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    //! `UnixStream` owned split support.
    //!
    //! A `UnixStream` can be split into an `OwnedReadHalf` and a `OwnedWriteHalf`
    //! with the `UnixStream::into_split` method.  `OwnedReadHalf` implements
    //! `AsyncRead` while `OwnedWriteHalf` implements `AsyncWrite`.
    //!
    //! Compared to the generic split of `AsyncRead + AsyncWrite`, this specialized
    //! split has no associated overhead and enforces all invariants at the type
    //! level.

    use crate::io::{AsyncRead, AsyncWrite, ReadBuf};
    use crate::net::UnixStream;

    use std::error::Error;
    use std::net::Shutdown;
    use std::pin::Pin;
    use std::sync::Arc;
    use std::task::{Context, Poll};
    use std::{fmt, io};

    /// Owned read half of a [`UnixStream`], created by [`into_split`].
    ///
    /// Reading from an `OwnedReadHalf` is usually done using the convenience methods found
    /// on the [`AsyncReadExt`] trait.
    ///
    /// [`UnixStream`]: crate::net::UnixStream
    /// [`into_split`]: crate::net::UnixStream::into_split()
    /// [`AsyncReadExt`]: trait@crate::io::AsyncReadExt
    #[derive(Debug)]
    pub struct OwnedReadHalf {
        inner: Arc<UnixStream>,
    }

    /// Owned write half of a [`UnixStream`], created by [`into_split`].
    ///
    /// Note that in the [`AsyncWrite`] implementation of this type,
    /// [`poll_shutdown`] will shut down the stream in the write direction.
    /// Dropping the write half will also shut down the write half of the stream.
    ///
    /// Writing to an `OwnedWriteHalf` is usually done using the convenience methods
    /// found on the [`AsyncWriteExt`] trait.
    ///
    /// [`UnixStream`]: crate::net::UnixStream
    /// [`into_split`]: crate::net::UnixStream::into_split()
    /// [`AsyncWrite`]: trait@crate::io::AsyncWrite
    /// [`poll_shutdown`]: fn@crate::io::AsyncWrite::poll_shutdown
    /// [`AsyncWriteExt`]: trait@crate::io::AsyncWriteExt
    #[derive(Debug)]
    pub struct OwnedWriteHalf {
        inner: Arc<UnixStream>,
        shutdown_on_drop: bool,
    }

    pub(crate) fn split_owned(stream: UnixStream) -> (OwnedReadHalf, OwnedWriteHalf) {
        let arc = Arc::new(stream);
        let read = OwnedReadHalf {
            inner: Arc::clone(&arc),
        };
        let write = OwnedWriteHalf {
            inner: arc,
            shutdown_on_drop: true,
        };
        (read, write)
    }

    pub(crate) fn reunite(
        read: OwnedReadHalf,
        write: OwnedWriteHalf,
    ) -> Result<UnixStream, ReuniteError> {
        if Arc::ptr_eq(&read.inner, &write.inner) {
            write.forget();
            // This unwrap cannot fail as the api does not allow creating more than two Arcs,
            // and we just dropped the other half.
            Ok(Arc::try_unwrap(read.inner).expect("UnixStream: try_unwrap failed in reunite"))
        } else {
            Err(ReuniteError(read, write))
        }
    }

    /// Error indicating that two halves were not from the same socket, and thus could
    /// not be reunited.
    #[derive(Debug)]
    pub struct ReuniteError(pub OwnedReadHalf, pub OwnedWriteHalf);

    impl fmt::Display for ReuniteError {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            write!(
                f,
                "tried to reunite halves that are not from the same socket"
            )
        }
    }

    impl Error for ReuniteError {}

    impl OwnedReadHalf {
        /// Attempts to put the two halves of a `UnixStream` back together and
        /// recover the original socket. Succeeds only if the two halves
        /// originated from the same call to [`into_split`].
        ///
        /// [`into_split`]: crate::net::UnixStream::into_split()
        pub fn reunite(self, other: OwnedWriteHalf) -> Result<UnixStream, ReuniteError> {
            reunite(self, other)
        }
    }

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

    impl OwnedWriteHalf {
        /// Attempts to put the two halves of a `UnixStream` back together and
        /// recover the original socket. Succeeds only if the two halves
        /// originated from the same call to [`into_split`].
        ///
        /// [`into_split`]: crate::net::UnixStream::into_split()
        pub fn reunite(self, other: OwnedReadHalf) -> Result<UnixStream, ReuniteError> {
            reunite(other, self)
        }

        /// Destroy the write half, but don't close the write half of the stream
        /// until the read half is dropped. If the read half has already been
        /// dropped, this closes the stream.
        pub fn forget(mut self) {
            self.shutdown_on_drop = false;
            drop(self);
        }
    }

    impl Drop for OwnedWriteHalf {
        fn drop(&mut self) {
            if self.shutdown_on_drop {
                let _ = self.inner.shutdown_std(Shutdown::Write);
            }
        }
    }

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

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

        fn is_write_vectored(&self) -> bool {
            self.inner.is_write_vectored()
        }

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

        // `poll_shutdown` on a write half shutdowns the stream in the "write" direction.
        fn poll_shutdown(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<io::Result<()>> {
            let res = self.inner.shutdown_std(Shutdown::Write);
            if res.is_ok() {
                Pin::into_inner(self).shutdown_on_drop = false;
            }
            res.into()
        }
    }

    impl AsRef<UnixStream> for OwnedReadHalf {
        fn as_ref(&self) -> &UnixStream {
            &*self.inner
        }
    }

    impl AsRef<UnixStream> for OwnedWriteHalf {
        fn as_ref(&self) -> &UnixStream {
            &*self.inner
        }
    }