use crate::future::poll_fn;
use crate::io::{AsyncRead, AsyncWrite, PollEvented};
use crate::net::tcp::split::{split, ReadHalf, WriteHalf};
use crate::net::tcp::split_owned::{split_owned, OwnedReadHalf, OwnedWriteHalf};
use crate::net::ToSocketAddrs;

use bytes::Buf;
use iovec::IoVec;
use std::convert::TryFrom;
use std::fmt;
use std::io::{self, Read, Write};
use std::mem::MaybeUninit;
use std::net::{self, Shutdown, SocketAddr};
use std::pin::Pin;
use std::task::{Context, Poll};
use std::time::Duration;

cfg_tcp! {
    /// 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].
    ///
    /// Reading and writing to a `TcpStream` is usually done using the
    /// convenience methods found on the [`AsyncReadExt`] and [`AsyncWriteExt`]
    /// traits. Examples import these traits through [the prelude].
    ///
    /// [`connect`]: method@TcpStream::connect
    /// [accepting]: method@super::TcpListener::accept
    /// [listener]: struct@super::TcpListener
    /// [`AsyncReadExt`]: trait@crate::io::AsyncReadExt
    /// [`AsyncWriteExt`]: trait@crate::io::AsyncWriteExt
    /// [the prelude]: crate::prelude
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use tokio::net::TcpStream;
    /// use tokio::prelude::*;
    /// 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 struct TcpStream {
        io: PollEvented<mio::net::TcpStream>,
    }
}

impl TcpStream {
    /// 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. Note that
    /// strings only implement this trait when the **`dns`** feature is enabled,
    /// as strings may contain domain names that need to be resolved.
    ///
    /// 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.
    ///
    /// [`ToSocketAddrs`]: trait@crate::net::ToSocketAddrs
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use tokio::net::TcpStream;
    /// use tokio::prelude::*;
    /// 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(())
    /// }
    /// ```
    ///
    /// Without the `dns` feature:
    ///
    /// ```no_run
    /// use tokio::net::TcpStream;
    /// use tokio::prelude::*;
    /// use std::error::Error;
    /// use std::net::Ipv4Addr;
    ///
    /// #[tokio::main]
    /// async fn main() -> Result<(), Box<dyn Error>> {
    ///     // Connect to a peer
    ///     let mut stream = TcpStream::connect((Ipv4Addr::new(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 = addr.to_socket_addrs().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)?;
        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.poll_write_ready(cx)).await?;

        if let Some(e) = stream.io.get_ref().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 will convert a TCP stream created by the standard library
    /// to a TCP stream ready to be used with the provided event loop handle.
    ///
    /// # 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")?;
    ///     let stream = TcpStream::from_std(std_stream)?;
    ///     Ok(())
    /// }
    /// ```
    ///
    /// # 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 [`Handle::enter`](crate::runtime::Handle::enter) function.
    pub fn from_std(stream: net::TcpStream) -> io::Result<TcpStream> {
        let io = mio::net::TcpStream::from_stream(stream)?;
        let io = PollEvented::new(io)?;
        Ok(TcpStream { io })
    }

    // Connects `TcpStream` asynchronously that may be built with a net2 `TcpBuilder`.
    //
    // This should be removed in favor of some in-crate TcpSocket builder API.
    #[doc(hidden)]
    pub async fn connect_std(stream: net::TcpStream, addr: &SocketAddr) -> io::Result<TcpStream> {
        let io = mio::net::TcpStream::connect_stream(stream, addr)?;
        let io = PollEvented::new(io)?;
        let stream = TcpStream { io };

        // 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.poll_write_ready(cx)).await?;

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

        Ok(stream)
    }

    /// 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.get_ref().local_addr()
    }

    /// 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.get_ref().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.
    ///
    /// # 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;
    /// use tokio::net::TcpStream;
    ///
    /// use futures::future::poll_fn;
    ///
    /// #[tokio::main]
    /// async fn main() -> io::Result<()> {
    ///     let mut stream = TcpStream::connect("127.0.0.1:8000").await?;
    ///     let mut buf = [0; 10];
    ///
    ///     poll_fn(|cx| {
    ///         stream.poll_peek(cx, &mut buf)
    ///     }).await?;
    ///
    ///     Ok(())
    /// }
    /// ```
    pub fn poll_peek(&mut self, cx: &mut Context<'_>, buf: &mut [u8]) -> Poll<io::Result<usize>> {
        self.poll_peek2(cx, buf)
    }

    pub(super) fn poll_peek2(
        &self,
        cx: &mut Context<'_>,
        buf: &mut [u8],
    ) -> Poll<io::Result<usize>> {
        ready!(self.io.poll_read_ready(cx, mio::Ready::readable()))?;

        match self.io.get_ref().peek(buf) {
            Ok(ret) => Poll::Ready(Ok(ret)),
            Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
                self.io.clear_read_ready(cx, mio::Ready::readable())?;
                Poll::Pending
            }
            Err(e) => Poll::Ready(Err(e)),
        }
    }

    /// 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::prelude::*;
    /// 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(&mut self, buf: &mut [u8]) -> io::Result<usize> {
        poll_fn(|cx| self.poll_peek(cx, 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`).
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use tokio::net::TcpStream;
    /// use std::error::Error;
    /// use std::net::Shutdown;
    ///
    /// #[tokio::main]
    /// async fn main() -> Result<(), Box<dyn Error>> {
    ///     // Connect to a peer
    ///     let stream = TcpStream::connect("127.0.0.1:8080").await?;
    ///
    ///     // Shutdown the stream
    ///     stream.shutdown(Shutdown::Write)?;
    ///
    ///     Ok(())
    /// }
    /// ```
    pub fn shutdown(&self, how: Shutdown) -> io::Result<()> {
        self.io.get_ref().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.get_ref().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.get_ref().set_nodelay(nodelay)
    }

    /// Gets the value of the `SO_RCVBUF` option on this socket.
    ///
    /// For more information about this option, see [`set_recv_buffer_size`].
    ///
    /// [`set_recv_buffer_size`]: TcpStream::set_recv_buffer_size
    ///
    /// # 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.recv_buffer_size()?);
    /// # Ok(())
    /// # }
    /// ```
    pub fn recv_buffer_size(&self) -> io::Result<usize> {
        self.io.get_ref().recv_buffer_size()
    }

    /// Sets the value of the `SO_RCVBUF` option on this socket.
    ///
    /// Changes the size of the operating system's receive buffer associated
    /// with the 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_recv_buffer_size(100)?;
    /// # Ok(())
    /// # }
    /// ```
    pub fn set_recv_buffer_size(&self, size: usize) -> io::Result<()> {
        self.io.get_ref().set_recv_buffer_size(size)
    }

    /// Gets the value of the `SO_SNDBUF` option on this socket.
    ///
    /// For more information about this option, see [`set_send_buffer_size`].
    ///
    /// [`set_send_buffer_size`]: TcpStream::set_send_buffer_size
    ///
    /// # 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.send_buffer_size()?);
    /// # Ok(())
    /// # }
    /// ```
    pub fn send_buffer_size(&self) -> io::Result<usize> {
        self.io.get_ref().send_buffer_size()
    }

    /// Sets the value of the `SO_SNDBUF` option on this socket.
    ///
    /// Changes the size of the operating system's send buffer associated with
    /// the 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_send_buffer_size(100)?;
    /// # Ok(())
    /// # }
    /// ```
    pub fn set_send_buffer_size(&self, size: usize) -> io::Result<()> {
        self.io.get_ref().set_send_buffer_size(size)
    }

    /// Returns whether keepalive messages are enabled on this socket, and if so
    /// the duration of time between them.
    ///
    /// For more information about this option, see [`set_keepalive`].
    ///
    /// [`set_keepalive`]: TcpStream::set_keepalive
    ///
    /// # 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.keepalive()?);
    /// # Ok(())
    /// # }
    /// ```
    pub fn keepalive(&self) -> io::Result<Option<Duration>> {
        self.io.get_ref().keepalive()
    }

    /// Sets whether keepalive messages are enabled to be sent on this socket.
    ///
    /// On Unix, this option will set the `SO_KEEPALIVE` as well as the
    /// `TCP_KEEPALIVE` or `TCP_KEEPIDLE` option (depending on your platform).
    /// On Windows, this will set the `SIO_KEEPALIVE_VALS` option.
    ///
    /// If `None` is specified then keepalive messages are disabled, otherwise
    /// the duration specified will be the time to remain idle before sending a
    /// TCP keepalive probe.
    ///
    /// Some platforms specify this value in seconds, so sub-second
    /// specifications may be omitted.
    ///
    /// # 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_keepalive(None)?;
    /// # Ok(())
    /// # }
    /// ```
    pub fn set_keepalive(&self, keepalive: Option<Duration>) -> io::Result<()> {
        self.io.get_ref().set_keepalive(keepalive)
    }

    /// 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.get_ref().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.get_ref().set_ttl(ttl)
    }

    /// 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>> {
        self.io.get_ref().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<()> {
        self.io.get_ref().set_linger(dur)
    }

    // 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(Write)`] on the `TcpStream`.
    ///
    /// [`split`]: TcpStream::split()
    /// [`shutdown(Write)`]: fn@crate::net::TcpStream::shutdown
    pub fn into_split(self) -> (OwnedReadHalf, OwnedWriteHalf) {
        split_owned(self)
    }

    // == Poll IO functions that takes `&self` ==
    //
    // They are not public because (taken from the doc of `PollEvented`):
    //
    // While `PollEvented` is `Sync` (if the underlying I/O type is `Sync`), the
    // caller must ensure that there are at most two tasks that use a
    // `PollEvented` instance concurrently. One for reading and one for writing.
    // While violating this requirement is "safe" from a Rust memory model point
    // of view, it will result in unexpected behavior in the form of lost
    // notifications and tasks hanging.

    pub(crate) fn poll_read_priv(
        &self,
        cx: &mut Context<'_>,
        buf: &mut [u8],
    ) -> Poll<io::Result<usize>> {
        ready!(self.io.poll_read_ready(cx, mio::Ready::readable()))?;

        match self.io.get_ref().read(buf) {
            Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
                self.io.clear_read_ready(cx, mio::Ready::readable())?;
                Poll::Pending
            }
            x => Poll::Ready(x),
        }
    }

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

        match self.io.get_ref().write(buf) {
            Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
                self.io.clear_write_ready(cx)?;
                Poll::Pending
            }
            x => Poll::Ready(x),
        }
    }

    pub(super) fn poll_write_buf_priv<B: Buf>(
        &self,
        cx: &mut Context<'_>,
        buf: &mut B,
    ) -> Poll<io::Result<usize>> {
        use std::io::IoSlice;

        ready!(self.io.poll_write_ready(cx))?;

        // The `IoVec` (v0.1.x) type can't have a zero-length size, so create
        // a dummy version from a 1-length slice which we'll overwrite with
        // the `bytes_vectored` method.
        static S: &[u8] = &[0];
        const MAX_BUFS: usize = 64;

        // IoSlice isn't Copy, so we must expand this manually ;_;
        let mut slices: [IoSlice<'_>; MAX_BUFS] = [
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
            IoSlice::new(S),
        ];
        let cnt = buf.bytes_vectored(&mut slices);

        let iovec = <&IoVec>::from(S);
        let mut vecs = [iovec; MAX_BUFS];
        for i in 0..cnt {
            vecs[i] = (*slices[i]).into();
        }

        match self.io.get_ref().write_bufs(&vecs[..cnt]) {
            Ok(n) => {
                buf.advance(n);
                Poll::Ready(Ok(n))
            }
            Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
                self.io.clear_write_ready(cx)?;
                Poll::Pending
            }
            Err(e) => Poll::Ready(Err(e)),
        }
    }
}

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

    /// Consumes value, returning the mio I/O object.
    ///
    /// See [`PollEvented::into_inner`] for more details about
    /// resource deregistration that happens during the call.
    ///
    /// [`PollEvented::into_inner`]: crate::io::PollEvented::into_inner
    fn try_from(value: TcpStream) -> Result<Self, Self::Error> {
        value.io.into_inner()
    }
}

impl TryFrom<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: net::TcpStream) -> Result<Self, Self::Error> {
        Self::from_std(stream)
    }
}

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

impl AsyncRead for TcpStream {
    unsafe fn prepare_uninitialized_buffer(&self, _: &mut [MaybeUninit<u8>]) -> bool {
        false
    }

    fn poll_read(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        buf: &mut [u8],
    ) -> Poll<io::Result<usize>> {
        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_buf<B: Buf>(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        buf: &mut B,
    ) -> Poll<io::Result<usize>> {
        self.poll_write_buf_priv(cx, buf)
    }

    #[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::net::Shutdown::Write)?;
        Poll::Ready(Ok(()))
    }
}

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

#[cfg(windows)]
mod sys {
    // TODO: let's land these upstream with mio and then we can add them here.
    //
    // use std::os::windows::prelude::*;
    // use super::TcpStream;
    //
    // impl AsRawHandle for TcpStream {
    //     fn as_raw_handle(&self) -> RawHandle {
    //         self.io.get_ref().as_raw_handle()
    //     }
    // }
}
