net/tcp/stream.rs

cfg_not_wasi! {
    use crate::net::{to_socket_addrs, ToSocketAddrs};
    use std::future::poll_fn;
    use std::time::Duration;
}

use crate::io::{AsyncRead, AsyncWrite, Interest, PollEvented, ReadBuf, Ready};
use crate::net::tcp::split::{split, ReadHalf, WriteHalf};
use crate::net::tcp::split_owned::{split_owned, OwnedReadHalf, OwnedWriteHalf};

use std::fmt;
use std::io;
use std::net::{Shutdown, SocketAddr};
use std::pin::Pin;
use std::task::{ready, Context, Poll};

cfg_io_util! {
    use bytes::BufMut;
}

cfg_net! {
    /// A TCP stream between a local and a remote socket.
    ///
    /// A TCP stream can either be created by connecting to an endpoint, via the
    /// [`connect`] method, or by [accepting] a connection from a [listener]. A
    /// TCP stream can also be created via the [`TcpSocket`] type.
    ///
    /// Reading and writing to a `TcpStream` is usually done using the
    /// convenience methods found on the [`AsyncReadExt`] and [`AsyncWriteExt`]
    /// traits.
    ///
    /// [`connect`]: method@TcpStream::connect
    /// [accepting]: method@crate::net::TcpListener::accept
    /// [listener]: struct@crate::net::TcpListener
    /// [`TcpSocket`]: struct@crate::net::TcpSocket
    /// [`AsyncReadExt`]: trait@crate::io::AsyncReadExt
    /// [`AsyncWriteExt`]: trait@crate::io::AsyncWriteExt
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use tokio::net::TcpStream;
    /// use tokio::io::AsyncWriteExt;
    /// use std::error::Error;
    ///
    /// #[tokio::main]
    /// async fn main() -> Result<(), Box<dyn Error>> {
    ///     // Connect to a peer
    ///     let mut stream = TcpStream::connect("127.0.0.1:8080").await?;
    ///
    ///     // Write some data.
    ///     stream.write_all(b"hello world!").await?;
    ///
    ///     Ok(())
    /// }
    /// ```
    ///
    /// The [`write_all`] method is defined on the [`AsyncWriteExt`] trait.
    ///
    /// [`write_all`]: fn@crate::io::AsyncWriteExt::write_all
    /// [`AsyncWriteExt`]: trait@crate::io::AsyncWriteExt
    ///
    /// To shut down the stream in the write direction, you can call the
    /// [`shutdown()`] method. This will cause the other peer to receive a read of
    /// length 0, indicating that no more data will be sent. This only closes
    /// the stream in one direction.
    ///
    /// [`shutdown()`]: fn@crate::io::AsyncWriteExt::shutdown
    pub struct TcpStream {
        io: PollEvented<mio::net::TcpStream>,
    }
}

impl TcpStream {
    cfg_not_wasi! {
        /// Opens a TCP connection to a remote host.
        ///
        /// `addr` is an address of the remote host. Anything which implements the
        /// [`ToSocketAddrs`] trait can be supplied as the address.  If `addr`
        /// yields multiple addresses, connect will be attempted with each of the
        /// addresses until a connection is successful. If none of the addresses
        /// result in a successful connection, the error returned from the last
        /// connection attempt (the last address) is returned.
        ///
        /// To configure the socket before connecting, you can use the [`TcpSocket`]
        /// type.
        ///
        /// [`ToSocketAddrs`]: trait@crate::net::ToSocketAddrs
        /// [`TcpSocket`]: struct@crate::net::TcpSocket
        ///
        /// # Examples
        ///
        /// ```no_run
        /// use tokio::net::TcpStream;
        /// use tokio::io::AsyncWriteExt;
        /// use std::error::Error;
        ///
        /// #[tokio::main]
        /// async fn main() -> Result<(), Box<dyn Error>> {
        ///     // Connect to a peer
        ///     let mut stream = TcpStream::connect("127.0.0.1:8080").await?;
        ///
        ///     // Write some data.
        ///     stream.write_all(b"hello world!").await?;
        ///
        ///     Ok(())
        /// }
        /// ```
        ///
        /// The [`write_all`] method is defined on the [`AsyncWriteExt`] trait.
        ///
        /// [`write_all`]: fn@crate::io::AsyncWriteExt::write_all
        /// [`AsyncWriteExt`]: trait@crate::io::AsyncWriteExt
        pub async fn connect<A: ToSocketAddrs>(addr: A) -> io::Result<TcpStream> {
            let addrs = to_socket_addrs(addr).await?;

            let mut last_err = None;

            for addr in addrs {
                match TcpStream::connect_addr(addr).await {
                    Ok(stream) => return Ok(stream),
                    Err(e) => last_err = Some(e),
                }
            }

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

        /// Establishes a connection to the specified `addr`.
        async fn connect_addr(addr: SocketAddr) -> io::Result<TcpStream> {
            let sys = mio::net::TcpStream::connect(addr)?;
            TcpStream::connect_mio(sys).await
        }

        pub(crate) async fn connect_mio(sys: mio::net::TcpStream) -> io::Result<TcpStream> {
            let stream = TcpStream::new(sys)?;

            // Once we've connected, wait for the stream to be writable as
            // that's when the actual connection has been initiated. Once we're
            // writable we check for `take_socket_error` to see if the connect
            // actually hit an error or not.
            //
            // If all that succeeded then we ship everything on up.
            poll_fn(|cx| stream.io.registration().poll_write_ready(cx)).await?;

            if let Some(e) = stream.io.take_error()? {
                return Err(e);
            }

            Ok(stream)
        }
    }

    pub(crate) fn new(connected: mio::net::TcpStream) -> io::Result<TcpStream> {
        let io = PollEvented::new(connected)?;
        Ok(TcpStream { io })
    }

    /// Creates new `TcpStream` from a `std::net::TcpStream`.
    ///
    /// This function is intended to be used to wrap a TCP stream from the
    /// standard library in the Tokio equivalent.
    ///
    /// # Notes
    ///
    /// The caller is responsible for ensuring that the stream is in
    /// non-blocking mode. Otherwise all I/O operations on the stream
    /// will block the thread, which will cause unexpected behavior.
    /// Non-blocking mode can be set using [`set_nonblocking`].
    ///
    /// [`set_nonblocking`]: std::net::TcpStream::set_nonblocking
    ///
    /// # Examples
    ///
    /// ```rust,no_run
    /// use std::error::Error;
    /// use tokio::net::TcpStream;
    ///
    /// #[tokio::main]
    /// async fn main() -> Result<(), Box<dyn Error>> {
    ///     let std_stream = std::net::TcpStream::connect("127.0.0.1:34254")?;
    ///     std_stream.set_nonblocking(true)?;
    ///     let stream = TcpStream::from_std(std_stream)?;
    ///     Ok(())
    /// }
    /// ```
    ///
    /// # Panics
    ///
    /// This function panics if it is not called from within a runtime with
    /// IO enabled.
    ///
    /// 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.
    #[track_caller]
    pub fn from_std(stream: std::net::TcpStream) -> io::Result<TcpStream> {
        let io = mio::net::TcpStream::from_std(stream);
        let io = PollEvented::new(io)?;
        Ok(TcpStream { io })
    }

    /// Turns a [`tokio::net::TcpStream`] into a [`std::net::TcpStream`].
    ///
    /// The returned [`std::net::TcpStream`] will have nonblocking mode set as `true`.
    /// Use [`set_nonblocking`] to change the blocking mode if needed.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::error::Error;
    /// use std::io::Read;
    /// use tokio::net::TcpListener;
    /// # use tokio::net::TcpStream;
    /// # use tokio::io::AsyncWriteExt;
    ///
    /// #[tokio::main]
    /// async fn main() -> Result<(), Box<dyn Error>> {
    ///     let mut data = [0u8; 12];
    /// #   if false {
    ///     let listener = TcpListener::bind("127.0.0.1:34254").await?;
    /// #   }
    /// #   let listener = TcpListener::bind("127.0.0.1:0").await?;
    /// #   let addr = listener.local_addr().unwrap();
    /// #   let handle = tokio::spawn(async move {
    /// #       let mut stream: TcpStream = TcpStream::connect(addr).await.unwrap();
    /// #       stream.write_all(b"Hello world!").await.unwrap();
    /// #   });
    ///     let (tokio_tcp_stream, _) = listener.accept().await?;
    ///     let mut std_tcp_stream = tokio_tcp_stream.into_std()?;
    /// #   handle.await.expect("The task being joined has panicked");
    ///     std_tcp_stream.set_nonblocking(false)?;
    ///     std_tcp_stream.read_exact(&mut data)?;
    /// #   assert_eq!(b"Hello world!", &data);
    ///     Ok(())
    /// }
    /// ```
    /// [`tokio::net::TcpStream`]: TcpStream
    /// [`std::net::TcpStream`]: std::net::TcpStream
    /// [`set_nonblocking`]: fn@std::net::TcpStream::set_nonblocking
    pub fn into_std(self) -> io::Result<std::net::TcpStream> {
        #[cfg(unix)]
        {
            use std::os::unix::io::{FromRawFd, IntoRawFd};
            self.io
                .into_inner()
                .map(IntoRawFd::into_raw_fd)
                .map(|raw_fd| unsafe { std::net::TcpStream::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::TcpStream::from_raw_socket(raw_socket) })
        }

        #[cfg(target_os = "wasi")]
        {
            use std::os::wasi::io::{FromRawFd, IntoRawFd};
            self.io
                .into_inner()
                .map(|io| io.into_raw_fd())
                .map(|raw_fd| unsafe { std::net::TcpStream::from_raw_fd(raw_fd) })
        }
    }

    /// Returns the local address that this stream is bound to.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use tokio::net::TcpStream;
    ///
    /// # async fn dox() -> Result<(), Box<dyn std::error::Error>> {
    /// let stream = TcpStream::connect("127.0.0.1:8080").await?;
    ///
    /// println!("{:?}", stream.local_addr()?);
    /// # Ok(())
    /// # }
    /// ```
    pub fn local_addr(&self) -> io::Result<SocketAddr> {
        self.io.local_addr()
    }

    /// Returns the value of the `SO_ERROR` option.
    pub fn take_error(&self) -> io::Result<Option<io::Error>> {
        self.io.take_error()
    }

    /// Returns the remote address that this stream is connected to.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use tokio::net::TcpStream;
    ///
    /// # async fn dox() -> Result<(), Box<dyn std::error::Error>> {
    /// let stream = TcpStream::connect("127.0.0.1:8080").await?;
    ///
    /// println!("{:?}", stream.peer_addr()?);
    /// # Ok(())
    /// # }
    /// ```
    pub fn peer_addr(&self) -> io::Result<SocketAddr> {
        self.io.peer_addr()
    }

    /// Attempts to receive data on the socket, without removing that data from
    /// the queue, registering the current task for wakeup if data is not yet
    /// available.
    ///
    /// Note that on multiple calls to `poll_peek`, `poll_read` or
    /// `poll_read_ready`, only the `Waker` from the `Context` passed to the
    /// most recent call is scheduled to receive a wakeup. (However,
    /// `poll_write` retains a second, independent waker.)
    ///
    /// # Return value
    ///
    /// The function returns:
    ///
    /// * `Poll::Pending` if data is not yet available.
    /// * `Poll::Ready(Ok(n))` if data is available. `n` is the number of bytes peeked.
    /// * `Poll::Ready(Err(e))` if an error is encountered.
    ///
    /// # Errors
    ///
    /// This function may encounter any standard I/O error except `WouldBlock`.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use tokio::io::{self, ReadBuf};
    /// use tokio::net::TcpStream;
    ///
    /// use std::future::poll_fn;
    ///
    /// #[tokio::main]
    /// async fn main() -> io::Result<()> {
    ///     let stream = TcpStream::connect("127.0.0.1:8000").await?;
    ///     let mut buf = [0; 10];
    ///     let mut buf = ReadBuf::new(&mut buf);
    ///
    ///     poll_fn(|cx| {
    ///         stream.poll_peek(cx, &mut buf)
    ///     }).await?;
    ///
    ///     Ok(())
    /// }
    /// ```
    pub fn poll_peek(
        &self,
        cx: &mut Context<'_>,
        buf: &mut ReadBuf<'_>,
    ) -> Poll<io::Result<usize>> {
        loop {
            let ev = ready!(self.io.registration().poll_read_ready(cx))?;

            let b = unsafe {
                &mut *(buf.unfilled_mut() as *mut [std::mem::MaybeUninit<u8>] as *mut [u8])
            };

            match self.io.peek(b) {
                Ok(ret) => {
                    unsafe { buf.assume_init(ret) };
                    buf.advance(ret);
                    return Poll::Ready(Ok(ret));
                }
                Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
                    self.io.registration().clear_readiness(ev);
                }
                Err(e) => return Poll::Ready(Err(e)),
            }
        }
    }

    /// Waits for any of the requested ready states.
    ///
    /// This function is usually paired with `try_read()` or `try_write()`. It
    /// can be used to concurrently read / write 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 read and write to the stream on the same task without
    /// splitting.
    ///
    /// ```no_run
    /// use tokio::io::Interest;
    /// use tokio::net::TcpStream;
    /// use std::error::Error;
    /// use std::io;
    ///
    /// #[tokio::main]
    /// async fn main() -> Result<(), Box<dyn Error>> {
    ///     let stream = TcpStream::connect("127.0.0.1:8080").await?;
    ///
    ///     loop {
    ///         let ready = stream.ready(Interest::READABLE | Interest::WRITABLE).await?;
    ///
    ///         if ready.is_readable() {
    ///             let mut data = vec![0; 1024];
    ///             // Try to read data, this may still fail with `WouldBlock`
    ///             // if the readiness event is a false positive.
    ///             match stream.try_read(&mut data) {
    ///                 Ok(n) => {
    ///                     println!("read {} bytes", n);
    ///                 }
    ///                 Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
    ///                     continue;
    ///                 }
    ///                 Err(e) => {
    ///                     return Err(e.into());
    ///                 }
    ///             }
    ///
    ///         }
    ///
    ///         if ready.is_writable() {
    ///             // Try to write data, this may still fail with `WouldBlock`
    ///             // if the readiness event is a false positive.
    ///             match stream.try_write(b"hello world") {
    ///                 Ok(n) => {
    ///                     println!("write {} bytes", n);
    ///                 }
    ///                 Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
    ///                     continue
    ///                 }
    ///                 Err(e) => {
    ///                     return Err(e.into());
    ///                 }
    ///             }
    ///         }
    ///     }
    /// }
    /// ```
    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 readable.
    ///
    /// This function is equivalent to `ready(Interest::READABLE)` and is usually
    /// paired with `try_read()`.
    ///
    /// # 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::TcpStream;
    /// use std::error::Error;
    /// use std::io;
    ///
    /// #[tokio::main]
    /// async fn main() -> Result<(), Box<dyn Error>> {
    ///     // Connect to a peer
    ///     let stream = TcpStream::connect("127.0.0.1:8080").await?;
    ///
    ///     let mut msg = vec![0; 1024];
    ///
    ///     loop {
    ///         // Wait for the socket to be readable
    ///         stream.readable().await?;
    ///
    ///         // Try to read data, this may still fail with `WouldBlock`
    ///         // if the readiness event is a false positive.
    ///         match stream.try_read(&mut msg) {
    ///             Ok(n) => {
    ///                 msg.truncate(n);
    ///                 break;
    ///             }
    ///             Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
    ///                 continue;
    ///             }
    ///             Err(e) => {
    ///                 return Err(e.into());
    ///             }
    ///         }
    ///     }
    ///
    ///     println!("GOT = {:?}", msg);
    ///     Ok(())
    /// }
    /// ```
    pub async fn readable(&self) -> io::Result<()> {
        self.ready(Interest::READABLE).await?;
        Ok(())
    }

    /// Polls for read readiness.
    ///
    /// If the tcp stream is not currently ready for reading, this method will
    /// store a clone of the `Waker` from the provided `Context`. When the tcp
    /// stream becomes ready for reading, `Waker::wake` will be called on the
    /// waker.
    ///
    /// Note that on multiple calls to `poll_read_ready`, `poll_read` or
    /// `poll_peek`, only the `Waker` from the `Context` passed to the most
    /// recent call is scheduled to receive a wakeup. (However,
    /// `poll_write_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 tcp stream is not ready for reading.
    /// * `Poll::Ready(Ok(()))` if the tcp 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_read_ready(&self, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
        self.io.registration().poll_read_ready(cx).map_ok(|_| ())
    }

    /// Tries to read data from the stream into the provided buffer, returning how
    /// many bytes were read.
    ///
    /// Receives any pending data from the socket but does not wait for new data
    /// to arrive. On success, returns the number of bytes read. Because
    /// `try_read()` is non-blocking, the buffer does not have to be stored by
    /// the async task and can exist entirely on the stack.
    ///
    /// Usually, [`readable()`] or [`ready()`] is used with this function.
    ///
    /// [`readable()`]: TcpStream::readable()
    /// [`ready()`]: TcpStream::ready()
    ///
    /// # Return
    ///
    /// If data is successfully read, `Ok(n)` is returned, where `n` is the
    /// number of bytes read. If `n` is `0`, then it can indicate one of two scenarios:
    ///
    /// 1. The stream's read half is closed and will no longer yield data.
    /// 2. The specified buffer was 0 bytes in length.
    ///
    /// If the stream is not ready to read data,
    /// `Err(io::ErrorKind::WouldBlock)` is returned.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use tokio::net::TcpStream;
    /// use std::error::Error;
    /// use std::io;
    ///
    /// #[tokio::main]
    /// async fn main() -> Result<(), Box<dyn Error>> {
    ///     // Connect to a peer
    ///     let stream = TcpStream::connect("127.0.0.1:8080").await?;
    ///
    ///     loop {
    ///         // Wait for the socket to be readable
    ///         stream.readable().await?;
    ///
    ///         // Creating the buffer **after** the `await` prevents it from
    ///         // being stored in the async task.
    ///         let mut buf = [0; 4096];
    ///
    ///         // Try to read data, this may still fail with `WouldBlock`
    ///         // if the readiness event is a false positive.
    ///         match stream.try_read(&mut buf) {
    ///             Ok(0) => break,
    ///             Ok(n) => {
    ///                 println!("read {} bytes", n);
    ///             }
    ///             Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
    ///                 continue;
    ///             }
    ///             Err(e) => {
    ///                 return Err(e.into());
    ///             }
    ///         }
    ///     }
    ///
    ///     Ok(())
    /// }
    /// ```
    pub fn try_read(&self, buf: &mut [u8]) -> io::Result<usize> {
        use std::io::Read;

        self.io
            .registration()
            .try_io(Interest::READABLE, || (&*self.io).read(buf))
    }

    /// Tries to read data from the stream into the provided buffers, returning
    /// how many bytes were read.
    ///
    /// Data is copied to fill each buffer in order, with the final buffer
    /// written to possibly being only partially filled. This method behaves
    /// equivalently to a single call to [`try_read()`] with concatenated
    /// buffers.
    ///
    /// Receives any pending data from the socket but does not wait for new data
    /// to arrive. On success, returns the number of bytes read. Because
    /// `try_read_vectored()` is non-blocking, the buffer does not have to be
    /// stored by the async task and can exist entirely on the stack.
    ///
    /// Usually, [`readable()`] or [`ready()`] is used with this function.
    ///
    /// [`try_read()`]: TcpStream::try_read()
    /// [`readable()`]: TcpStream::readable()
    /// [`ready()`]: TcpStream::ready()
    ///
    /// # Return
    ///
    /// If data is successfully read, `Ok(n)` is returned, where `n` is the
    /// number of bytes read. `Ok(0)` indicates the stream's read half is closed
    /// and will no longer yield data. If the stream is not ready to read data
    /// `Err(io::ErrorKind::WouldBlock)` is returned.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use tokio::net::TcpStream;
    /// use std::error::Error;
    /// use std::io::{self, IoSliceMut};
    ///
    /// #[tokio::main]
    /// async fn main() -> Result<(), Box<dyn Error>> {
    ///     // Connect to a peer
    ///     let stream = TcpStream::connect("127.0.0.1:8080").await?;
    ///
    ///     loop {
    ///         // Wait for the socket to be readable
    ///         stream.readable().await?;
    ///
    ///         // Creating the buffer **after** the `await` prevents it from
    ///         // being stored in the async task.
    ///         let mut buf_a = [0; 512];
    ///         let mut buf_b = [0; 1024];
    ///         let mut bufs = [
    ///             IoSliceMut::new(&mut buf_a),
    ///             IoSliceMut::new(&mut buf_b),
    ///         ];
    ///
    ///         // Try to read data, this may still fail with `WouldBlock`
    ///         // if the readiness event is a false positive.
    ///         match stream.try_read_vectored(&mut bufs) {
    ///             Ok(0) => break,
    ///             Ok(n) => {
    ///                 println!("read {} bytes", n);
    ///             }
    ///             Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
    ///                 continue;
    ///             }
    ///             Err(e) => {
    ///                 return Err(e.into());
    ///             }
    ///         }
    ///     }
    ///
    ///     Ok(())
    /// }
    /// ```
    pub fn try_read_vectored(&self, bufs: &mut [io::IoSliceMut<'_>]) -> io::Result<usize> {
        use std::io::Read;

        self.io
            .registration()
            .try_io(Interest::READABLE, || (&*self.io).read_vectored(bufs))
    }

    cfg_io_util! {
        /// Tries to read data from the stream into the provided buffer, advancing the
        /// buffer's internal cursor, returning how many bytes were read.
        ///
        /// Receives any pending data from the socket but does not wait for new data
        /// to arrive. On success, returns the number of bytes read. Because
        /// `try_read_buf()` is non-blocking, the buffer does not have to be stored by
        /// the async task and can exist entirely on the stack.
        ///
        /// Usually, [`readable()`] or [`ready()`] is used with this function.
        ///
        /// [`readable()`]: TcpStream::readable()
        /// [`ready()`]: TcpStream::ready()
        ///
        /// # Return
        ///
        /// If data is successfully read, `Ok(n)` is returned, where `n` is the
        /// number of bytes read. `Ok(0)` indicates the stream's read half is closed
        /// and will no longer yield data. If the stream is not ready to read data
        /// `Err(io::ErrorKind::WouldBlock)` is returned.
        ///
        /// # Examples
        ///
        /// ```no_run
        /// use tokio::net::TcpStream;
        /// use std::error::Error;
        /// use std::io;
        ///
        /// #[tokio::main]
        /// async fn main() -> Result<(), Box<dyn Error>> {
        ///     // Connect to a peer
        ///     let stream = TcpStream::connect("127.0.0.1:8080").await?;
        ///
        ///     loop {
        ///         // Wait for the socket to be readable
        ///         stream.readable().await?;
        ///
        ///         let mut buf = Vec::with_capacity(4096);
        ///
        ///         // Try to read data, this may still fail with `WouldBlock`
        ///         // if the readiness event is a false positive.
        ///         match stream.try_read_buf(&mut buf) {
        ///             Ok(0) => break,
        ///             Ok(n) => {
        ///                 println!("read {} bytes", n);
        ///             }
        ///             Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
        ///                 continue;
        ///             }
        ///             Err(e) => {
        ///                 return Err(e.into());
        ///             }
        ///         }
        ///     }
        ///
        ///     Ok(())
        /// }
        /// ```
        pub fn try_read_buf<B: BufMut>(&self, buf: &mut B) -> io::Result<usize> {
            self.io.registration().try_io(Interest::READABLE, || {
                use std::io::Read;

                let dst = buf.chunk_mut();
                let dst =
                    unsafe { &mut *(dst as *mut _ as *mut [std::mem::MaybeUninit<u8>] as *mut [u8]) };

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

                unsafe {
                    buf.advance_mut(n);
                }

                Ok(n)
            })
        }
    }

    /// Waits for the socket to become writable.
    ///
    /// This function is equivalent to `ready(Interest::WRITABLE)` and is usually
    /// paired with `try_write()`.
    ///
    /// # 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::TcpStream;
    /// use std::error::Error;
    /// use std::io;
    ///
    /// #[tokio::main]
    /// async fn main() -> Result<(), Box<dyn Error>> {
    ///     // Connect to a peer
    ///     let stream = TcpStream::connect("127.0.0.1:8080").await?;
    ///
    ///     loop {
    ///         // Wait for the socket to be writable
    ///         stream.writable().await?;
    ///
    ///         // Try to write data, this may still fail with `WouldBlock`
    ///         // if the readiness event is a false positive.
    ///         match stream.try_write(b"hello world") {
    ///             Ok(n) => {
    ///                 break;
    ///             }
    ///             Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
    ///                 continue;
    ///             }
    ///             Err(e) => {
    ///                 return Err(e.into());
    ///             }
    ///         }
    ///     }
    ///
    ///     Ok(())
    /// }
    /// ```
    pub async fn writable(&self) -> io::Result<()> {
        self.ready(Interest::WRITABLE).await?;
        Ok(())
    }

    /// Polls for write readiness.
    ///
    /// If the tcp stream is not currently ready for writing, this method will
    /// store a clone of the `Waker` from the provided `Context`. When the tcp
    /// stream becomes ready for writing, `Waker::wake` will be called on the
    /// waker.
    ///
    /// Note that on multiple calls to `poll_write_ready` or `poll_write`, only
    /// the `Waker` from the `Context` passed to the most recent call is
    /// scheduled to receive a wakeup. (However, `poll_read_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 tcp stream is not ready for writing.
    /// * `Poll::Ready(Ok(()))` if the tcp 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_write_ready(&self, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
        self.io.registration().poll_write_ready(cx).map_ok(|_| ())
    }

    /// Try to write a buffer to the stream, returning how many bytes were
    /// written.
    ///
    /// The function will attempt to write the entire contents of `buf`, but
    /// only part of the buffer may be written.
    ///
    /// This function is usually paired with `writable()`.
    ///
    /// # Return
    ///
    /// If data is successfully written, `Ok(n)` is returned, where `n` is the
    /// number of bytes written. If the stream is not ready to write data,
    /// `Err(io::ErrorKind::WouldBlock)` is returned.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use tokio::net::TcpStream;
    /// use std::error::Error;
    /// use std::io;
    ///
    /// #[tokio::main]
    /// async fn main() -> Result<(), Box<dyn Error>> {
    ///     // Connect to a peer
    ///     let stream = TcpStream::connect("127.0.0.1:8080").await?;
    ///
    ///     loop {
    ///         // Wait for the socket to be writable
    ///         stream.writable().await?;
    ///
    ///         // Try to write data, this may still fail with `WouldBlock`
    ///         // if the readiness event is a false positive.
    ///         match stream.try_write(b"hello world") {
    ///             Ok(n) => {
    ///                 break;
    ///             }
    ///             Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
    ///                 continue;
    ///             }
    ///             Err(e) => {
    ///                 return Err(e.into());
    ///             }
    ///         }
    ///     }
    ///
    ///     Ok(())
    /// }
    /// ```
    pub fn try_write(&self, buf: &[u8]) -> io::Result<usize> {
        use std::io::Write;

        self.io
            .registration()
            .try_io(Interest::WRITABLE, || (&*self.io).write(buf))
    }

    /// Tries to write several buffers to the stream, returning how many bytes
    /// were written.
    ///
    /// Data is written from each buffer in order, with the final buffer read
    /// from possible being only partially consumed. This method behaves
    /// equivalently to a single call to [`try_write()`] with concatenated
    /// buffers.
    ///
    /// This function is usually paired with `writable()`.
    ///
    /// [`try_write()`]: TcpStream::try_write()
    ///
    /// # Return
    ///
    /// If data is successfully written, `Ok(n)` is returned, where `n` is the
    /// number of bytes written. If the stream is not ready to write data,
    /// `Err(io::ErrorKind::WouldBlock)` is returned.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use tokio::net::TcpStream;
    /// use std::error::Error;
    /// use std::io;
    ///
    /// #[tokio::main]
    /// async fn main() -> Result<(), Box<dyn Error>> {
    ///     // Connect to a peer
    ///     let stream = TcpStream::connect("127.0.0.1:8080").await?;
    ///
    ///     let bufs = [io::IoSlice::new(b"hello "), io::IoSlice::new(b"world")];
    ///
    ///     loop {
    ///         // Wait for the socket to be writable
    ///         stream.writable().await?;
    ///
    ///         // Try to write data, this may still fail with `WouldBlock`
    ///         // if the readiness event is a false positive.
    ///         match stream.try_write_vectored(&bufs) {
    ///             Ok(n) => {
    ///                 break;
    ///             }
    ///             Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
    ///                 continue;
    ///             }
    ///             Err(e) => {
    ///                 return Err(e.into());
    ///             }
    ///         }
    ///     }
    ///
    ///     Ok(())
    /// }
    /// ```
    pub fn try_write_vectored(&self, bufs: &[io::IoSlice<'_>]) -> io::Result<usize> {
        use std::io::Write;

        self.io
            .registration()
            .try_io(Interest::WRITABLE, || (&*self.io).write_vectored(bufs))
    }

    /// 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 `TcpStream` 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()`]: TcpStream::readable()
    /// [`writable()`]: TcpStream::writable()
    /// [`ready()`]: TcpStream::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))
    }

    /// Reads or writes from the socket using a user-provided IO operation.
    ///
    /// The readiness of the socket is awaited and when 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. In such case the
    /// readiness flag is cleared and the socket readiness is awaited again.
    /// This loop is repeated until the closure returns an `Ok` or an error
    /// other than `WouldBlock`.
    ///
    /// 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 `TcpStream` 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.
    pub async fn async_io<R>(
        &self,
        interest: Interest,
        mut f: impl FnMut() -> io::Result<R>,
    ) -> io::Result<R> {
        self.io
            .registration()
            .async_io(interest, || self.io.try_io(&mut f))
            .await
    }

    /// Receives data on the socket from the remote address to which it is
    /// connected, without removing that data from the queue. On success,
    /// returns the number of bytes peeked.
    ///
    /// Successive calls return the same data. This is accomplished by passing
    /// `MSG_PEEK` as a flag to the underlying `recv` system call.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use tokio::net::TcpStream;
    /// use tokio::io::AsyncReadExt;
    /// use std::error::Error;
    ///
    /// #[tokio::main]
    /// async fn main() -> Result<(), Box<dyn Error>> {
    ///     // Connect to a peer
    ///     let mut stream = TcpStream::connect("127.0.0.1:8080").await?;
    ///
    ///     let mut b1 = [0; 10];
    ///     let mut b2 = [0; 10];
    ///
    ///     // Peek at the data
    ///     let n = stream.peek(&mut b1).await?;
    ///
    ///     // Read the data
    ///     assert_eq!(n, stream.read(&mut b2[..n]).await?);
    ///     assert_eq!(&b1[..n], &b2[..n]);
    ///
    ///     Ok(())
    /// }
    /// ```
    ///
    /// The [`read`] method is defined on the [`AsyncReadExt`] trait.
    ///
    /// [`read`]: fn@crate::io::AsyncReadExt::read
    /// [`AsyncReadExt`]: trait@crate::io::AsyncReadExt
    pub async fn peek(&self, buf: &mut [u8]) -> io::Result<usize> {
        self.io
            .registration()
            .async_io(Interest::READABLE, || self.io.peek(buf))
            .await
    }

    /// Shuts down the read, write, or both halves of this connection.
    ///
    /// This function will cause all pending and future I/O on the specified
    /// portions to return immediately with an appropriate value (see the
    /// documentation of `Shutdown`).
    pub(super) fn shutdown_std(&self, how: Shutdown) -> io::Result<()> {
        self.io.shutdown(how)
    }

    /// Gets the value of the `TCP_NODELAY` option on this socket.
    ///
    /// For more information about this option, see [`set_nodelay`].
    ///
    /// [`set_nodelay`]: TcpStream::set_nodelay
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use tokio::net::TcpStream;
    ///
    /// # async fn dox() -> Result<(), Box<dyn std::error::Error>> {
    /// let stream = TcpStream::connect("127.0.0.1:8080").await?;
    ///
    /// println!("{:?}", stream.nodelay()?);
    /// # Ok(())
    /// # }
    /// ```
    pub fn nodelay(&self) -> io::Result<bool> {
        self.io.nodelay()
    }

    /// Sets the value of the `TCP_NODELAY` option on this socket.
    ///
    /// If set, this option disables the Nagle algorithm. This means that
    /// segments are always sent as soon as possible, even if there is only a
    /// small amount of data. When not set, data is buffered until there is a
    /// sufficient amount to send out, thereby avoiding the frequent sending of
    /// small packets.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use tokio::net::TcpStream;
    ///
    /// # async fn dox() -> Result<(), Box<dyn std::error::Error>> {
    /// let stream = TcpStream::connect("127.0.0.1:8080").await?;
    ///
    /// stream.set_nodelay(true)?;
    /// # Ok(())
    /// # }
    /// ```
    pub fn set_nodelay(&self, nodelay: bool) -> io::Result<()> {
        self.io.set_nodelay(nodelay)
    }

    cfg_not_wasi! {
        /// Reads the linger duration for this socket by getting the `SO_LINGER`
        /// option.
        ///
        /// For more information about this option, see [`set_linger`].
        ///
        /// [`set_linger`]: TcpStream::set_linger
        ///
        /// # Examples
        ///
        /// ```no_run
        /// use tokio::net::TcpStream;
        ///
        /// # async fn dox() -> Result<(), Box<dyn std::error::Error>> {
        /// let stream = TcpStream::connect("127.0.0.1:8080").await?;
        ///
        /// println!("{:?}", stream.linger()?);
        /// # Ok(())
        /// # }
        /// ```
        pub fn linger(&self) -> io::Result<Option<Duration>> {
            socket2::SockRef::from(self).linger()
        }

        /// Sets the linger duration of this socket by setting the `SO_LINGER` option.
        ///
        /// This option controls the action taken when a stream has unsent messages and the stream is
        /// closed. If `SO_LINGER` is set, the system shall block the process until it can transmit the
        /// data or until the time expires.
        ///
        /// If `SO_LINGER` is not specified, and the stream is closed, the system handles the call in a
        /// way that allows the process to continue as quickly as possible.
        ///
        /// # Examples
        ///
        /// ```no_run
        /// use tokio::net::TcpStream;
        ///
        /// # async fn dox() -> Result<(), Box<dyn std::error::Error>> {
        /// let stream = TcpStream::connect("127.0.0.1:8080").await?;
        ///
        /// stream.set_linger(None)?;
        /// # Ok(())
        /// # }
        /// ```
        pub fn set_linger(&self, dur: Option<Duration>) -> io::Result<()> {
            socket2::SockRef::from(self).set_linger(dur)
        }
    }

    /// Gets the value of the `IP_TTL` option for this socket.
    ///
    /// For more information about this option, see [`set_ttl`].
    ///
    /// [`set_ttl`]: TcpStream::set_ttl
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use tokio::net::TcpStream;
    ///
    /// # async fn dox() -> Result<(), Box<dyn std::error::Error>> {
    /// let stream = TcpStream::connect("127.0.0.1:8080").await?;
    ///
    /// println!("{:?}", stream.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::TcpStream;
    ///
    /// # async fn dox() -> Result<(), Box<dyn std::error::Error>> {
    /// let stream = TcpStream::connect("127.0.0.1:8080").await?;
    ///
    /// stream.set_ttl(123)?;
    /// # Ok(())
    /// # }
    /// ```
    pub fn set_ttl(&self, ttl: u32) -> io::Result<()> {
        self.io.set_ttl(ttl)
    }

    // These lifetime markers also appear in the generated documentation, and make
    // it more clear that this is a *borrowed* split.
    #[allow(clippy::needless_lifetimes)]
    /// Splits a `TcpStream` into a read half and a write half, which can be used
    /// to read and write the stream concurrently.
    ///
    /// This method is more efficient than [`into_split`], but the halves cannot be
    /// moved into independently spawned tasks.
    ///
    /// [`into_split`]: TcpStream::into_split()
    pub fn split<'a>(&'a mut self) -> (ReadHalf<'a>, WriteHalf<'a>) {
        split(self)
    }

    /// Splits a `TcpStream` into a read half and a write half, which can be used
    /// to read and write the stream concurrently.
    ///
    /// Unlike [`split`], the owned halves can be moved to separate tasks, however
    /// this comes at the cost of a heap allocation.
    ///
    /// **Note:** Dropping the write half will shut down the write half of the TCP
    /// stream. This is equivalent to calling [`shutdown()`] on the `TcpStream`.
    ///
    /// [`split`]: TcpStream::split()
    /// [`shutdown()`]: fn@crate::io::AsyncWriteExt::shutdown
    pub fn into_split(self) -> (OwnedReadHalf, OwnedWriteHalf) {
        split_owned(self)
    }

    // == Poll IO functions that takes `&self` ==
    //
    // To read or write without mutable access to the `TcpStream`, combine the
    // `poll_read_ready` or `poll_write_ready` methods with the `try_read` or
    // `try_write` methods.

    pub(crate) fn poll_read_priv(
        &self,
        cx: &mut Context<'_>,
        buf: &mut ReadBuf<'_>,
    ) -> Poll<io::Result<()>> {
        // Safety: `TcpStream::read` correctly handles reads into uninitialized memory
        unsafe { self.io.poll_read(cx, buf) }
    }

    pub(super) fn poll_write_priv(
        &self,
        cx: &mut Context<'_>,
        buf: &[u8],
    ) -> Poll<io::Result<usize>> {
        self.io.poll_write(cx, buf)
    }

    pub(super) fn poll_write_vectored_priv(
        &self,
        cx: &mut Context<'_>,
        bufs: &[io::IoSlice<'_>],
    ) -> Poll<io::Result<usize>> {
        self.io.poll_write_vectored(cx, bufs)
    }
}

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

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

// ===== impl Read / Write =====

impl AsyncRead for TcpStream {
    fn poll_read(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        buf: &mut ReadBuf<'_>,
    ) -> Poll<io::Result<()>> {
        self.poll_read_priv(cx, buf)
    }
}

impl AsyncWrite for TcpStream {
    fn poll_write(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        buf: &[u8],
    ) -> Poll<io::Result<usize>> {
        self.poll_write_priv(cx, buf)
    }

    fn poll_write_vectored(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        bufs: &[io::IoSlice<'_>],
    ) -> Poll<io::Result<usize>> {
        self.poll_write_vectored_priv(cx, bufs)
    }

    fn is_write_vectored(&self) -> bool {
        true
    }

    #[inline]
    fn poll_flush(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<io::Result<()>> {
        // tcp flush is a no-op
        Poll::Ready(Ok(()))
    }

    fn poll_shutdown(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<io::Result<()>> {
        self.shutdown_std(std::net::Shutdown::Write)?;
        Poll::Ready(Ok(()))
    }
}

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

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

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

    impl AsFd for TcpStream {
        fn as_fd(&self) -> BorrowedFd<'_> {
            unsafe { BorrowedFd::borrow_raw(self.as_raw_fd()) }
        }
    }
}

cfg_windows! {
    use crate::os::windows::io::{AsRawSocket, RawSocket, AsSocket, BorrowedSocket};

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

    impl AsSocket for TcpStream {
        fn as_socket(&self) -> BorrowedSocket<'_> {
            unsafe { BorrowedSocket::borrow_raw(self.as_raw_socket()) }
        }
    }
}

#[cfg(all(tokio_unstable, target_os = "wasi"))]
mod sys {
    use super::TcpStream;
    use std::os::wasi::prelude::*;

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

    impl AsFd for TcpStream {
        fn as_fd(&self) -> BorrowedFd<'_> {
            unsafe { BorrowedFd::borrow_raw(self.as_raw_fd()) }
        }
    }
}