udp.rs

use crate::io::{Interest, PollEvented, ReadBuf, Ready};
use crate::net::{to_socket_addrs, ToSocketAddrs};

use std::convert::TryFrom;
use std::fmt;
use std::io;
use std::net::{self, Ipv4Addr, Ipv6Addr, SocketAddr};
use std::task::{Context, Poll};
use crate::net::unix::SocketAddr;

cfg_io_util! {
    use bytes::BufMut;
}

cfg_net! {
    /// A UDP socket.
    ///
    /// UDP is "connectionless", unlike TCP. Meaning, regardless of what address you've bound to, a `UdpSocket`
    /// is free to communicate with many different remotes. In tokio there are basically two main ways to use `UdpSocket`:
    ///
    /// * one to many: [`bind`](`UdpSocket::bind`) and use [`send_to`](`UdpSocket::send_to`)
    ///   and [`recv_from`](`UdpSocket::recv_from`) to communicate with many different addresses
    /// * one to one: [`connect`](`UdpSocket::connect`) and associate with a single address, using [`send`](`UdpSocket::send`)
    ///   and [`recv`](`UdpSocket::recv`) to communicate only with that remote address
    ///
    /// This type does not provide a `split` method, because this functionality
    /// can be achieved by instead wrapping the socket in an [`Arc`]. Note that
    /// you do not need a `Mutex` to share the `UdpSocket` — an `Arc<UdpSocket>`
    /// is enough. This is because all of the methods take `&self` instead of
    /// `&mut self`. Once you have wrapped it in an `Arc`, you can call
    /// `.clone()` on the `Arc<UdpSocket>` to get multiple shared handles to the
    /// same socket. An example of such usage can be found further down.
    ///
    /// [`Arc`]: std::sync::Arc
    ///
    /// # Streams
    ///
    /// If you need to listen over UDP and produce a [`Stream`], you can look
    /// at [`UdpFramed`].
    ///
    /// [`UdpFramed`]: https://docs.rs/tokio-util/latest/tokio_util/udp/struct.UdpFramed.html
    /// [`Stream`]: https://docs.rs/futures/0.3/futures/stream/trait.Stream.html
    ///
    /// # Example: one to many (bind)
    ///
    /// Using `bind` we can create a simple echo server that sends and recv's with many different clients:
    /// ```no_run
    /// use tokio::net::UdpSocket;
    /// use std::io;
    ///
    /// #[tokio::main]
    /// async fn main() -> io::Result<()> {
    ///     let sock = UdpSocket::bind("0.0.0.0:8080").await?;
    ///     let mut buf = [0; 1024];
    ///     loop {
    ///         let (len, addr) = sock.recv_from(&mut buf).await?;
    ///         println!("{:?} bytes received from {:?}", len, addr);
    ///
    ///         let len = sock.send_to(&buf[..len], addr).await?;
    ///         println!("{:?} bytes sent", len);
    ///     }
    /// }
    /// ```
    ///
    /// # Example: one to one (connect)
    ///
    /// Or using `connect` we can echo with a single remote address using `send` and `recv`:
    /// ```no_run
    /// use tokio::net::UdpSocket;
    /// use std::io;
    ///
    /// #[tokio::main]
    /// async fn main() -> io::Result<()> {
    ///     let sock = UdpSocket::bind("0.0.0.0:8080").await?;
    ///
    ///     let remote_addr = "127.0.0.1:59611";
    ///     sock.connect(remote_addr).await?;
    ///     let mut buf = [0; 1024];
    ///     loop {
    ///         let len = sock.recv(&mut buf).await?;
    ///         println!("{:?} bytes received from {:?}", len, remote_addr);
    ///
    ///         let len = sock.send(&buf[..len]).await?;
    ///         println!("{:?} bytes sent", len);
    ///     }
    /// }
    /// ```
    ///
    /// # Example: Splitting with `Arc`
    ///
    /// Because `send_to` and `recv_from` take `&self`. It's perfectly alright
    /// to use an `Arc<UdpSocket>` and share the references to multiple tasks.
    /// Here is a similar "echo" example that supports concurrent
    /// sending/receiving:
    ///
    /// ```no_run
    /// use tokio::{net::UdpSocket, sync::mpsc};
    /// use std::{io, net::SocketAddr, sync::Arc};
    ///
    /// #[tokio::main]
    /// async fn main() -> io::Result<()> {
    ///     let sock = UdpSocket::bind("0.0.0.0:8080".parse::<SocketAddr>().unwrap()).await?;
    ///     let r = Arc::new(sock);
    ///     let s = r.clone();
    ///     let (tx, mut rx) = mpsc::channel::<(Vec<u8>, SocketAddr)>(1_000);
    ///
    ///     tokio::spawn(async move {
    ///         while let Some((bytes, addr)) = rx.recv().await {
    ///             let len = s.send_to(&bytes, &addr).await.unwrap();
    ///             println!("{:?} bytes sent", len);
    ///         }
    ///     });
    ///
    ///     let mut buf = [0; 1024];
    ///     loop {
    ///         let (len, addr) = r.recv_from(&mut buf).await?;
    ///         println!("{:?} bytes received from {:?}", len, addr);
    ///         tx.send((buf[..len].to_vec(), addr)).await.unwrap();
    ///     }
    /// }
    /// ```
    ///
    pub struct UdpSocket {
        io: PollEvented<mio::net::UdpSocket>,
    }
}

impl UdpSocket {
    /// This function will create a new UDP socket and attempt to bind it to
    /// the `addr` provided.
    ///
    /// Binding with a port number of 0 will request that the OS assigns a port
    /// to this listener. The port allocated can be queried via the `local_addr`
    /// method.
    ///
    /// # Example
    ///
    /// ```no_run
    /// use tokio::net::UdpSocket;
    /// use std::io;
    ///
    /// #[tokio::main]
    /// async fn main() -> io::Result<()> {
    ///     let sock = UdpSocket::bind("0.0.0.0:8080").await?;
    ///     // use `sock`
    /// #   let _ = sock;
    ///     Ok(())
    /// }
    /// ```
    pub async fn bind<A: ToSocketAddrs>(addr: A) -> io::Result<UdpSocket> {
        let addrs = to_socket_addrs(addr).await?;
        let mut last_err = None;

        for addr in addrs {
            match UdpSocket::bind_addr(addr) {
                Ok(socket) => return Ok(socket),
                Err(e) => last_err = Some(e),
            }
        }

        Err(last_err.unwrap_or_else(|| {
            io::Error::new(
                io::ErrorKind::InvalidInput,
                "could not resolve to any address",
            )
        }))
    }

    fn bind_addr(addr: SocketAddr) -> io::Result<UdpSocket> {
        let sys = mio::net::UdpSocket::bind(addr)?;
        UdpSocket::new(sys)
    }

    #[track_caller]
    fn new(socket: mio::net::UdpSocket) -> io::Result<UdpSocket> {
        let io = PollEvented::new(socket)?;
        Ok(UdpSocket { io })
    }

    /// Creates new `UdpSocket` from a previously bound `std::net::UdpSocket`.
    ///
    /// This function is intended to be used to wrap a UDP socket from the
    /// standard library in the Tokio equivalent.
    ///
    /// This can be used in conjunction with socket2's `Socket` interface to
    /// configure a socket before it's handed off, such as setting options like
    /// `reuse_address` or binding to multiple addresses.
    ///
    /// # Notes
    ///
    /// The caller is responsible for ensuring that the socket is in
    /// non-blocking mode. Otherwise all I/O operations on the socket
    /// will block the thread, which will cause unexpected behavior.
    /// Non-blocking mode can be set using [`set_nonblocking`].
    ///
    /// [`set_nonblocking`]: std::net::UdpSocket::set_nonblocking
    ///
    /// # Panics
    ///
    /// This function panics if thread-local runtime is not set.
    ///
    /// The runtime is usually set implicitly when this function is called
    /// from a future driven by a tokio runtime, otherwise runtime can be set
    /// explicitly with [`Runtime::enter`](crate::runtime::Runtime::enter) function.
    ///
    /// # Example
    ///
    /// ```no_run
    /// use tokio::net::UdpSocket;
    /// # use std::{io, net::SocketAddr};
    ///
    /// # #[tokio::main]
    /// # async fn main() -> io::Result<()> {
    /// let addr = "0.0.0.0:8080".parse::<SocketAddr>().unwrap();
    /// let std_sock = std::net::UdpSocket::bind(addr)?;
    /// std_sock.set_nonblocking(true)?;
    /// let sock = UdpSocket::from_std(std_sock)?;
    /// // use `sock`
    /// # Ok(())
    /// # }
    /// ```
    #[track_caller]
    pub fn from_std(socket: net::UdpSocket) -> io::Result<UdpSocket> {
        let io = mio::net::UdpSocket::from_std(socket);
        UdpSocket::new(io)
    }

    /// Turns a [`tokio::net::UdpSocket`] into a [`std::net::UdpSocket`].
    ///
    /// The returned [`std::net::UdpSocket`] will have nonblocking mode set as
    /// `true`.  Use [`set_nonblocking`] to change the blocking mode if needed.
    ///
    /// # Examples
    ///
    /// ```rust,no_run
    /// use std::error::Error;
    ///
    /// #[tokio::main]
    /// async fn main() -> Result<(), Box<dyn Error>> {
    ///     let tokio_socket = tokio::net::UdpSocket::bind("127.0.0.1:0").await?;
    ///     let std_socket = tokio_socket.into_std()?;
    ///     std_socket.set_nonblocking(false)?;
    ///     Ok(())
    /// }
    /// ```
    ///
    /// [`tokio::net::UdpSocket`]: UdpSocket
    /// [`std::net::UdpSocket`]: std::net::UdpSocket
    /// [`set_nonblocking`]: fn@std::net::UdpSocket::set_nonblocking
    pub fn into_std(self) -> io::Result<std::net::UdpSocket> {
        #[cfg(unix)]
        {
            use std::os::unix::io::{FromRawFd, IntoRawFd};
            self.io
                .into_inner()
                .map(|io| io.into_raw_fd())
                .map(|raw_fd| unsafe { std::net::UdpSocket::from_raw_fd(raw_fd) })
        }

        #[cfg(windows)]
        {
            use std::os::windows::io::{FromRawSocket, IntoRawSocket};
            self.io
                .into_inner()
                .map(|io| io.into_raw_socket())
                .map(|raw_socket| unsafe { std::net::UdpSocket::from_raw_socket(raw_socket) })
        }
    }

    fn as_socket(&self) -> socket2::SockRef<'_> {
        socket2::SockRef::from(self)
    }

    /// Returns the local address that this socket is bound to.
    ///
    /// # Example
    ///
    /// ```no_run
    /// use tokio::net::UdpSocket;
    /// # use std::{io, net::SocketAddr};
    ///
    /// # #[tokio::main]
    /// # async fn main() -> io::Result<()> {
    /// let addr = "0.0.0.0:8080".parse::<SocketAddr>().unwrap();
    /// let sock = UdpSocket::bind(addr).await?;
    /// // the address the socket is bound to
    /// let local_addr = sock.local_addr()?;
    /// # Ok(())
    /// # }
    /// ```
    pub fn local_addr(&self) -> io::Result<SocketAddr> {
        self.io.local_addr()
    }

    /// Returns the socket address of the remote peer this socket was connected to.
    ///
    /// # Example
    ///
    /// ```
    /// use tokio::net::UdpSocket;
    ///
    /// # use std::{io, net::SocketAddr};
    /// # #[tokio::main]
    /// # async fn main() -> io::Result<()> {
    /// let addr = "0.0.0.0:8080".parse::<SocketAddr>().unwrap();
    /// let peer = "127.0.0.1:11100".parse::<SocketAddr>().unwrap();
    /// let sock = UdpSocket::bind(addr).await?;
    /// sock.connect(peer).await?;
    /// assert_eq!(peer, sock.peer_addr()?);
    /// #    Ok(())
    /// # }
    /// ```
    pub fn peer_addr(&self) -> io::Result<SocketAddr> {
        self.io.peer_addr()
    }

    /// Connects the UDP socket setting the default destination for send() and
    /// limiting packets that are read via recv from the address specified in
    /// `addr`.
    ///
    /// # Example
    ///
    /// ```no_run
    /// use tokio::net::UdpSocket;
    /// # use std::{io, net::SocketAddr};
    ///
    /// # #[tokio::main]
    /// # async fn main() -> io::Result<()> {
    /// let sock = UdpSocket::bind("0.0.0.0:8080".parse::<SocketAddr>().unwrap()).await?;
    ///
    /// let remote_addr = "127.0.0.1:59600".parse::<SocketAddr>().unwrap();
    /// sock.connect(remote_addr).await?;
    /// let mut buf = [0u8; 32];
    /// // recv from remote_addr
    /// let len = sock.recv(&mut buf).await?;
    /// // send to remote_addr
    /// let _len = sock.send(&buf[..len]).await?;
    /// # Ok(())
    /// # }
    /// ```
    pub async fn connect<A: ToSocketAddrs>(&self, addr: A) -> io::Result<()> {
        let addrs = to_socket_addrs(addr).await?;
        let mut last_err = None;

        for addr in addrs {
            match self.io.connect(addr) {
                Ok(_) => return Ok(()),
                Err(e) => last_err = Some(e),
            }
        }

        Err(last_err.unwrap_or_else(|| {
            io::Error::new(
                io::ErrorKind::InvalidInput,
                "could not resolve to any address",
            )
        }))
    }

    /// Waits for any of the requested ready states.
    ///
    /// This function is usually paired with `try_recv()` or `try_send()`. It
    /// can be used to concurrently recv / send to the same socket on a single
    /// task without splitting the socket.
    ///
    /// The function may complete without the socket being ready. This is a
    /// false-positive and attempting an operation will return with
    /// `io::ErrorKind::WouldBlock`. The function can also return with an empty
    /// [`Ready`] set, so you should always check the returned value and possibly
    /// wait again if the requested states are not set.
    ///
    /// # Cancel safety
    ///
    /// This method is cancel safe. Once a readiness event occurs, the method
    /// will continue to return immediately until the readiness event is
    /// consumed by an attempt to read or write that fails with `WouldBlock` or
    /// `Poll::Pending`.
    ///
    /// # Examples
    ///
    /// Concurrently receive from and send to the socket on the same task
    /// without splitting.
    ///
    /// ```no_run
    /// use tokio::io::{self, Interest};
    /// use tokio::net::UdpSocket;
    ///
    /// #[tokio::main]
    /// async fn main() -> io::Result<()> {
    ///     let socket = UdpSocket::bind("127.0.0.1:8080").await?;
    ///     socket.connect("127.0.0.1:8081").await?;
    ///
    ///     loop {
    ///         let ready = socket.ready(Interest::READABLE | Interest::WRITABLE).await?;
    ///
    ///         if ready.is_readable() {
    ///             // The buffer is **not** included in the async task and will only exist
    ///             // on the stack.
    ///             let mut data = [0; 1024];
    ///             match socket.try_recv(&mut data[..]) {
    ///                 Ok(n) => {
    ///                     println!("received {:?}", &data[..n]);
    ///                 }
    ///                 // False-positive, continue
    ///                 Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {}
    ///                 Err(e) => {
    ///                     return Err(e);
    ///                 }
    ///             }
    ///         }
    ///
    ///         if ready.is_writable() {
    ///             // Write some data
    ///             match socket.try_send(b"hello world") {
    ///                 Ok(n) => {
    ///                     println!("sent {} bytes", n);
    ///                 }
    ///                 // False-positive, continue
    ///                 Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {}
    ///                 Err(e) => {
    ///                     return Err(e);
    ///                 }
    ///             }
    ///         }
    ///     }
    /// }
    /// ```
    pub async fn ready(&self, interest: Interest) -> io::Result<Ready> {
        let event = self.io.registration().readiness(interest).await?;
        Ok(event.ready)
    }

    /// Waits for the socket to become writable.
    ///
    /// This function is equivalent to `ready(Interest::WRITABLE)` and is
    /// usually paired with `try_send()` or `try_send_to()`.
    ///
    /// The function may complete without the socket being writable. This is a
    /// false-positive and attempting a `try_send()` will return with
    /// `io::ErrorKind::WouldBlock`.
    ///
    /// # Cancel safety
    ///
    /// This method is cancel safe. Once a readiness event occurs, the method
    /// will continue to return immediately until the readiness event is
    /// consumed by an attempt to write that fails with `WouldBlock` or
    /// `Poll::Pending`.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use tokio::net::UdpSocket;
    /// use std::io;
    ///
    /// #[tokio::main]
    /// async fn main() -> io::Result<()> {
    ///     // Bind socket
    ///     let socket = UdpSocket::bind("127.0.0.1:8080").await?;
    ///     socket.connect("127.0.0.1:8081").await?;
    ///
    ///     loop {
    ///         // Wait for the socket to be writable
    ///         socket.writable().await?;
    ///
    ///         // Try to send data, this may still fail with `WouldBlock`
    ///         // if the readiness event is a false positive.
    ///         match socket.try_send(b"hello world") {
    ///             Ok(n) => {
    ///                 break;
    ///             }
    ///             Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
    ///                 continue;
    ///             }
    ///             Err(e) => {
    ///                 return Err(e);
    ///             }
    ///         }
    ///     }
    ///
    ///     Ok(())
    /// }
    /// ```
    pub async fn writable(&self) -> io::Result<()> {
        self.ready(Interest::WRITABLE).await?;
        Ok(())
    }

    /// Polls for write/send readiness.
    ///
    /// If the udp stream is not currently ready for sending, this method will
    /// store a clone of the `Waker` from the provided `Context`. When the udp
    /// stream becomes ready for sending, `Waker::wake` will be called on the
    /// waker.
    ///
    /// Note that on multiple calls to `poll_send_ready` or `poll_send`, only
    /// the `Waker` from the `Context` passed to the most recent call is
    /// scheduled to receive a wakeup. (However, `poll_recv_ready` retains a
    /// second, independent waker.)
    ///
    /// This function is intended for cases where creating and pinning a future
    /// via [`writable`] is not feasible. Where possible, using [`writable`] is
    /// preferred, as this supports polling from multiple tasks at once.
    ///
    /// # Return value
    ///
    /// The function returns:
    ///
    /// * `Poll::Pending` if the udp stream is not ready for writing.
    /// * `Poll::Ready(Ok(()))` if the udp stream is ready for writing.
    /// * `Poll::Ready(Err(e))` if an error is encountered.
    ///
    /// # Errors
    ///
    /// This function may encounter any standard I/O error except `WouldBlock`.
    ///
    /// [`writable`]: method@Self::writable
    pub fn poll_send_ready(&self, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
        self.io.registration().poll_write_ready(cx).map_ok(|_| ())
    }

    /// Sends data on the socket to the remote address that the socket is
    /// connected to.
    ///
    /// The [`connect`] method will connect this socket to a remote address.
    /// This method will fail if the socket is not connected.
    ///
    /// [`connect`]: method@Self::connect
    ///
    /// # Return
    ///
    /// On success, the number of bytes sent is returned, otherwise, the
    /// encountered error is returned.
    ///
    /// # Cancel safety
    ///
    /// This method is cancel safe. If `send` is used as the event in a
    /// [`tokio::select!`](crate::select) statement and some other branch
    /// completes first, then it is guaranteed that the message was not sent.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use tokio::io;
    /// use tokio::net::UdpSocket;
    ///
    /// #[tokio::main]
    /// async fn main() -> io::Result<()> {
    ///     // Bind socket
    ///     let socket = UdpSocket::bind("127.0.0.1:8080").await?;
    ///     socket.connect("127.0.0.1:8081").await?;
    ///
    ///     // Send a message
    ///     socket.send(b"hello world").await?;
    ///
    ///     Ok(())
    /// }
    /// ```
    pub async fn send(&self, buf: &[u8]) -> io::Result<usize> {
        self.io
            .registration()
            .async_io(Interest::WRITABLE, || self.io.send(buf))
            .await
    }

    /// Attempts to send data on the socket to the remote address to which it
    /// was previously `connect`ed.
    ///
    /// The [`connect`] method will connect this socket to a remote address.
    /// This method will fail if the socket is not connected.
    ///
    /// Note that on multiple calls to a `poll_*` method in the send direction,
    /// only the `Waker` from the `Context` passed to the most recent call will
    /// be scheduled to receive a wakeup.
    ///
    /// # Return value
    ///
    /// The function returns:
    ///
    /// * `Poll::Pending` if the socket is not available to write
    /// * `Poll::Ready(Ok(n))` `n` is the number of bytes sent
    /// * `Poll::Ready(Err(e))` if an error is encountered.
    ///
    /// # Errors
    ///
    /// This function may encounter any standard I/O error except `WouldBlock`.
    ///
    /// [`connect`]: method@Self::connect
    pub fn poll_send(&self, cx: &mut Context<'_>, buf: &[u8]) -> Poll<io::Result<usize>> {
        self.io
            .registration()
            .poll_write_io(cx, || self.io.send(buf))
    }

    /// Tries to send data on the socket to the remote address to which it is
    /// connected.
    ///
    /// When the socket buffer is full, `Err(io::ErrorKind::WouldBlock)` is
    /// returned. This function is usually paired with `writable()`.
    ///
    /// # Returns
    ///
    /// If successful, `Ok(n)` is returned, where `n` is the number of bytes
    /// sent. If the socket is not ready to send data,
    /// `Err(ErrorKind::WouldBlock)` is returned.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use tokio::net::UdpSocket;
    /// use std::io;
    ///
    /// #[tokio::main]
    /// async fn main() -> io::Result<()> {
    ///     // Bind a UDP socket
    ///     let socket = UdpSocket::bind("127.0.0.1:8080").await?;
    ///
    ///     // Connect to a peer
    ///     socket.connect("127.0.0.1:8081").await?;
    ///
    ///     loop {
    ///         // Wait for the socket to be writable
    ///         socket.writable().await?;
    ///
    ///         // Try to send data, this may still fail with `WouldBlock`
    ///         // if the readiness event is a false positive.
    ///         match socket.try_send(b"hello world") {
    ///             Ok(n) => {
    ///                 break;
    ///             }
    ///             Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
    ///                 continue;
    ///             }
    ///             Err(e) => {
    ///                 return Err(e);
    ///             }
    ///         }
    ///     }
    ///
    ///     Ok(())
    /// }
    /// ```
    pub fn try_send(&self, buf: &[u8]) -> io::Result<usize> {
        self.io
            .registration()
            .try_io(Interest::WRITABLE, || self.io.send(buf))
    }

    /// Waits for the socket to become readable.
    ///
    /// This function is equivalent to `ready(Interest::READABLE)` and is usually
    /// paired with `try_recv()`.
    ///
    /// The function may complete without the socket being readable. This is a
    /// false-positive and attempting a `try_recv()` will return with
    /// `io::ErrorKind::WouldBlock`.
    ///
    /// # Cancel safety
    ///
    /// This method is cancel safe. Once a readiness event occurs, the method
    /// will continue to return immediately until the readiness event is
    /// consumed by an attempt to read that fails with `WouldBlock` or
    /// `Poll::Pending`.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use tokio::net::UdpSocket;
    /// use std::io;
    ///
    /// #[tokio::main]
    /// async fn main() -> io::Result<()> {
    ///     // Connect to a peer
    ///     let socket = UdpSocket::bind("127.0.0.1:8080").await?;
    ///     socket.connect("127.0.0.1:8081").await?;
    ///
    ///     loop {
    ///         // Wait for the socket to be readable
    ///         socket.readable().await?;
    ///
    ///         // The buffer is **not** included in the async task and will
    ///         // only exist on the stack.
    ///         let mut buf = [0; 1024];
    ///
    ///         // Try to recv data, this may still fail with `WouldBlock`
    ///         // if the readiness event is a false positive.
    ///         match socket.try_recv(&mut buf) {
    ///             Ok(n) => {
    ///                 println!("GOT {:?}", &buf[..n]);
    ///                 break;
    ///             }
    ///             Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
    ///                 continue;
    ///             }
    ///             Err(e) => {
    ///                 return Err(e);
    ///             }
    ///         }
    ///     }
    ///
    ///     Ok(())
    /// }
    /// ```
    pub async fn readable(&self) -> io::Result<()> {
        self.ready(Interest::READABLE).await?;
        Ok(())
    }

    /// Polls for read/receive readiness.
    ///
    /// If the udp stream is not currently ready for receiving, this method will
    /// store a clone of the `Waker` from the provided `Context`. When the udp
    /// socket becomes ready for reading, `Waker::wake` will be called on the
    /// waker.
    ///
    /// Note that on multiple calls to `poll_recv_ready`, `poll_recv` or
    /// `poll_peek`, only the `Waker` from the `Context` passed to the most
    /// recent call is scheduled to receive a wakeup. (However,
    /// `poll_send_ready` retains a second, independent waker.)
    ///
    /// This function is intended for cases where creating and pinning a future
    /// via [`readable`] is not feasible. Where possible, using [`readable`] is
    /// preferred, as this supports polling from multiple tasks at once.
    ///
    /// # Return value
    ///
    /// The function returns:
    ///
    /// * `Poll::Pending` if the udp stream is not ready for reading.
    /// * `Poll::Ready(Ok(()))` if the udp stream is ready for reading.
    /// * `Poll::Ready(Err(e))` if an error is encountered.
    ///
    /// # Errors
    ///
    /// This function may encounter any standard I/O error except `WouldBlock`.
    ///
    /// [`readable`]: method@Self::readable
    pub fn poll_recv_ready(&self, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
        self.io.registration().poll_read_ready(cx).map_ok(|_| ())
    }

    /// Receives a single datagram message on the socket from the remote address
    /// to which it is connected. On success, returns the number of bytes read.
    ///
    /// The function must be called with valid byte array `buf` of sufficient
    /// size to hold the message bytes. If a message is too long to fit in the
    /// supplied buffer, excess bytes may be discarded.
    ///
    /// The [`connect`] method will connect this socket to a remote address.
    /// This method will fail if the socket is not connected.
    ///
    /// # Cancel safety
    ///
    /// This method is cancel safe. If `recv` is used as the event in a
    /// [`tokio::select!`](crate::select) statement and some other branch
    /// completes first, it is guaranteed that no messages were received on this
    /// socket.
    ///
    /// [`connect`]: method@Self::connect
    ///
    /// ```no_run
    /// use tokio::net::UdpSocket;
    /// use std::io;
    ///
    /// #[tokio::main]
    /// async fn main() -> io::Result<()> {
    ///     // Bind socket
    ///     let socket = UdpSocket::bind("127.0.0.1:8080").await?;
    ///     socket.connect("127.0.0.1:8081").await?;
    ///
    ///     let mut buf = vec![0; 10];
    ///     let n = socket.recv(&mut buf).await?;
    ///
    ///     println!("received {} bytes {:?}", n, &buf[..n]);
    ///
    ///     Ok(())
    /// }
    /// ```
    pub async fn recv(&self, buf: &mut [u8]) -> io::Result<usize> {
        self.io
            .registration()
            .async_io(Interest::READABLE, || self.io.recv(buf))
            .await
    }

    /// Attempts to receive a single datagram message on the socket from the remote
    /// address to which it is `connect`ed.
    ///
    /// The [`connect`] method will connect this socket to a remote address. This method
    /// resolves to an error if the socket is not connected.
    ///
    /// Note that on multiple calls to a `poll_*` method in the recv direction, only the
    /// `Waker` from the `Context` passed to the most recent call will be scheduled to
    /// receive a wakeup.
    ///
    /// # Return value
    ///
    /// The function returns:
    ///
    /// * `Poll::Pending` if the socket is not ready to read
    /// * `Poll::Ready(Ok(()))` reads data `ReadBuf` if the socket is ready
    /// * `Poll::Ready(Err(e))` if an error is encountered.
    ///
    /// # Errors
    ///
    /// This function may encounter any standard I/O error except `WouldBlock`.
    ///
    /// [`connect`]: method@Self::connect
    pub fn poll_recv(&self, cx: &mut Context<'_>, buf: &mut ReadBuf<'_>) -> Poll<io::Result<()>> {
        let n = ready!(self.io.registration().poll_read_io(cx, || {
            // Safety: will not read the maybe uninitialized bytes.
            let b = unsafe {
                &mut *(buf.unfilled_mut() as *mut [std::mem::MaybeUninit<u8>] as *mut [u8])
            };

            self.io.recv(b)
        }))?;

        // Safety: We trust `recv` to have filled up `n` bytes in the buffer.
        unsafe {
            buf.assume_init(n);
        }
        buf.advance(n);
        Poll::Ready(Ok(()))
    }

    /// Tries to receive a single datagram message on the socket from the remote
    /// address to which it is connected. On success, returns the number of
    /// bytes read.
    ///
    /// The function must be called with valid byte array buf of sufficient size
    /// to hold the message bytes. If a message is too long to fit in the
    /// supplied buffer, excess bytes may be discarded.
    ///
    /// When there is no pending data, `Err(io::ErrorKind::WouldBlock)` is
    /// returned. This function is usually paired with `readable()`.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use tokio::net::UdpSocket;
    /// use std::io;
    ///
    /// #[tokio::main]
    /// async fn main() -> io::Result<()> {
    ///     // Connect to a peer
    ///     let socket = UdpSocket::bind("127.0.0.1:8080").await?;
    ///     socket.connect("127.0.0.1:8081").await?;
    ///
    ///     loop {
    ///         // Wait for the socket to be readable
    ///         socket.readable().await?;
    ///
    ///         // The buffer is **not** included in the async task and will
    ///         // only exist on the stack.
    ///         let mut buf = [0; 1024];
    ///
    ///         // Try to recv data, this may still fail with `WouldBlock`
    ///         // if the readiness event is a false positive.
    ///         match socket.try_recv(&mut buf) {
    ///             Ok(n) => {
    ///                 println!("GOT {:?}", &buf[..n]);
    ///                 break;
    ///             }
    ///             Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
    ///                 continue;
    ///             }
    ///             Err(e) => {
    ///                 return Err(e);
    ///             }
    ///         }
    ///     }
    ///
    ///     Ok(())
    /// }
    /// ```
    pub fn try_recv(&self, buf: &mut [u8]) -> io::Result<usize> {
        self.io
            .registration()
            .try_io(Interest::READABLE, || self.io.recv(buf))
    }

    cfg_io_util! {
        /// Tries to receive data from the stream into the provided buffer, advancing the
        /// buffer's internal cursor, returning how many bytes were read.
        ///
        /// The function must be called with valid byte array buf of sufficient size
        /// to hold the message bytes. If a message is too long to fit in the
        /// supplied buffer, excess bytes may be discarded.
        ///
        /// When there is no pending data, `Err(io::ErrorKind::WouldBlock)` is
        /// returned. This function is usually paired with `readable()`.
        ///
        /// # Examples
        ///
        /// ```no_run
        /// use tokio::net::UdpSocket;
        /// use std::io;
        ///
        /// #[tokio::main]
        /// async fn main() -> io::Result<()> {
        ///     // Connect to a peer
        ///     let socket = UdpSocket::bind("127.0.0.1:8080").await?;
        ///     socket.connect("127.0.0.1:8081").await?;
        ///
        ///     loop {
        ///         // Wait for the socket to be readable
        ///         socket.readable().await?;
        ///
        ///         let mut buf = Vec::with_capacity(1024);
        ///
        ///         // Try to recv data, this may still fail with `WouldBlock`
        ///         // if the readiness event is a false positive.
        ///         match socket.try_recv_buf(&mut buf) {
        ///             Ok(n) => {
        ///                 println!("GOT {:?}", &buf[..n]);
        ///                 break;
        ///             }
        ///             Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
        ///                 continue;
        ///             }
        ///             Err(e) => {
        ///                 return Err(e);
        ///             }
        ///         }
        ///     }
        ///
        ///     Ok(())
        /// }
        /// ```
        pub fn try_recv_buf<B: BufMut>(&self, buf: &mut B) -> io::Result<usize> {
            self.io.registration().try_io(Interest::READABLE, || {
                let dst = buf.chunk_mut();
                let dst =
                    unsafe { &mut *(dst as *mut _ as *mut [std::mem::MaybeUninit<u8>] as *mut [u8]) };

                // Safety: We trust `UdpSocket::recv` to have filled up `n` bytes in the
                // buffer.
                let n = (*self.io).recv(dst)?;

                unsafe {
                    buf.advance_mut(n);
                }

                Ok(n)
            })
        }

        /// Tries to receive a single datagram message on the socket. On success,
        /// returns the number of bytes read and the origin.
        ///
        /// The function must be called with valid byte array buf of sufficient size
        /// to hold the message bytes. If a message is too long to fit in the
        /// supplied buffer, excess bytes may be discarded.
        ///
        /// When there is no pending data, `Err(io::ErrorKind::WouldBlock)` is
        /// returned. This function is usually paired with `readable()`.
        ///
        /// # Examples
        ///
        /// ```no_run
        /// use tokio::net::UdpSocket;
        /// use std::io;
        ///
        /// #[tokio::main]
        /// async fn main() -> io::Result<()> {
        ///     // Connect to a peer
        ///     let socket = UdpSocket::bind("127.0.0.1:8080").await?;
        ///
        ///     loop {
        ///         // Wait for the socket to be readable
        ///         socket.readable().await?;
        ///
        ///         let mut buf = Vec::with_capacity(1024);
        ///
        ///         // Try to recv data, this may still fail with `WouldBlock`
        ///         // if the readiness event is a false positive.
        ///         match socket.try_recv_buf_from(&mut buf) {
        ///             Ok((n, _addr)) => {
        ///                 println!("GOT {:?}", &buf[..n]);
        ///                 break;
        ///             }
        ///             Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
        ///                 continue;
        ///             }
        ///             Err(e) => {
        ///                 return Err(e);
        ///             }
        ///         }
        ///     }
        ///
        ///     Ok(())
        /// }
        /// ```
        pub fn try_recv_buf_from<B: BufMut>(&self, buf: &mut B) -> io::Result<(usize, SocketAddr)> {
            self.io.registration().try_io(Interest::READABLE, || {
                let dst = buf.chunk_mut();
                let dst =
                    unsafe { &mut *(dst as *mut _ as *mut [std::mem::MaybeUninit<u8>] as *mut [u8]) };

                // Safety: We trust `UdpSocket::recv_from` to have filled up `n` bytes in the
                // buffer.
                let (n, addr) = (*self.io).recv_from(dst)?;

                unsafe {
                    buf.advance_mut(n);
                }

                Ok((n, addr))
            })
        }
    }

    /// Sends data on the socket to the given address. On success, returns the
    /// number of bytes written.
    ///
    /// Address type can be any implementor of [`ToSocketAddrs`] trait. See its
    /// documentation for concrete examples.
    ///
    /// It is possible for `addr` to yield multiple addresses, but `send_to`
    /// will only send data to the first address yielded by `addr`.
    ///
    /// This will return an error when the IP version of the local socket does
    /// not match that returned from [`ToSocketAddrs`].
    ///
    /// [`ToSocketAddrs`]: crate::net::ToSocketAddrs
    ///
    /// # Cancel safety
    ///
    /// This method is cancel safe. If `send_to` is used as the event in a
    /// [`tokio::select!`](crate::select) statement and some other branch
    /// completes first, then it is guaranteed that the message was not sent.
    ///
    /// # Example
    ///
    /// ```no_run
    /// use tokio::net::UdpSocket;
    /// use std::io;
    ///
    /// #[tokio::main]
    /// async fn main() -> io::Result<()> {
    ///     let socket = UdpSocket::bind("127.0.0.1:8080").await?;
    ///     let len = socket.send_to(b"hello world", "127.0.0.1:8081").await?;
    ///
    ///     println!("Sent {} bytes", len);
    ///
    ///     Ok(())
    /// }
    /// ```
    pub async fn send_to<A: ToSocketAddrs>(&self, buf: &[u8], target: A) -> io::Result<usize> {
        let mut addrs = to_socket_addrs(target).await?;

        match addrs.next() {
            Some(target) => self.send_to_addr(buf, target).await,
            None => Err(io::Error::new(
                io::ErrorKind::InvalidInput,
                "no addresses to send data to",
            )),
        }
    }

    /// Attempts to send data on the socket to a given address.
    ///
    /// Note that on multiple calls to a `poll_*` method in the send direction, only the
    /// `Waker` from the `Context` passed to the most recent call will be scheduled to
    /// receive a wakeup.
    ///
    /// # Return value
    ///
    /// The function returns:
    ///
    /// * `Poll::Pending` if the socket is not ready to write
    /// * `Poll::Ready(Ok(n))` `n` is the number of bytes sent.
    /// * `Poll::Ready(Err(e))` if an error is encountered.
    ///
    /// # Errors
    ///
    /// This function may encounter any standard I/O error except `WouldBlock`.
    pub fn poll_send_to(
        &self,
        cx: &mut Context<'_>,
        buf: &[u8],
        target: SocketAddr,
    ) -> Poll<io::Result<usize>> {
        self.io
            .registration()
            .poll_write_io(cx, || self.io.send_to(buf, target))
    }

    /// Tries to send data on the socket to the given address, but if the send is
    /// blocked this will return right away.
    ///
    /// This function is usually paired with `writable()`.
    ///
    /// # Returns
    ///
    /// If successful, returns the number of bytes sent
    ///
    /// Users should ensure that when the remote cannot receive, the
    /// [`ErrorKind::WouldBlock`] is properly handled. An error can also occur
    /// if the IP version of the socket does not match that of `target`.
    ///
    /// [`ErrorKind::WouldBlock`]: std::io::ErrorKind::WouldBlock
    ///
    /// # Example
    ///
    /// ```no_run
    /// use tokio::net::UdpSocket;
    /// use std::error::Error;
    /// use std::io;
    ///
    /// #[tokio::main]
    /// async fn main() -> Result<(), Box<dyn Error>> {
    ///     let socket = UdpSocket::bind("127.0.0.1:8080").await?;
    ///
    ///     let dst = "127.0.0.1:8081".parse()?;
    ///
    ///     loop {
    ///         socket.writable().await?;
    ///
    ///         match socket.try_send_to(&b"hello world"[..], dst) {
    ///             Ok(sent) => {
    ///                 println!("sent {} bytes", sent);
    ///                 break;
    ///             }
    ///             Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
    ///                 // Writable false positive.
    ///                 continue;
    ///             }
    ///             Err(e) => return Err(e.into()),
    ///         }
    ///     }
    ///
    ///     Ok(())
    /// }
    /// ```
    pub fn try_send_to(&self, buf: &[u8], target: SocketAddr) -> io::Result<usize> {
        self.io
            .registration()
            .try_io(Interest::WRITABLE, || self.io.send_to(buf, target))
    }

    async fn send_to_addr(&self, buf: &[u8], target: SocketAddr) -> io::Result<usize> {
        self.io
            .registration()
            .async_io(Interest::WRITABLE, || self.io.send_to(buf, target))
            .await
    }

    /// Receives a single datagram message on the socket. On success, returns
    /// the number of bytes read and the origin.
    ///
    /// The function must be called with valid byte array `buf` of sufficient
    /// size to hold the message bytes. If a message is too long to fit in the
    /// supplied buffer, excess bytes may be discarded.
    ///
    /// # Cancel safety
    ///
    /// This method is cancel safe. If `recv_from` is used as the event in a
    /// [`tokio::select!`](crate::select) statement and some other branch
    /// completes first, it is guaranteed that no messages were received on this
    /// socket.
    ///
    /// # Example
    ///
    /// ```no_run
    /// use tokio::net::UdpSocket;
    /// use std::io;
    ///
    /// #[tokio::main]
    /// async fn main() -> io::Result<()> {
    ///     let socket = UdpSocket::bind("127.0.0.1:8080").await?;
    ///
    ///     let mut buf = vec![0u8; 32];
    ///     let (len, addr) = socket.recv_from(&mut buf).await?;
    ///
    ///     println!("received {:?} bytes from {:?}", len, addr);
    ///
    ///     Ok(())
    /// }
    /// ```
    pub async fn recv_from(&self, buf: &mut [u8]) -> io::Result<(usize, SocketAddr)> {
        self.io
            .registration()
            .async_io(Interest::READABLE, || self.io.recv_from(buf))
            .await
    }

    /// Attempts to receive a single datagram on the socket.
    ///
    /// Note that on multiple calls to a `poll_*` method in the recv direction, only the
    /// `Waker` from the `Context` passed to the most recent call will be scheduled to
    /// receive a wakeup.
    ///
    /// # Return value
    ///
    /// The function returns:
    ///
    /// * `Poll::Pending` if the socket is not ready to read
    /// * `Poll::Ready(Ok(addr))` reads data from `addr` into `ReadBuf` if the socket is ready
    /// * `Poll::Ready(Err(e))` if an error is encountered.
    ///
    /// # Errors
    ///
    /// This function may encounter any standard I/O error except `WouldBlock`.
    pub fn poll_recv_from(
        &self,
        cx: &mut Context<'_>,
        buf: &mut ReadBuf<'_>,
    ) -> Poll<io::Result<SocketAddr>> {
        let (n, addr) = ready!(self.io.registration().poll_read_io(cx, || {
            // Safety: will not read the maybe uninitialized bytes.
            let b = unsafe {
                &mut *(buf.unfilled_mut() as *mut [std::mem::MaybeUninit<u8>] as *mut [u8])
            };

            self.io.recv_from(b)
        }))?;

        // Safety: We trust `recv` to have filled up `n` bytes in the buffer.
        unsafe {
            buf.assume_init(n);
        }
        buf.advance(n);
        Poll::Ready(Ok(addr))
    }

    /// Tries to receive a single datagram message on the socket. On success,
    /// returns the number of bytes read and the origin.
    ///
    /// The function must be called with valid byte array buf of sufficient size
    /// to hold the message bytes. If a message is too long to fit in the
    /// supplied buffer, excess bytes may be discarded.
    ///
    /// When there is no pending data, `Err(io::ErrorKind::WouldBlock)` is
    /// returned. This function is usually paired with `readable()`.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use tokio::net::UdpSocket;
    /// use std::io;
    ///
    /// #[tokio::main]
    /// async fn main() -> io::Result<()> {
    ///     // Connect to a peer
    ///     let socket = UdpSocket::bind("127.0.0.1:8080").await?;
    ///
    ///     loop {
    ///         // Wait for the socket to be readable
    ///         socket.readable().await?;
    ///
    ///         // The buffer is **not** included in the async task and will
    ///         // only exist on the stack.
    ///         let mut buf = [0; 1024];
    ///
    ///         // Try to recv data, this may still fail with `WouldBlock`
    ///         // if the readiness event is a false positive.
    ///         match socket.try_recv_from(&mut buf) {
    ///             Ok((n, _addr)) => {
    ///                 println!("GOT {:?}", &buf[..n]);
    ///                 break;
    ///             }
    ///             Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
    ///                 continue;
    ///             }
    ///             Err(e) => {
    ///                 return Err(e);
    ///             }
    ///         }
    ///     }
    ///
    ///     Ok(())
    /// }
    /// ```
    pub fn try_recv_from(&self, buf: &mut [u8]) -> io::Result<(usize, SocketAddr)> {
        self.io
            .registration()
            .try_io(Interest::READABLE, || self.io.recv_from(buf))
    }

    /// Tries to read or write from the socket using a user-provided IO operation.
    ///
    /// If the socket is ready, the provided closure is called. The closure
    /// should attempt to perform IO operation on the socket by manually
    /// calling the appropriate syscall. If the operation fails because the
    /// socket is not actually ready, then the closure should return a
    /// `WouldBlock` error and the readiness flag is cleared. The return value
    /// of the closure is then returned by `try_io`.
    ///
    /// If the socket is not ready, then the closure is not called
    /// and a `WouldBlock` error is returned.
    ///
    /// The closure should only return a `WouldBlock` error if it has performed
    /// an IO operation on the socket that failed due to the socket not being
    /// ready. Returning a `WouldBlock` error in any other situation will
    /// incorrectly clear the readiness flag, which can cause the socket to
    /// behave incorrectly.
    ///
    /// The closure should not perform the IO operation using any of the methods
    /// defined on the Tokio `UdpSocket` type, as this will mess with the
    /// readiness flag and can cause the socket to behave incorrectly.
    ///
    /// This method is not intended to be used with combined interests.
    /// The closure should perform only one type of IO operation, so it should not
    /// require more than one ready state. This method may panic or sleep forever
    /// if it is called with a combined interest.
    ///
    /// Usually, [`readable()`], [`writable()`] or [`ready()`] is used with this function.
    ///
    /// [`readable()`]: UdpSocket::readable()
    /// [`writable()`]: UdpSocket::writable()
    /// [`ready()`]: UdpSocket::ready()
    pub fn try_io<R>(
        &self,
        interest: Interest,
        f: impl FnOnce() -> io::Result<R>,
    ) -> io::Result<R> {
        self.io
            .registration()
            .try_io(interest, || self.io.try_io(f))
    }

    /// Receives data from the socket, without removing it from the input queue.
    /// On success, returns the number of bytes read and the address from whence
    /// the data came.
    ///
    /// # Notes
    ///
    /// On Windows, if the data is larger than the buffer specified, the buffer
    /// is filled with the first part of the data, and peek_from returns the error
    /// WSAEMSGSIZE(10040). The excess data is lost.
    /// Make sure to always use a sufficiently large buffer to hold the
    /// maximum UDP packet size, which can be up to 65536 bytes in size.
    ///
    /// MacOS will return an error if you pass a zero-sized buffer.
    ///
    /// If you're merely interested in learning the sender of the data at the head of the queue,
    /// try [`peek_sender`].
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use tokio::net::UdpSocket;
    /// use std::io;
    ///
    /// #[tokio::main]
    /// async fn main() -> io::Result<()> {
    ///     let socket = UdpSocket::bind("127.0.0.1:8080").await?;
    ///
    ///     let mut buf = vec![0u8; 32];
    ///     let (len, addr) = socket.peek_from(&mut buf).await?;
    ///
    ///     println!("peeked {:?} bytes from {:?}", len, addr);
    ///
    ///     Ok(())
    /// }
    /// ```
    ///
    /// [`peek_sender`]: method@Self::peek_sender
    pub async fn peek_from(&self, buf: &mut [u8]) -> io::Result<(usize, SocketAddr)> {
        self.io
            .registration()
            .async_io(Interest::READABLE, || self.io.peek_from(buf))
            .await
    }

    /// Receives data from the socket, without removing it from the input queue.
    /// On success, returns the number of bytes read.
    ///
    /// # Notes
    ///
    /// Note that on multiple calls to a `poll_*` method in the recv direction, only the
    /// `Waker` from the `Context` passed to the most recent call will be scheduled to
    /// receive a wakeup
    ///
    /// On Windows, if the data is larger than the buffer specified, the buffer
    /// is filled with the first part of the data, and peek returns the error
    /// WSAEMSGSIZE(10040). The excess data is lost.
    /// Make sure to always use a sufficiently large buffer to hold the
    /// maximum UDP packet size, which can be up to 65536 bytes in size.
    ///
    /// MacOS will return an error if you pass a zero-sized buffer.
    ///
    /// If you're merely interested in learning the sender of the data at the head of the queue,
    /// try [`poll_peek_sender`].
    ///
    /// # Return value
    ///
    /// The function returns:
    ///
    /// * `Poll::Pending` if the socket is not ready to read
    /// * `Poll::Ready(Ok(addr))` reads data from `addr` into `ReadBuf` if the socket is ready
    /// * `Poll::Ready(Err(e))` if an error is encountered.
    ///
    /// # Errors
    ///
    /// This function may encounter any standard I/O error except `WouldBlock`.
    ///
    /// [`poll_peek_sender`]: method@Self::poll_peek_sender
    pub fn poll_peek_from(
        &self,
        cx: &mut Context<'_>,
        buf: &mut ReadBuf<'_>,
    ) -> Poll<io::Result<SocketAddr>> {
        let (n, addr) = ready!(self.io.registration().poll_read_io(cx, || {
            // Safety: will not read the maybe uninitialized bytes.
            let b = unsafe {
                &mut *(buf.unfilled_mut() as *mut [std::mem::MaybeUninit<u8>] as *mut [u8])
            };

            self.io.peek_from(b)
        }))?;

        // Safety: We trust `recv` to have filled up `n` bytes in the buffer.
        unsafe {
            buf.assume_init(n);
        }
        buf.advance(n);
        Poll::Ready(Ok(addr))
    }

    /// Retrieve the sender of the data at the head of the input queue, waiting if empty.
    ///
    /// This is equivalent to calling [`peek_from`] with a zero-sized buffer,
    /// but suppresses the `WSAEMSGSIZE` error on Windows and the "invalid argument" error on macOS.
    ///
    /// [`peek_from`]: method@Self::peek_from
    pub async fn peek_sender(&self) -> io::Result<SocketAddr> {
        self.io
            .registration()
            .async_io(Interest::READABLE, || self.try_peek_sender())
            .await
    }

    /// Retrieve the sender of the data at the head of the input queue,
    /// scheduling a wakeup if empty.
    ///
    /// This is equivalent to calling [`poll_peek_from`] with a zero-sized buffer,
    /// but suppresses the `WSAEMSGSIZE` error on Windows and the "invalid argument" error on macOS.
    ///
    /// # Notes
    ///
    /// Note that on multiple calls to a `poll_*` method in the recv direction, only the
    /// `Waker` from the `Context` passed to the most recent call will be scheduled to
    /// receive a wakeup.
    ///
    /// [`poll_peek_from`]: method@Self::poll_peek_from
    pub fn poll_peek_sender(&self, cx: &mut Context<'_>) -> Poll<io::Result<SocketAddr>> {
        self.io
            .registration()
            .poll_read_io(cx,  || self.try_peek_sender())
    }

    #[inline]
    fn try_peek_sender(&self) -> io::Result<SocketAddr> {
        self
            .as_socket()
            .peek_sender()?
            // May be `None` if the platform doesn't populate the sender for some reason.
            // In testing, that only occurred on macOS if you pass a zero-sized buffer,
            // but the implementation of `Socket::peek_sender()` covers that.
            .as_socket()
            .ok_or_else(|| io::Error::new(io::ErrorKind::Other, "sender not available"))
    }

    /// Gets the value of the `SO_BROADCAST` option for this socket.
    ///
    /// For more information about this option, see [`set_broadcast`].
    ///
    /// [`set_broadcast`]: method@Self::set_broadcast
    pub fn broadcast(&self) -> io::Result<bool> {
        self.io.broadcast()
    }

    /// Sets the value of the `SO_BROADCAST` option for this socket.
    ///
    /// When enabled, this socket is allowed to send packets to a broadcast
    /// address.
    pub fn set_broadcast(&self, on: bool) -> io::Result<()> {
        self.io.set_broadcast(on)
    }

    /// Gets the value of the `IP_MULTICAST_LOOP` option for this socket.
    ///
    /// For more information about this option, see [`set_multicast_loop_v4`].
    ///
    /// [`set_multicast_loop_v4`]: method@Self::set_multicast_loop_v4
    pub fn multicast_loop_v4(&self) -> io::Result<bool> {
        self.io.multicast_loop_v4()
    }

    /// Sets the value of the `IP_MULTICAST_LOOP` option for this socket.
    ///
    /// If enabled, multicast packets will be looped back to the local socket.
    ///
    /// # Note
    ///
    /// This may not have any affect on IPv6 sockets.
    pub fn set_multicast_loop_v4(&self, on: bool) -> io::Result<()> {
        self.io.set_multicast_loop_v4(on)
    }

    /// Gets the value of the `IP_MULTICAST_TTL` option for this socket.
    ///
    /// For more information about this option, see [`set_multicast_ttl_v4`].
    ///
    /// [`set_multicast_ttl_v4`]: method@Self::set_multicast_ttl_v4
    pub fn multicast_ttl_v4(&self) -> io::Result<u32> {
        self.io.multicast_ttl_v4()
    }

    /// Sets the value of the `IP_MULTICAST_TTL` option for this socket.
    ///
    /// Indicates the time-to-live value of outgoing multicast packets for
    /// this socket. The default value is 1 which means that multicast packets
    /// don't leave the local network unless explicitly requested.
    ///
    /// # Note
    ///
    /// This may not have any affect on IPv6 sockets.
    pub fn set_multicast_ttl_v4(&self, ttl: u32) -> io::Result<()> {
        self.io.set_multicast_ttl_v4(ttl)
    }

    /// Gets the value of the `IPV6_MULTICAST_LOOP` option for this socket.
    ///
    /// For more information about this option, see [`set_multicast_loop_v6`].
    ///
    /// [`set_multicast_loop_v6`]: method@Self::set_multicast_loop_v6
    pub fn multicast_loop_v6(&self) -> io::Result<bool> {
        self.io.multicast_loop_v6()
    }

    /// Sets the value of the `IPV6_MULTICAST_LOOP` option for this socket.
    ///
    /// Controls whether this socket sees the multicast packets it sends itself.
    ///
    /// # Note
    ///
    /// This may not have any affect on IPv4 sockets.
    pub fn set_multicast_loop_v6(&self, on: bool) -> io::Result<()> {
        self.io.set_multicast_loop_v6(on)
    }

    /// Gets the value of the `IP_TTL` option for this socket.
    ///
    /// For more information about this option, see [`set_ttl`].
    ///
    /// [`set_ttl`]: method@Self::set_ttl
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use tokio::net::UdpSocket;
    /// # use std::io;
    ///
    /// # async fn dox() -> io::Result<()> {
    /// let sock = UdpSocket::bind("127.0.0.1:8080").await?;
    ///
    /// println!("{:?}", sock.ttl()?);
    /// # Ok(())
    /// # }
    /// ```
    pub fn ttl(&self) -> io::Result<u32> {
        self.io.ttl()
    }

    /// Sets the value for the `IP_TTL` option on this socket.
    ///
    /// This value sets the time-to-live field that is used in every packet sent
    /// from this socket.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use tokio::net::UdpSocket;
    /// # use std::io;
    ///
    /// # async fn dox() -> io::Result<()> {
    /// let sock = UdpSocket::bind("127.0.0.1:8080").await?;
    /// sock.set_ttl(60)?;
    ///
    /// # Ok(())
    /// # }
    /// ```
    pub fn set_ttl(&self, ttl: u32) -> io::Result<()> {
        self.io.set_ttl(ttl)
    }

    /// Gets the value of the `IP_TOS` option for this socket.
    ///
    /// For more information about this option, see [`set_tos`].
    ///
    /// **NOTE:** On Windows, `IP_TOS` is only supported on [Windows 8+ or
    /// Windows Server 2012+.](https://docs.microsoft.com/en-us/windows/win32/winsock/ipproto-ip-socket-options)
    ///
    /// [`set_tos`]: Self::set_tos
    // https://docs.rs/socket2/0.4.2/src/socket2/socket.rs.html#1178
    #[cfg(not(any(
        target_os = "fuchsia",
        target_os = "redox",
        target_os = "solaris",
        target_os = "illumos",
    )))]
    #[cfg_attr(
        docsrs,
        doc(cfg(not(any(
            target_os = "fuchsia",
            target_os = "redox",
            target_os = "solaris",
            target_os = "illumos",
        ))))
    )]
    pub fn tos(&self) -> io::Result<u32> {
        self.as_socket().tos()
    }

    /// Sets the value for the `IP_TOS` option on this socket.
    ///
    /// This value sets the type-of-service field that is used in every packet
    /// sent from this socket.
    ///
    /// **NOTE:** On Windows, `IP_TOS` is only supported on [Windows 8+ or
    /// Windows Server 2012+.](https://docs.microsoft.com/en-us/windows/win32/winsock/ipproto-ip-socket-options)
    // https://docs.rs/socket2/0.4.2/src/socket2/socket.rs.html#1178
    #[cfg(not(any(
        target_os = "fuchsia",
        target_os = "redox",
        target_os = "solaris",
        target_os = "illumos",
    )))]
    #[cfg_attr(
        docsrs,
        doc(cfg(not(any(
            target_os = "fuchsia",
            target_os = "redox",
            target_os = "solaris",
            target_os = "illumos",
        ))))
    )]
    pub fn set_tos(&self, tos: u32) -> io::Result<()> {
        self.as_socket().set_tos(tos)
    }

    /// Gets the value for the `SO_BINDTODEVICE` option on this socket
    ///
    /// This value gets the socket-bound device's interface name.
    #[cfg(any(target_os = "android", target_os = "fuchsia", target_os = "linux",))]
    #[cfg_attr(
        docsrs,
        doc(cfg(any(target_os = "android", target_os = "fuchsia", target_os = "linux",)))
    )]
    pub fn device(&self) -> io::Result<Option<Vec<u8>>> {
        self.as_socket().device()
    }

    /// Sets the value for the `SO_BINDTODEVICE` option on this socket
    ///
    /// If a socket is bound to an interface, only packets received from that
    /// particular interface are processed by the socket. Note that this only
    /// works for some socket types, particularly `AF_INET` sockets.
    ///
    /// If `interface` is `None` or an empty string it removes the binding.
    #[cfg(all(any(target_os = "android", target_os = "fuchsia", target_os = "linux")))]
    #[cfg_attr(
        docsrs,
        doc(cfg(all(any(target_os = "android", target_os = "fuchsia", target_os = "linux"))))
    )]
    pub fn bind_device(&self, interface: Option<&[u8]>) -> io::Result<()> {
        self.as_socket().bind_device(interface)
    }

    /// Executes an operation of the `IP_ADD_MEMBERSHIP` type.
    ///
    /// This function specifies a new multicast group for this socket to join.
    /// The address must be a valid multicast address, and `interface` is the
    /// address of the local interface with which the system should join the
    /// multicast group. If it's equal to `INADDR_ANY` then an appropriate
    /// interface is chosen by the system.
    pub fn join_multicast_v4(&self, multiaddr: Ipv4Addr, interface: Ipv4Addr) -> io::Result<()> {
        self.io.join_multicast_v4(&multiaddr, &interface)
    }

    /// Executes an operation of the `IPV6_ADD_MEMBERSHIP` type.
    ///
    /// This function specifies a new multicast group for this socket to join.
    /// The address must be a valid multicast address, and `interface` is the
    /// index of the interface to join/leave (or 0 to indicate any interface).
    pub fn join_multicast_v6(&self, multiaddr: &Ipv6Addr, interface: u32) -> io::Result<()> {
        self.io.join_multicast_v6(multiaddr, interface)
    }

    /// Executes an operation of the `IP_DROP_MEMBERSHIP` type.
    ///
    /// For more information about this option, see [`join_multicast_v4`].
    ///
    /// [`join_multicast_v4`]: method@Self::join_multicast_v4
    pub fn leave_multicast_v4(&self, multiaddr: Ipv4Addr, interface: Ipv4Addr) -> io::Result<()> {
        self.io.leave_multicast_v4(&multiaddr, &interface)
    }

    /// Executes an operation of the `IPV6_DROP_MEMBERSHIP` type.
    ///
    /// For more information about this option, see [`join_multicast_v6`].
    ///
    /// [`join_multicast_v6`]: method@Self::join_multicast_v6
    pub fn leave_multicast_v6(&self, multiaddr: &Ipv6Addr, interface: u32) -> io::Result<()> {
        self.io.leave_multicast_v6(multiaddr, interface)
    }

    /// Returns the value of the `SO_ERROR` option.
    ///
    /// # Examples
    /// ```
    /// use tokio::net::UdpSocket;
    /// use std::io;
    ///
    /// #[tokio::main]
    /// async fn main() -> io::Result<()> {
    ///     // Create a socket
    ///     let socket = UdpSocket::bind("0.0.0.0:8080").await?;
    ///
    ///     if let Ok(Some(err)) = socket.take_error() {
    ///         println!("Got error: {:?}", err);
    ///     }
    ///
    ///     Ok(())
    /// }
    /// ```
    pub fn take_error(&self) -> io::Result<Option<io::Error>> {
        self.io.take_error()
    }
}

impl TryFrom<std::net::UdpSocket> for UdpSocket {
    type Error = io::Error;

    /// Consumes stream, returning the tokio I/O object.
    ///
    /// This is equivalent to
    /// [`UdpSocket::from_std(stream)`](UdpSocket::from_std).
    fn try_from(stream: std::net::UdpSocket) -> Result<Self, Self::Error> {
        Self::from_std(stream)
    }
}

impl fmt::Debug for UdpSocket {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        self.io.fmt(f)
    }
}

#[cfg(all(unix))]
mod sys {
    use super::UdpSocket;
    use std::os::unix::prelude::*;

    impl AsRawFd for UdpSocket {
        fn as_raw_fd(&self) -> RawFd {
            self.io.as_raw_fd()
        }
    }
}

#[cfg(windows)]
mod sys {
    use super::UdpSocket;
    use std::os::windows::prelude::*;

    impl AsRawSocket for UdpSocket {
        fn as_raw_socket(&self) -> RawSocket {
            self.io.as_raw_socket()
        }
    }
}