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    use crate::{event, sys, Events, Interest, Token};
    use log::trace;
    #[cfg(unix)]
    use std::os::unix::io::{AsRawFd, RawFd};
    use std::time::Duration;
    use std::{fmt, io};

    /// Polls for readiness events on all registered values.
    ///
    /// `Poll` allows a program to monitor a large number of [`event::Source`]s,
    /// waiting until one or more become "ready" for some class of operations; e.g.
    /// reading and writing. An event source is considered ready if it is possible
    /// to immediately perform a corresponding operation; e.g. [`read`] or
    /// [`write`].
    ///
    /// To use `Poll`, an `event::Source` must first be registered with the `Poll`
    /// instance using the [`register`] method on its associated `Register`,
    /// 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
    /// event source that generated the event.
    ///
    /// [`event::Source`]: ./event/trait.Source.html
    /// [`read`]: ./net/struct.TcpStream.html#method.read
    /// [`write`]: ./net/struct.TcpStream.html#method.write
    /// [`register`]: struct.Registry.html#method.register
    ///
    /// # Examples
    ///
    /// A basic example -- establishing a `TcpStream` connection.
    ///
    #[cfg_attr(all(feature = "os-poll", features = "net"), doc = "```")]
    #[cfg_attr(not(all(feature = "os-poll", features = "net")), doc = "```ignore")]
    /// # use std::error::Error;
    /// # fn main() -> Result<(), Box<dyn Error>> {
    /// use mio::{Events, Poll, Interest, Token};
    /// use mio::net::TcpStream;
    ///
    /// use std::net::{self, SocketAddr};
    ///
    /// // Bind a server socket to connect to.
    /// let addr: SocketAddr = "127.0.0.1:0".parse()?;
    /// let server = net::TcpListener::bind(addr)?;
    ///
    /// // Construct a new `Poll` handle as well as the `Events` we'll store into
    /// let mut poll = Poll::new()?;
    /// let mut events = Events::with_capacity(1024);
    ///
    /// // Connect the stream
    /// let mut stream = TcpStream::connect(server.local_addr()?)?;
    ///
    /// // Register the stream with `Poll`
    /// poll.registry().register(&mut stream, Token(0), Interest::READABLE | Interest::WRITABLE)?;
    ///
    /// // 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.is_writable() {
    ///             // The socket connected (probably, it could still be a spurious
    ///             // wakeup)
    ///             return Ok(());
    ///         }
    ///     }
    /// }
    /// # }
    /// ```
    ///
    /// # 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
    /// event source 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 event 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
    ///
    /// 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 event source.
    ///
    /// [`WouldBlock`]: std::io::ErrorKind::WouldBlock
    ///
    /// ### 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`]
    /// interest and either an error or close 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 `read_closed`, `write_closed`, 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 [`read_closed`],
    /// [`write_closed`], or [`error`] readiness should be doing so as an
    /// **optimization** and always be able to handle an error or close situation
    /// when performing the actual read operation.
    ///
    /// [`readable`]: ./event/struct.Event.html#method.is_readable
    /// [`writable`]: ./event/struct.Event.html#method.is_writable
    /// [`error`]: ./event/struct.Event.html#method.is_error
    /// [`read_closed`]: ./event/struct.Event.html#method.is_read_closed
    /// [`write_closed`]: ./event/struct.Event.html#method.is_write_closed
    ///
    /// ### Registering handles
    ///
    /// Unless otherwise noted, it should be assumed that types implementing
    /// [`event::Source`] will never become ready unless they are registered with
    /// `Poll`.
    ///
    /// For example:
    ///
    #[cfg_attr(all(feature = "os-poll", features = "net"), doc = "```")]
    #[cfg_attr(not(all(feature = "os-poll", features = "net")), doc = "```ignore")]
    /// # use std::error::Error;
    /// # use std::net;
    /// # fn main() -> Result<(), Box<dyn Error>> {
    /// use mio::{Poll, Interest, Token};
    /// use mio::net::TcpStream;
    /// use std::net::SocketAddr;
    /// use std::time::Duration;
    /// use std::thread;
    ///
    /// let address: SocketAddr = "127.0.0.1:0".parse()?;
    /// let listener = net::TcpListener::bind(address)?;
    /// let mut sock = TcpStream::connect(listener.local_addr()?)?;
    ///
    /// 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.registry().register(&mut sock, Token(0), Interest::READABLE | Interest::WRITABLE)?;
    /// #     Ok(())
    /// # }
    /// ```
    ///
    /// ### Dropping `Poll`
    ///
    /// When the `Poll` instance is dropped it may cancel in-flight operations for
    /// the registered [event sources], meaning that no further events for them may
    /// be received. It also means operations on the registered event sources may no
    /// longer work. It is up to the user to keep the `Poll` instance alive while
    /// registered event sources are being used.
    ///
    /// [event sources]: ./event/trait.Source.html
    ///
    /// # Implementation notes
    ///
    /// `Poll` is backed by the selector provided by the operating system.
    ///
    /// |      OS       |  Selector |
    /// |---------------|-----------|
    /// | Android       | [epoll]   |
    /// | DragonFly BSD | [kqueue]  |
    /// | FreeBSD       | [kqueue]  |
    /// | Linux         | [epoll]   |
    /// | NetBSD        | [kqueue]  |
    /// | OpenBSD       | [kqueue]  |
    /// | Solaris       | [epoll]   |
    /// | illumos       | [epoll]   |
    /// | Windows       | [IOCP]    |
    /// | iOS           | [kqueue]  |
    /// | macOS         | [kqueue]  |
    ///
    /// On all supported platforms, socket operations are handled by using the
    /// system selector. Platform specific extensions (e.g. [`SourceFd`]) 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 [`SourceFd`].
    ///
    /// 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.
    ///
    /// [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
    /// [`SourceFd`]: unix/struct.SourceFd.html
    /// [`Poll::poll`]: struct.Poll.html#method.poll
    pub struct Poll {
        registry: Registry,
    }

    /// Registers I/O resources.
    pub struct Registry {
        selector: sys::Selector,
    }

    impl Poll {
        /// Create a separate `Registry` which can be used to register
        /// `event::Source`s.
        pub fn registry(&self) -> &Registry {
            &self.registry
        }

        /// Wait for readiness events
        ///
        /// Blocks the current thread and waits for readiness events for any of the
        /// [`event::Source`]s 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 event source. 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.
        ///
        /// See the [struct] level documentation for a higher level discussion of
        /// polling.
        ///
        /// [`event::Source`]: ./event/trait.Source.html
        /// [`readable`]: struct.Interest.html#associatedconstant.READABLE
        /// [`writable`]: struct.Interest.html#associatedconstant.WRITABLE
        /// [struct]: struct.Poll.html
        /// [`iter`]: ./event/struct.Events.html#method.iter
        ///
        /// # Notes
        ///
        /// This returns any errors without attempting to retry, previous versions
        /// of Mio would automatically retry the poll call if it was interrupted
        /// (if `EINTR` was returned).
        ///
        /// # Examples
        ///
        /// A basic example -- establishing a `TcpStream` connection.
        ///
        #[cfg_attr(all(feature = "os-poll", features = "net"), doc = "```")]
        #[cfg_attr(not(all(feature = "os-poll", features = "net")), doc = "```ignore")]
        /// # use std::error::Error;
        /// # fn main() -> Result<(), Box<dyn Error>> {
        /// use mio::{Events, Poll, Interest, 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 mut poll = Poll::new()?;
        /// let mut events = Events::with_capacity(1024);
        ///
        /// // Connect the stream
        /// let mut stream = TcpStream::connect(addr)?;
        ///
        /// // Register the stream with `Poll`
        /// poll.registry().register(
        ///     &mut stream,
        ///     Token(0),
        ///     Interest::READABLE | Interest::WRITABLE)?;
        ///
        /// // 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.is_writable() {
        ///             // The socket connected (probably, it could still be a spurious
        ///             // wakeup)
        ///             return Ok(());
        ///         }
        ///     }
        /// }
        /// # }
        /// ```
        ///
        /// [struct]: #
        pub fn poll(&mut self, events: &mut Events, timeout: Option<Duration>) -> io::Result<()> {
            self.registry.selector.select(events.sys(), timeout)
        }
    }

    cfg_os_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 main() -> Result<(), Box<dyn Error>> {
            /// use mio::{Poll, Events};
            /// use std::time::Duration;
            ///
            /// let mut 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
            /// // `event::Source`s have been registered with this `Poll` instance.
            /// poll.poll(&mut events, Some(Duration::from_millis(500)))?;
            /// assert!(events.is_empty());
            /// #     Ok(())
            /// # }
            /// ```
            pub fn new() -> io::Result<Poll> {
                sys::Selector::new().map(|selector| Poll {
                    registry: Registry { selector },
                })
            }
        }
    }

    #[cfg(unix)]
    impl AsRawFd for Poll {
        fn as_raw_fd(&self) -> RawFd {
            self.registry.as_raw_fd()
        }
    }

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

    impl Registry {
        /// Register an [`event::Source`] with the `Poll` instance.
        ///
        /// Once registered, the `Poll` instance will monitor the event source 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 [`Poll`] docs for a high level overview.
        ///
        /// # Arguments
        ///
        /// `source: &S: event::Source`: This is the source of events 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 source. The token
        /// associated with the `event::Source` can be changed at any time by
        /// calling [`reregister`].
        ///
        /// See documentation on [`Token`] for an example showing how to pick
        /// [`Token`] values.
        ///
        /// `interest: Interest`: 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 `event::Source` can be changed at any time
        /// by calling [`reregister`].
        ///
        /// # Notes
        ///
        /// Callers must ensure that if a source being registered with a `Poll`
        /// instance was previously registered with that `Poll` instance, then a
        /// call to [`deregister`] has already occurred. Consecutive calls to
        /// `register` is undefined behavior.
        ///
        /// Unless otherwise specified, the caller should assume that once an event
        /// source is registered with a `Poll` instance, it is bound to that `Poll`
        /// instance for the lifetime of the event source. This remains true even
        /// if the event source is deregistered from the poll instance using
        /// [`deregister`].
        ///
        /// [`event::Source`]: ./event/trait.Source.html
        /// [`poll`]: struct.Poll.html#method.poll
        /// [`reregister`]: struct.Registry.html#method.reregister
        /// [`deregister`]: struct.Registry.html#method.deregister
        /// [`Token`]: struct.Token.html
        ///
        /// # Examples
        ///
        #[cfg_attr(all(feature = "os-poll", features = "net"), doc = "```")]
        #[cfg_attr(not(all(feature = "os-poll", features = "net")), doc = "```ignore")]
        /// # use std::error::Error;
        /// # use std::net;
        /// # fn main() -> Result<(), Box<dyn Error>> {
        /// use mio::{Events, Poll, Interest, Token};
        /// use mio::net::TcpStream;
        /// use std::net::SocketAddr;
        /// use std::time::{Duration, Instant};
        ///
        /// let mut poll = Poll::new()?;
        ///
        /// let address: SocketAddr = "127.0.0.1:0".parse()?;
        /// let listener = net::TcpListener::bind(address)?;
        /// let mut socket = TcpStream::connect(listener.local_addr()?)?;
        ///
        /// // Register the socket with `poll`
        /// poll.registry().register(
        ///     &mut socket,
        ///     Token(0),
        ///     Interest::READABLE | Interest::WRITABLE)?;
        ///
        /// 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(());
        ///         }
        ///     }
        /// }
        /// # }
        /// ```
        pub fn register<S>(&self, source: &mut S, token: Token, interests: Interest) -> io::Result<()>
        where
            S: event::Source + ?Sized,
        {
            trace!(
                "registering event source with poller: token={:?}, interests={:?}",
                token,
                interests
            );
            source.register(self, token, interests)
        }

        /// Re-register an [`event::Source`] with the `Poll` instance.
        ///
        /// Re-registering an event source allows changing the details of the
        /// registration. Specifically, it allows updating the associated `token`
        /// and `interests` 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 event source must have previously been registered with this instance
        /// of `Poll`, otherwise the behavior is undefined.
        ///
        /// See the [`register`] documentation for details about the function
        /// arguments and see the [`struct`] docs for a high level overview of
        /// polling.
        ///
        /// # Examples
        ///
        #[cfg_attr(all(feature = "os-poll", features = "net"), doc = "```")]
        #[cfg_attr(not(all(feature = "os-poll", features = "net")), doc = "```ignore")]
        /// # use std::error::Error;
        /// # use std::net;
        /// # fn main() -> Result<(), Box<dyn Error>> {
        /// use mio::{Poll, Interest, Token};
        /// use mio::net::TcpStream;
        /// use std::net::SocketAddr;
        ///
        /// let poll = Poll::new()?;
        ///
        /// let address: SocketAddr = "127.0.0.1:0".parse()?;
        /// let listener = net::TcpListener::bind(address)?;
        /// let mut socket = TcpStream::connect(listener.local_addr()?)?;
        ///
        /// // Register the socket with `poll`, requesting readable
        /// poll.registry().register(
        ///     &mut socket,
        ///     Token(0),
        ///     Interest::READABLE)?;
        ///
        /// // Reregister the socket specifying write interest instead. Even though
        /// // the token is the same it must be specified.
        /// poll.registry().reregister(
        ///     &mut socket,
        ///     Token(0),
        ///     Interest::WRITABLE)?;
        /// #     Ok(())
        /// # }
        /// ```
        ///
        /// [`event::Source`]: ./event/trait.Source.html
        /// [`struct`]: struct.Poll.html
        /// [`register`]: struct.Registry.html#method.register
        /// [`readable`]: ./event/struct.Event.html#is_readable
        /// [`writable`]: ./event/struct.Event.html#is_writable
        pub fn reregister<S>(&self, source: &mut S, token: Token, interests: Interest) -> io::Result<()>
        where
            S: event::Source + ?Sized,
        {
            trace!(
                "reregistering event source with poller: token={:?}, interests={:?}",
                token,
                interests
            );
            source.reregister(self, token, interests)
        }

        /// Deregister an [`event::Source`] with the `Poll` instance.
        ///
        /// When an event source is deregistered, the `Poll` instance will no longer
        /// monitor it for readiness state changes. Deregistering clears up any
        /// internal resources needed to track the handle.  After an explicit call
        /// to this method completes, it is guaranteed that the token previously
        /// registered to this handle will not be returned by a future poll, so long
        /// as a happens-before relationship is established between this call and
        /// the poll.
        ///
        /// The event source must have previously been registered with this instance
        /// of `Poll`, otherwise the behavior is undefined.
        ///
        /// A handle can be passed back to `register` after it has been
        /// deregistered; however, it must be passed back to the **same** `Poll`
        /// instance, otherwise the behavior is undefined.
        ///
        /// # Examples
        ///
        #[cfg_attr(all(feature = "os-poll", features = "net"), doc = "```")]
        #[cfg_attr(not(all(feature = "os-poll", features = "net")), doc = "```ignore")]
        /// # use std::error::Error;
        /// # use std::net;
        /// # fn main() -> Result<(), Box<dyn Error>> {
        /// use mio::{Events, Poll, Interest, Token};
        /// use mio::net::TcpStream;
        /// use std::net::SocketAddr;
        /// use std::time::Duration;
        ///
        /// let mut poll = Poll::new()?;
        ///
        /// let address: SocketAddr = "127.0.0.1:0".parse()?;
        /// let listener = net::TcpListener::bind(address)?;
        /// let mut socket = TcpStream::connect(listener.local_addr()?)?;
        ///
        /// // Register the socket with `poll`
        /// poll.registry().register(
        ///     &mut socket,
        ///     Token(0),
        ///     Interest::READABLE)?;
        ///
        /// poll.registry().deregister(&mut socket)?;
        ///
        /// let mut events = Events::with_capacity(1024);
        ///
        /// // Set a timeout because this poll should never receive any events.
        /// poll.poll(&mut events, Some(Duration::from_secs(1)))?;
        /// assert!(events.is_empty());
        /// #     Ok(())
        /// # }
        /// ```
        pub fn deregister<S>(&self, source: &mut S) -> io::Result<()>
        where
            S: event::Source + ?Sized,
        {
            trace!("deregistering event source from poller");
            source.deregister(self)
        }

        /// Creates a new independently owned `Registry`.
        ///
        /// Event sources registered with this `Registry` will be registered with
        /// the original `Registry` and `Poll` instance.
        pub fn try_clone(&self) -> io::Result<Registry> {
            self.selector
                .try_clone()
                .map(|selector| Registry { selector })
        }

        /// Internal check to ensure only a single `Waker` is active per [`Poll`]
        /// instance.
        #[cfg(debug_assertions)]
        pub(crate) fn register_waker(&self) {
            if self.selector.register_waker() {
                panic!("Only a single `Waker` can be active per `Poll` instance");
            }
        }
    }

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

    #[cfg(unix)]
    impl AsRawFd for Registry {
        fn as_raw_fd(&self) -> RawFd {
            self.selector.as_raw_fd()
        }
    }

    /// Get access to the `sys::Selector` from `Registry`.
    pub(crate) fn selector(registry: &Registry) -> &sys::Selector {
        &registry.selector
    }

    cfg_os_poll! {
        #[cfg(unix)]
        #[test]
        pub fn as_raw_fd() {
            let poll = Poll::new().unwrap();
            assert!(poll.as_raw_fd() > 0);
        }
    }