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    //! In-process memory IO types.

    use crate::io::{AsyncRead, AsyncWrite, ReadBuf};
    use crate::loom::sync::Mutex;

    use bytes::{Buf, BytesMut};
    use std::{
        pin::Pin,
        sync::Arc,
        task::{self, Poll, Waker},
    };

    /// A bidirectional pipe to read and write bytes in memory.
    ///
    /// A pair of `DuplexStream`s are created together, and they act as a "channel"
    /// that can be used as in-memory IO types. Writing to one of the pairs will
    /// allow that data to be read from the other, and vice versa.
    ///
    /// # Example
    ///
    /// ```
    /// # async fn ex() -> std::io::Result<()> {
    /// # use tokio::io::{AsyncReadExt, AsyncWriteExt};
    /// let (mut client, mut server) = tokio::io::duplex(64);
    ///
    /// client.write_all(b"ping").await?;
    ///
    /// let mut buf = [0u8; 4];
    /// server.read_exact(&mut buf).await?;
    /// assert_eq!(&buf, b"ping");
    ///
    /// server.write_all(b"pong").await?;
    ///
    /// client.read_exact(&mut buf).await?;
    /// assert_eq!(&buf, b"pong");
    /// # Ok(())
    /// # }
    /// ```
    #[derive(Debug)]
    pub struct DuplexStream {
        read: Arc<Mutex<Pipe>>,
        write: Arc<Mutex<Pipe>>,
    }

    /// A unidirectional IO over a piece of memory.
    ///
    /// Data can be written to the pipe, and reading will return that data.
    #[derive(Debug)]
    struct Pipe {
        /// The buffer storing the bytes written, also read from.
        ///
        /// Using a `BytesMut` because it has efficient `Buf` and `BufMut`
        /// functionality already. Additionally, it can try to copy data in the
        /// same buffer if there read index has advanced far enough.
        buffer: BytesMut,
        /// Determines if the write side has been closed.
        is_closed: bool,
        /// The maximum amount of bytes that can be written before returning
        /// `Poll::Pending`.
        max_buf_size: usize,
        /// If the `read` side has been polled and is pending, this is the waker
        /// for that parked task.
        read_waker: Option<Waker>,
        /// If the `write` side has filled the `max_buf_size` and returned
        /// `Poll::Pending`, this is the waker for that parked task.
        write_waker: Option<Waker>,
    }

    // ===== impl DuplexStream =====

    /// Create a new pair of `DuplexStream`s that act like a pair of connected sockets.
    ///
    /// The `max_buf_size` argument is the maximum amount of bytes that can be
    /// written to a side before the write returns `Poll::Pending`.
    pub fn duplex(max_buf_size: usize) -> (DuplexStream, DuplexStream) {
        let one = Arc::new(Mutex::new(Pipe::new(max_buf_size)));
        let two = Arc::new(Mutex::new(Pipe::new(max_buf_size)));

        (
            DuplexStream {
                read: one.clone(),
                write: two.clone(),
            },
            DuplexStream {
                read: two,
                write: one,
            },
        )
    }

    impl AsyncRead for DuplexStream {
        // Previous rustc required this `self` to be `mut`, even though newer
        // versions recognize it isn't needed to call `lock()`. So for
        // compatibility, we include the `mut` and `allow` the lint.
        //
        // See https://github.com/rust-lang/rust/issues/73592
        #[allow(unused_mut)]
        fn poll_read(
            mut self: Pin<&mut Self>,
            cx: &mut task::Context<'_>,
            buf: &mut ReadBuf<'_>,
        ) -> Poll<std::io::Result<()>> {
            Pin::new(&mut *self.read.lock()).poll_read(cx, buf)
        }
    }

    impl AsyncWrite for DuplexStream {
        #[allow(unused_mut)]
        fn poll_write(
            mut self: Pin<&mut Self>,
            cx: &mut task::Context<'_>,
            buf: &[u8],
        ) -> Poll<std::io::Result<usize>> {
            Pin::new(&mut *self.write.lock()).poll_write(cx, buf)
        }

        #[allow(unused_mut)]
        fn poll_flush(
            mut self: Pin<&mut Self>,
            cx: &mut task::Context<'_>,
        ) -> Poll<std::io::Result<()>> {
            Pin::new(&mut *self.write.lock()).poll_flush(cx)
        }

        #[allow(unused_mut)]
        fn poll_shutdown(
            mut self: Pin<&mut Self>,
            cx: &mut task::Context<'_>,
        ) -> Poll<std::io::Result<()>> {
            Pin::new(&mut *self.write.lock()).poll_shutdown(cx)
        }
    }

    impl Drop for DuplexStream {
        fn drop(&mut self) {
            // notify the other side of the closure
            self.write.lock().close();
        }
    }

    // ===== impl Pipe =====

    impl Pipe {
        fn new(max_buf_size: usize) -> Self {
            Pipe {
                buffer: BytesMut::new(),
                is_closed: false,
                max_buf_size,
                read_waker: None,
                write_waker: None,
            }
        }

        fn close(&mut self) {
            self.is_closed = true;
            if let Some(waker) = self.read_waker.take() {
                waker.wake();
            }
        }
    }

    impl AsyncRead for Pipe {
        fn poll_read(
            mut self: Pin<&mut Self>,
            cx: &mut task::Context<'_>,
            buf: &mut ReadBuf<'_>,
        ) -> Poll<std::io::Result<()>> {
            if self.buffer.has_remaining() {
                let max = self.buffer.remaining().min(buf.remaining());
                buf.put_slice(&self.buffer[..max]);
                self.buffer.advance(max);
                if max > 0 {
                    // The passed `buf` might have been empty, don't wake up if
                    // no bytes have been moved.
                    if let Some(waker) = self.write_waker.take() {
                        waker.wake();
                    }
                }
                Poll::Ready(Ok(()))
            } else if self.is_closed {
                Poll::Ready(Ok(()))
            } else {
                self.read_waker = Some(cx.waker().clone());
                Poll::Pending
            }
        }
    }

    impl AsyncWrite for Pipe {
        fn poll_write(
            mut self: Pin<&mut Self>,
            cx: &mut task::Context<'_>,
            buf: &[u8],
        ) -> Poll<std::io::Result<usize>> {
            if self.is_closed {
                return Poll::Ready(Err(std::io::ErrorKind::BrokenPipe.into()));
            }
            let avail = self.max_buf_size - self.buffer.len();
            if avail == 0 {
                self.write_waker = Some(cx.waker().clone());
                return Poll::Pending;
            }

            let len = buf.len().min(avail);
            self.buffer.extend_from_slice(&buf[..len]);
            if let Some(waker) = self.read_waker.take() {
                waker.wake();
            }
            Poll::Ready(Ok(len))
        }

        fn poll_flush(self: Pin<&mut Self>, _: &mut task::Context<'_>) -> Poll<std::io::Result<()>> {
            Poll::Ready(Ok(()))
        }

        fn poll_shutdown(
            mut self: Pin<&mut Self>,
            _: &mut task::Context<'_>,
        ) -> Poll<std::io::Result<()>> {
            self.close();
            Poll::Ready(Ok(()))
        }
    }