#
actix_codec/
framed.rs

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    use std::{
        fmt, io,
        pin::Pin,
        task::{Context, Poll},
    };

    use bitflags::bitflags;
    use bytes::{Buf, BytesMut};
    use futures_core::{ready, Stream};
    use futures_sink::Sink;
    use pin_project_lite::pin_project;

    use crate::{AsyncRead, AsyncWrite, Decoder, Encoder};

    /// Low-water mark
    const LW: usize = 1024;
    /// High-water mark
    const HW: usize = 8 * 1024;

    bitflags! {
        #[derive(Debug, Clone, Copy)]
        struct Flags: u8 {
            const EOF = 0b0001;
            const READABLE = 0b0010;
        }
    }

    pin_project! {
        /// A unified `Stream` and `Sink` interface to an underlying I/O object, using the `Encoder` and
        /// `Decoder` traits to encode and decode frames.
        ///
        /// Raw I/O objects work with byte sequences, but higher-level code usually wants to batch these
        /// into meaningful chunks, called "frames". This method layers framing on top of an I/O object,
        /// by using the `Encoder`/`Decoder` traits to handle encoding and decoding of message frames.
        /// Note that the incoming and outgoing frame types may be distinct.
        pub struct Framed<T, U> {
            #[pin]
            io: T,
            codec: U,
            flags: Flags,
            read_buf: BytesMut,
            write_buf: BytesMut,
        }
    }

    impl<T, U> Framed<T, U>
    where
        T: AsyncRead + AsyncWrite,
        U: Decoder,
    {
        /// This function returns a *single* object that is both `Stream` and `Sink`; grouping this into
        /// a single object is often useful for layering things like gzip or TLS, which require both
        /// read and write access to the underlying object.
        pub fn new(io: T, codec: U) -> Framed<T, U> {
            Framed {
                io,
                codec,
                flags: Flags::empty(),
                read_buf: BytesMut::with_capacity(HW),
                write_buf: BytesMut::with_capacity(HW),
            }
        }
    }

    impl<T, U> Framed<T, U> {
        /// Returns a reference to the underlying codec.
        pub fn codec_ref(&self) -> &U {
            &self.codec
        }

        /// Returns a mutable reference to the underlying codec.
        pub fn codec_mut(&mut self) -> &mut U {
            &mut self.codec
        }

        /// Returns a reference to the underlying I/O stream wrapped by `Frame`.
        ///
        /// Note that care should be taken to not tamper with the underlying stream of data coming in as
        /// it may corrupt the stream of frames otherwise being worked with.
        pub fn io_ref(&self) -> &T {
            &self.io
        }

        /// Returns a mutable reference to the underlying I/O stream.
        ///
        /// Note that care should be taken to not tamper with the underlying stream of data coming in as
        /// it may corrupt the stream of frames otherwise being worked with.
        pub fn io_mut(&mut self) -> &mut T {
            &mut self.io
        }

        /// Returns a `Pin` of a mutable reference to the underlying I/O stream.
        pub fn io_pin(self: Pin<&mut Self>) -> Pin<&mut T> {
            self.project().io
        }

        /// Check if read buffer is empty.
        pub fn is_read_buf_empty(&self) -> bool {
            self.read_buf.is_empty()
        }

        /// Check if write buffer is empty.
        pub fn is_write_buf_empty(&self) -> bool {
            self.write_buf.is_empty()
        }

        /// Check if write buffer is full.
        pub fn is_write_buf_full(&self) -> bool {
            self.write_buf.len() >= HW
        }

        /// Check if framed is able to write more data.
        ///
        /// `Framed` object considers ready if there is free space in write buffer.
        pub fn is_write_ready(&self) -> bool {
            self.write_buf.len() < HW
        }

        /// Consume the `Frame`, returning `Frame` with different codec.
        pub fn replace_codec<U2>(self, codec: U2) -> Framed<T, U2> {
            Framed {
                codec,
                io: self.io,
                flags: self.flags,
                read_buf: self.read_buf,
                write_buf: self.write_buf,
            }
        }

        /// Consume the `Frame`, returning `Frame` with different io.
        pub fn into_map_io<F, T2>(self, f: F) -> Framed<T2, U>
        where
            F: Fn(T) -> T2,
        {
            Framed {
                io: f(self.io),
                codec: self.codec,
                flags: self.flags,
                read_buf: self.read_buf,
                write_buf: self.write_buf,
            }
        }

        /// Consume the `Frame`, returning `Frame` with different codec.
        pub fn into_map_codec<F, U2>(self, f: F) -> Framed<T, U2>
        where
            F: Fn(U) -> U2,
        {
            Framed {
                io: self.io,
                codec: f(self.codec),
                flags: self.flags,
                read_buf: self.read_buf,
                write_buf: self.write_buf,
            }
        }
    }

    impl<T, U> Framed<T, U> {
        /// Serialize item and write to the inner buffer
        pub fn write<I>(mut self: Pin<&mut Self>, item: I) -> Result<(), <U as Encoder<I>>::Error>
        where
            T: AsyncWrite,
            U: Encoder<I>,
        {
            let this = self.as_mut().project();
            let remaining = this.write_buf.capacity() - this.write_buf.len();
            if remaining < LW {
                this.write_buf.reserve(HW - remaining);
            }

            this.codec.encode(item, this.write_buf)?;
            Ok(())
        }

        /// Try to read underlying I/O stream and decode item.
        pub fn next_item(
            mut self: Pin<&mut Self>,
            cx: &mut Context<'_>,
        ) -> Poll<Option<Result<<U as Decoder>::Item, U::Error>>>
        where
            T: AsyncRead,
            U: Decoder,
        {
            loop {
                let this = self.as_mut().project();
                // Repeatedly call `decode` or `decode_eof` as long as it is "readable". Readable is
                // defined as not having returned `None`. If the upstream has returned EOF, and the
                // decoder is no longer readable, it can be assumed that the decoder will never become
                // readable again, at which point the stream is terminated.

                if this.flags.contains(Flags::READABLE) {
                    if this.flags.contains(Flags::EOF) {
                        match this.codec.decode_eof(this.read_buf) {
                            Ok(Some(frame)) => return Poll::Ready(Some(Ok(frame))),
                            Ok(None) => return Poll::Ready(None),
                            Err(err) => return Poll::Ready(Some(Err(err))),
                        }
                    }

                    tracing::trace!("attempting to decode a frame");

                    match this.codec.decode(this.read_buf) {
                        Ok(Some(frame)) => {
                            tracing::trace!("frame decoded from buffer");
                            return Poll::Ready(Some(Ok(frame)));
                        }
                        Err(err) => return Poll::Ready(Some(Err(err))),
                        _ => (), // Need more data
                    }

                    this.flags.remove(Flags::READABLE);
                }

                debug_assert!(!this.flags.contains(Flags::EOF));

                // Otherwise, try to read more data and try again. Make sure we've got room.
                let remaining = this.read_buf.capacity() - this.read_buf.len();
                if remaining < LW {
                    this.read_buf.reserve(HW - remaining)
                }

                let cnt = match tokio_util::io::poll_read_buf(this.io, cx, this.read_buf) {
                    Poll::Pending => return Poll::Pending,
                    Poll::Ready(Err(err)) => return Poll::Ready(Some(Err(err.into()))),
                    Poll::Ready(Ok(cnt)) => cnt,
                };

                if cnt == 0 {
                    this.flags.insert(Flags::EOF);
                }
                this.flags.insert(Flags::READABLE);
            }
        }

        /// Flush write buffer to underlying I/O stream.
        pub fn flush<I>(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), U::Error>>
        where
            T: AsyncWrite,
            U: Encoder<I>,
        {
            let mut this = self.as_mut().project();
            tracing::trace!("flushing framed transport");

            while !this.write_buf.is_empty() {
                tracing::trace!("writing; remaining={}", this.write_buf.len());

                let n = ready!(this.io.as_mut().poll_write(cx, this.write_buf))?;

                if n == 0 {
                    return Poll::Ready(Err(io::Error::new(
                        io::ErrorKind::WriteZero,
                        "failed to write frame to transport",
                    )
                    .into()));
                }

                // remove written data
                this.write_buf.advance(n);
            }

            // Try flushing the underlying IO
            ready!(this.io.poll_flush(cx))?;

            tracing::trace!("framed transport flushed");
            Poll::Ready(Ok(()))
        }

        /// Flush write buffer and shutdown underlying I/O stream.
        pub fn close<I>(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), U::Error>>
        where
            T: AsyncWrite,
            U: Encoder<I>,
        {
            let mut this = self.as_mut().project();
            ready!(this.io.as_mut().poll_flush(cx))?;
            ready!(this.io.as_mut().poll_shutdown(cx))?;
            Poll::Ready(Ok(()))
        }
    }

    impl<T, U> Stream for Framed<T, U>
    where
        T: AsyncRead,
        U: Decoder,
    {
        type Item = Result<U::Item, U::Error>;

        fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
            self.next_item(cx)
        }
    }

    impl<T, U, I> Sink<I> for Framed<T, U>
    where
        T: AsyncWrite,
        U: Encoder<I>,
        U::Error: From<io::Error>,
    {
        type Error = U::Error;

        fn poll_ready(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
            if self.is_write_ready() {
                Poll::Ready(Ok(()))
            } else {
                self.flush(cx)
            }
        }

        fn start_send(self: Pin<&mut Self>, item: I) -> Result<(), Self::Error> {
            self.write(item)
        }

        fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
            self.flush(cx)
        }

        fn poll_close(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
            self.close(cx)
        }
    }

    impl<T, U> fmt::Debug for Framed<T, U>
    where
        T: fmt::Debug,
        U: fmt::Debug,
    {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("Framed")
                .field("io", &self.io)
                .field("codec", &self.codec)
                .finish()
        }
    }

    impl<T, U> Framed<T, U> {
        /// This function returns a *single* object that is both `Stream` and `Sink`; grouping this into
        /// a single object is often useful for layering things like gzip or TLS, which require both
        /// read and write access to the underlying object.
        ///
        /// These objects take a stream, a read buffer and a write buffer. These fields can be obtained
        /// from an existing `Framed` with the `into_parts` method.
        pub fn from_parts(parts: FramedParts<T, U>) -> Framed<T, U> {
            Framed {
                io: parts.io,
                codec: parts.codec,
                flags: parts.flags,
                write_buf: parts.write_buf,
                read_buf: parts.read_buf,
            }
        }

        /// Consumes the `Frame`, returning its underlying I/O stream, the buffer with unprocessed data,
        /// and the codec.
        ///
        /// Note that care should be taken to not tamper with the underlying stream of data coming in as
        /// it may corrupt the stream of frames otherwise being worked with.
        pub fn into_parts(self) -> FramedParts<T, U> {
            FramedParts {
                io: self.io,
                codec: self.codec,
                flags: self.flags,
                read_buf: self.read_buf,
                write_buf: self.write_buf,
            }
        }
    }

    /// `FramedParts` contains an export of the data of a Framed transport.
    ///
    /// It can be used to construct a new `Framed` with a different codec. It contains all current
    /// buffers and the inner transport.
    #[derive(Debug)]
    pub struct FramedParts<T, U> {
        /// The inner transport used to read bytes to and write bytes to.
        pub io: T,

        /// The codec object.
        pub codec: U,

        /// The buffer with read but unprocessed data.
        pub read_buf: BytesMut,

        /// A buffer with unprocessed data which are not written yet.
        pub write_buf: BytesMut,

        flags: Flags,
    }

    impl<T, U> FramedParts<T, U> {
        /// Creates a new default `FramedParts`.
        pub fn new(io: T, codec: U) -> FramedParts<T, U> {
            FramedParts {
                io,
                codec,
                flags: Flags::empty(),
                read_buf: BytesMut::new(),
                write_buf: BytesMut::new(),
            }
        }

        /// Creates a new `FramedParts` with read buffer.
        pub fn with_read_buf(io: T, codec: U, read_buf: BytesMut) -> FramedParts<T, U> {
            FramedParts {
                io,
                codec,
                read_buf,
                flags: Flags::empty(),
                write_buf: BytesMut::new(),
            }
        }
    }