() && !readiness.is_empty() {
                            next.disarm();
                        }
                    } else if readiness.is_empty() {
                        next.set_dequeued();
                    }

                    // Ensure `token_read_pos` is set to `token_write_pos` so that
                    // we read the most up to date token value.
                    next.update_token_read_pos();

                    if state == next {
                        break;
                    }

                    let actual = node.state.compare_and_swap(state, next, AcqRel);

                    if actual == state {
                        break;
                    }

                    state = actual;
                }

                // If the queued flag is still set, then the node must be requeued.
                // This typically happens when using level-triggered notifications.
                if next.is_queued() {
                    if until.is_null() {
                        // We never want to see the node again
                        until = ptr;
                    }

                    // Requeue the node
                    self.inner.enqueue_node(node);
                }

                if !readiness.is_empty() {
                    // Get the token
                    let token = unsafe { token(node, next.token_read_pos()) };

                    // Push the event
                    dst.push_event(Event::new(readiness, token));
                }
            }
        }

        /// Prepare the queue for the `Poll::poll` thread to block in the system
        /// selector. This involves changing `head_readiness` to `sleep_marker`.
        /// Returns true if successful and `poll` can block.
        fn prepare_for_sleep(&self) -> bool {
            let end_marker = self.inner.end_marker();
            let sleep_marker = self.inner.sleep_marker();

            let tail = unsafe { *self.inner.tail_readiness.get() };

            // If the tail is currently set to the sleep_marker, then check if the
            // head is as well. If it is, then the queue is currently ready to
            // sleep. If it is not, then the queue is not empty and there should be
            // no sleeping.
            if tail == sleep_marker {
                return self.inner.head_readiness.load(Acquire) == sleep_marker;
            }

            // If the tail is not currently set to `end_marker`, then the queue is
            // not empty.
            if tail != end_marker {
                return false;
            }

            // The sleep marker is *not* currently in the readiness queue.
            //
            // The sleep marker is only inserted in this function. It is also only
            // inserted in the tail position. This is guaranteed by first checking
            // that the end marker is in the tail position, pushing the sleep marker
            // after the end marker, then removing the end marker.
            //
            // Before inserting a node into the queue, the next pointer has to be
            // set to null. Again, this is only safe to do when the node is not
            // currently in the queue, but we already have ensured this.
            self.inner.sleep_marker.next_readiness.store(ptr::null_mut(), Relaxed);

            let actual = self.inner.head_readiness.compare_and_swap(
                end_marker, sleep_marker, AcqRel);

            debug_assert!(actual != sleep_marker);

            if actual != end_marker {
                // The readiness queue is not empty
                return false;
            }

            // The current tail should be pointing to `end_marker`
            debug_assert!(unsafe { *self.inner.tail_readiness.get() == end_marker });
            // The `end_marker` next pointer should be null
            debug_assert!(self.inner.end_marker.next_readiness.load(Relaxed).is_null());

            // Update tail pointer.
            unsafe { *self.inner.tail_readiness.get() = sleep_marker; }
            true
        }
    }

    impl Drop for ReadinessQueue {
        fn drop(&mut self) {
            // Close the queue by enqueuing the closed node
            self.inner.enqueue_node(&*self.inner.closed_marker);

            loop {
                // Free any nodes that happen to be left in the readiness queue
                let ptr = match unsafe { self.inner.dequeue_node(ptr::null_mut()) } {
                    Dequeue::Empty => break,
                    Dequeue::Inconsistent => {
                        // This really shouldn't be possible as all other handles to
                        // `ReadinessQueueInner` are dropped, but handle this by
                        // spinning I guess?
                        continue;
                    }
                    Dequeue::Data(ptr) => ptr,
                };

                let node = unsafe { &*ptr };

                let state = node.state.load(Acquire);

                debug_assert!(state.is_queued());

                release_node(ptr);
            }
        }
    }

    impl ReadinessQueueInner {
        fn wakeup(&self) -> io::Result<()> {
            self.awakener.wakeup()
        }

        /// Prepend the given node to the head of the readiness queue. This is done
        /// with relaxed ordering. Returns true if `Poll` needs to be woken up.
        fn enqueue_node_with_wakeup(&self, node: &ReadinessNode) -> io::Result<()> {
            if self.enqueue_node(node) {
                self.wakeup()?;
            }

            Ok(())
        }

        /// Push the node into the readiness queue
        fn enqueue_node(&self, node: &ReadinessNode) -> bool {
            // This is the 1024cores.net intrusive MPSC queue [1] "push" function.
            let node_ptr = node as *const _ as *mut _;

            // Relaxed used as the ordering is "released" when swapping
            // `head_readiness`
            node.next_readiness.store(ptr::null_mut(), Relaxed);

            unsafe {
                let mut prev = self.head_readiness.load(Acquire);

                loop {
                    if prev == self.closed_marker() {
                        debug_assert!(node_ptr != self.closed_marker());
                        // debug_assert!(node_ptr != self.end_marker());
                        debug_assert!(node_ptr != self.sleep_marker());

                        if node_ptr != self.end_marker() {
                            // The readiness queue is shutdown, but the enqueue flag was
                            // set. This means that we are responsible for decrementing
                            // the ready queue's ref count
                            debug_assert!(node.ref_count.load(Relaxed) >= 2);
                            release_node(node_ptr);
                        }

                        return false;
                    }

                    let act = self.head_readiness.compare_and_swap(prev, node_ptr, AcqRel);

                    if prev == act {
                        break;
                    }

                    prev = act;
                }

                debug_assert!((*prev).next_readiness.load(Relaxed).is_null());

                (*prev).next_readiness.store(node_ptr, Release);

                prev == self.sleep_marker()
            }
        }

        fn clear_sleep_marker(&self) {
            let end_marker = self.end_marker();
            let sleep_marker = self.sleep_marker();

            unsafe {
                let tail = *self.tail_readiness.get();

                if tail != self.sleep_marker() {
                    return;
                }

                // The empty markeer is *not* currently in the readiness queue
                // (since the sleep markeris).
                self.end_marker.next_readiness.store(ptr::null_mut(), Relaxed);

                let actual = self.head_readiness.compare_and_swap(
                    sleep_marker, end_marker, AcqRel);

                debug_assert!(actual != end_marker);

                if actual != sleep_marker {
                    // The readiness queue is not empty, we cannot remove the sleep
                    // markeer
                    return;
                }

                // Update the tail pointer.
                *self.tail_readiness.get() = end_marker;
            }
        }

        /// Must only be called in `poll` or `drop`
        unsafe fn dequeue_node(&self, until: *mut ReadinessNode) -> Dequeue {
            // This is the 1024cores.net intrusive MPSC queue [1] "pop" function
            // with the modifications mentioned at the top of the file.
            let mut tail = *self.tail_readiness.get();
            let mut next = (*tail).next_readiness.load(Acquire);

            if tail == self.end_marker() || tail == self.sleep_marker() || tail == self.closed_marker() {
                if next.is_null() {
                    // Make sure the sleep marker is removed (as we are no longer
                    // sleeping
                    self.clear_sleep_marker();

                    return Dequeue::Empty;
                }

                *self.tail_readiness.get() = next;
                tail = next;
                next = (*next).next_readiness.load(Acquire);
            }

            // Only need to check `until` at this point. `until` is either null,
            // which will never match tail OR it is a node that was pushed by
            // the current thread. This means that either:
            //
            // 1) The queue is inconsistent, which is handled explicitly
            // 2) We encounter `until` at this point in dequeue
            // 3) we will pop a different node
            if tail == until {
                return Dequeue::Empty;
            }

            if !next.is_null() {
                *self.tail_readiness.get() = next;
                return Dequeue::Data(tail);
            }

            if self.head_readiness.load(Acquire) != tail {
                return Dequeue::Inconsistent;
            }

            // Push the stub node
            self.enqueue_node(&*self.end_marker);

            next = (*tail).next_readiness.load(Acquire);

            if !next.is_null() {
                *self.tail_readiness.get() = next;
                return Dequeue::Data(tail);
            }

            Dequeue::Inconsistent
        }

        fn end_marker(&self) -> *mut ReadinessNode {
            &*self.end_marker as *const ReadinessNode as *mut ReadinessNode
        }

        fn sleep_marker(&self) -> *mut ReadinessNode {
            &*self.sleep_marker as *const ReadinessNode as *mut ReadinessNode
        }

        fn closed_marker(&self) -> *mut ReadinessNode {
            &*self.closed_marker as *const ReadinessNode as *mut ReadinessNode
        }
    }

    impl ReadinessNode {
        /// Return a new `ReadinessNode`, initialized with a ref_count of 3.
        fn new(queue: *mut (),
               token: Token,
               interest: Ready,
               opt: PollOpt,
               ref_count: usize) -> ReadinessNode
        {
            ReadinessNode {
                state: AtomicState::new(interest, opt),
                // Only the first token is set, the others are initialized to 0
                token_0: UnsafeCell::new(token),
                token_1: UnsafeCell::new(Token(0)),
                token_2: UnsafeCell::new(Token(0)),
                next_readiness: AtomicPtr::new(ptr::null_mut()),
                update_lock: AtomicBool::new(false),
                readiness_queue: AtomicPtr::new(queue),
                ref_count: AtomicUsize::new(ref_count),
            }
        }

        fn marker() -> ReadinessNode {
            ReadinessNode {
                state: AtomicState::new(Ready::empty(), PollOpt::empty()),
                token_0: UnsafeCell::new(Token(0)),
                token_1: UnsafeCell::new(Token(0)),
                token_2: UnsafeCell::new(Token(0)),
                next_readiness: AtomicPtr::new(ptr::null_mut()),
                update_lock: AtomicBool::new(false),
                readiness_queue: AtomicPtr::new(ptr::null_mut()),
                ref_count: AtomicUsize::new(0),
            }
        }

        fn enqueue_with_wakeup(&self) -> io::Result<()> {
            let queue = self.readiness_queue.load(Acquire);

            if queue.is_null() {
                // Not associated with a queue, nothing to do
                return Ok(());
            }

            enqueue_with_wakeup(queue, self)
        }
    }

    fn enqueue_with_wakeup(queue: *mut (), node: &ReadinessNode) -> io::Result<()> {
        debug_assert!(!queue.is_null());
        // This is ugly... but we don't want to bump the ref count.
        let queue: &Arc<ReadinessQueueInner> = unsafe {
            &*(&queue as *const *mut () as *const Arc<ReadinessQueueInner>)
        };
        queue.enqueue_node_with_wakeup(node)
    }

    unsafe fn token(node: &ReadinessNode, pos: usize) -> Token {
        match pos {
            0 => *node.token_0.get(),
            1 => *node.token_1.get(),
            2 => *node.token_2.get(),
            _ => unreachable!(),
        }
    }

    fn release_node(ptr: *mut ReadinessNode) {
        unsafe {
            // `AcqRel` synchronizes with other `release_node` functions and ensures
            // that the drop happens after any reads / writes on other threads.
            if (*ptr).ref_count.fetch_sub(1, AcqRel) != 1 {
                return;
            }

            let node = Box::from_raw(ptr);

            // Decrement the readiness_queue Arc
            let queue = node.readiness_queue.load(Acquire);

            if queue.is_null() {
                return;
            }

            let _: Arc<ReadinessQueueInner> = mem::transmute(queue);
        }
    }

    impl AtomicState {
        fn new(interest: Ready, opt: PollOpt) -> AtomicState {
            let state = ReadinessState::new(interest, opt);

            AtomicState {
                inner: AtomicUsize::new(state.into()),
            }
        }

        /// Loads the current `ReadinessState`
        fn load(&self, order: Ordering) -> ReadinessState {
            self.inner.load(order).into()
        }

        /// Stores a state if the current state is the same as `current`.
        fn compare_and_swap(&self, current: ReadinessState, new: ReadinessState, order: Ordering) -> ReadinessState {
            self.inner.compare_and_swap(current.into(), new.into(), order).into()
        }

        // Returns `true` if the node should be queued
        fn flag_as_dropped(&self) -> bool {
            let prev: ReadinessState = self.inner.fetch_or(DROPPED_MASK | QUEUED_MASK, Release).into();
            // The flag should not have been previously set
            debug_assert!(!prev.is_dropped());

            !prev.is_queued()
        }
    }

    impl ReadinessState {
        // Create a `ReadinessState` initialized with the provided arguments
        #[inline]
        fn new(interest: Ready, opt: PollOpt) -> ReadinessState {
            let interest = event::ready_as_usize(interest);
            let opt = event::opt_as_usize(opt);

            debug_assert!(interest <= MASK_4);
            debug_assert!(opt <= MASK_4);

            let mut val = interest << INTEREST_SHIFT;
            val |= opt << POLL_OPT_SHIFT;

            ReadinessState(val)
        }

        #[inline]
        fn get(self, mask: usize, shift: usize) -> usize{
            (self.0 >> shift) & mask
        }

        #[inline]
        fn set(&mut self, val: usize, mask: usize, shift: usize) {
            self.0 = (self.0 & !(mask << shift)) | (val << shift)
        }

        /// Get the readiness
        #[inline]
        fn readiness(self) -> Ready {
            let v = self.get(MASK_4, READINESS_SHIFT);
            event::ready_from_usize(v)
        }

        #[inline]
        fn effective_readiness(self) -> Ready {
            self.readiness() & self.interest()
        }

        /// Set the readiness
        #[inline]
        fn set_readiness(&mut self, v: Ready) {
            self.set(event::ready_as_usize(v), MASK_4, READINESS_SHIFT);
        }

        /// Get the interest
        #[inline]
        fn interest(self) -> Ready {
            let v = self.get(MASK_4, INTEREST_SHIFT);
            event::ready_from_usize(v)
        }

        /// Set the interest
        #[inline]
        fn set_interest(&mut self, v: Ready) {
            self.set(event::ready_as_usize(v), MASK_4, INTEREST_SHIFT);
        }

        #[inline]
        fn disarm(&mut self) {
            self.set_interest(Ready::empty());
        }

        /// Get the poll options
        #[inline]
        fn poll_opt(self) -> PollOpt {
            let v = self.get(MASK_4, POLL_OPT_SHIFT);
            event::opt_from_usize(v)
        }

        /// Set the poll options
        #[inline]
        fn set_poll_opt(&mut self, v: PollOpt) {
            self.set(event::opt_as_usize(v), MASK_4, POLL_OPT_SHIFT);
        }

        #[inline]
        fn is_queued(self) -> bool {
            self.0 & QUEUED_MASK == QUEUED_MASK
        }

        /// Set the queued flag
        #[inline]
        fn set_queued(&mut self) {
            // Dropped nodes should never be queued
            debug_assert!(!self.is_dropped());
            self.0 |= QUEUED_MASK;
        }

        #[inline]
        fn set_dequeued(&mut self) {
            debug_assert!(self.is_queued());
            self.0 &= !QUEUED_MASK
        }

        #[inline]
        fn is_dropped(self) -> bool {
            self.0 & DROPPED_MASK == DROPPED_MASK
        }

        #[inline]
        fn token_read_pos(self) -> usize {
            self.get(MASK_2, TOKEN_RD_SHIFT)
        }

        #[inline]
        fn token_write_pos(self) -> usize {
            self.get(MASK_2, TOKEN_WR_SHIFT)
        }

        #[inline]
        fn next_token_pos(self) -> usize {
            let rd = self.token_read_pos();
            let wr = self.token_write_pos();

            match wr {
                0 => {
                    match rd {
                        1 => 2,
                        2 => 1,
                        0 => 1,
                        _ => unreachable!(),
                    }
                }
                1 => {
                    match rd {
                        0 => 2,
                        2 => 0,
                        1 => 2,
                        _ => unreachable!(),
                    }
                }
                2 => {
                    match rd {
                        0 => 1,
                        1 => 0,
                        2 => 0,
                        _ => unreachable!(),
                    }
                }
                _ => unreachable!(),
            }
        }

        #[inline]
        fn set_token_write_pos(&mut self, val: usize) {
            self.set(val, MASK_2, TOKEN_WR_SHIFT);
        }

        #[inline]
        fn update_token_read_pos(&mut self) {
            let val = self.token_write_pos();
            self.set(val, MASK_2, TOKEN_RD_SHIFT);
        }
    }

    impl From<ReadinessState> for usize {
        fn from(src: ReadinessState) -> usize {
            src.0
        }
    }

    impl From<usize> for ReadinessState {
        fn from(src: usize) -> ReadinessState {
            ReadinessState(src)
        }
    }

    fn is_send<T: Send>() {}
    fn is_sync<T: Sync>() {}

    impl SelectorId {
        pub fn new() -> SelectorId {
            SelectorId {
                id: AtomicUsize::new(0),
            }
        }

        pub fn associate_selector(&self, poll: &Poll) -> io::Result<()> {
            let selector_id = self.id.load(Ordering::SeqCst);

            if selector_id != 0 && selector_id != poll.selector.id() {
                Err(io::Error::new(io::ErrorKind::Other, "socket already registered"))
            } else {
                self.id.store(poll.selector.id(), Ordering::SeqCst);
                Ok(())
            }
        }
    }

    impl Clone for SelectorId {
        fn clone(&self) -> SelectorId {
            SelectorId {
                id: AtomicUsize::new(self.id.load(Ordering::SeqCst)),
            }
        }
    }

    #[test]
    #[cfg(all(unix, not(target_os = "fuchsia")))]
    pub fn as_raw_fd() {
        let poll = Poll::new().unwrap();
        assert!(poll.as_raw_fd() > 0);
    }

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