rust-libp2p/core/src/muxing.rs

// Copyright 2018 Parity Technologies (UK) Ltd.
//
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//! Muxing is the process of splitting a connection into multiple substreams.
//!
//! The main item of this module is the `StreamMuxer` trait. An implementation of `StreamMuxer`
//! has ownership of a connection, lets you open and close substreams, and read/write data
//! on open substreams.
//!
//! > **Note**: You normally don't need to use the methods of the `StreamMuxer` directly, as this
//! >           is managed by the library's internals.
//!
//! Each substream of a connection is an isolated stream of data. All the substreams are muxed
//! together so that the data read from or written to each substream doesn't influence the other
//! substreams.
//!
//! In the context of libp2p, each substream can use a different protocol. Contrary to opening a
//! connection, opening a substream is almost free in terms of resources. This means that you
//! shouldn't hesitate to rapidly open and close substreams, and to design protocols that don't
//! require maintaining long-lived channels of communication.
//!
//! > **Example**: The Kademlia protocol opens a new substream for each request it wants to
//! >              perform. Multiple requests can be performed simultaneously by opening multiple
//! >              substreams, without having to worry about associating responses with the
//! >              right request.
//!
//! # Implementing a muxing protocol
//!
//! In order to implement a muxing protocol, create an object that implements the `UpgradeInfo`,
//! `InboundUpgrade` and `OutboundUpgrade` traits. See the `upgrade` module for more information.
//! The `Output` associated type of the `InboundUpgrade` and `OutboundUpgrade` traits should be
//! identical, and should be an object that implements the `StreamMuxer` trait.
//!
//! The upgrade process will take ownership of the connection, which makes it possible for the
//! implementation of `StreamMuxer` to control everything that happens on the wire.

use futures::{future, prelude::*, task::Context, task::Poll};
use multiaddr::Multiaddr;
use std::{fmt, io, ops::Deref, pin::Pin};

pub use self::boxed::StreamMuxerBox;
pub use self::singleton::SingletonMuxer;

mod boxed;
mod singleton;

/// Implemented on objects that can open and manage substreams.
///
/// The state of a muxer, as exposed by this API, is the following:
///
/// - A connection to the remote. The `poll_event`, `flush_all` and `close` methods operate
///   on this.
/// - A list of substreams that are open. The `poll_outbound`, `read_substream`, `write_substream`,
///   `flush_substream`, `shutdown_substream` and `destroy_substream` methods allow controlling
///   these entries.
/// - A list of outbound substreams being opened. The `open_outbound`, `poll_outbound` and
///   `destroy_outbound` methods allow controlling these entries.
///
pub trait StreamMuxer {
    /// Type of the object that represents the raw substream where data can be read and written.
    type Substream;

    /// Future that will be resolved when the outgoing substream is open.
    type OutboundSubstream;

    /// Error type of the muxer
    type Error: Into<io::Error>;

    /// Polls for a connection-wide event.
    ///
    /// This function behaves the same as a `Stream`.
    ///
    /// If `Pending` is returned, then the current task will be notified once the muxer
    /// is ready to be polled, similar to the API of `Stream::poll()`.
    /// Only the latest task that was used to call this method may be notified.
    ///
    /// It is permissible and common to use this method to perform background
    /// work, such as processing incoming packets and polling timers.
    ///
    /// An error can be generated if the connection has been closed.
    fn poll_event(
        &self,
        cx: &mut Context<'_>,
    ) -> Poll<Result<StreamMuxerEvent<Self::Substream>, Self::Error>>;

    /// Opens a new outgoing substream, and produces the equivalent to a future that will be
    /// resolved when it becomes available.
    ///
    /// The API of `OutboundSubstream` is totally opaque, and the object can only be interfaced
    /// through the methods on the `StreamMuxer` trait.
    fn open_outbound(&self) -> Self::OutboundSubstream;

    /// Polls the outbound substream.
    ///
    /// If `Pending` is returned, then the current task will be notified once the substream
    /// is ready to be polled, similar to the API of `Future::poll()`.
    /// However, for each individual outbound substream, only the latest task that was used to
    /// call this method may be notified.
    ///
    /// May panic or produce an undefined result if an earlier polling of the same substream
    /// returned `Ready` or `Err`.
    fn poll_outbound(
        &self,
        cx: &mut Context<'_>,
        s: &mut Self::OutboundSubstream,
    ) -> Poll<Result<Self::Substream, Self::Error>>;

    /// Destroys an outbound substream future. Use this after the outbound substream has finished,
    /// or if you want to interrupt it.
    fn destroy_outbound(&self, s: Self::OutboundSubstream);

    /// Reads data from a substream. The behaviour is the same as `futures::AsyncRead::poll_read`.
    ///
    /// If `Pending` is returned, then the current task will be notified once the substream
    /// is ready to be read. However, for each individual substream, only the latest task that
    /// was used to call this method may be notified.
    ///
    /// If `Async::Ready(0)` is returned, the substream has been closed by the remote and should
    /// no longer be read afterwards.
    ///
    /// An error can be generated if the connection has been closed, or if a protocol misbehaviour
    /// happened.
    fn read_substream(
        &self,
        cx: &mut Context<'_>,
        s: &mut Self::Substream,
        buf: &mut [u8],
    ) -> Poll<Result<usize, Self::Error>>;

    /// Write data to a substream. The behaviour is the same as `futures::AsyncWrite::poll_write`.
    ///
    /// If `Pending` is returned, then the current task will be notified once the substream
    /// is ready to be read. For each individual substream, only the latest task that was used to
    /// call this method may be notified.
    ///
    /// Calling `write_substream` does not guarantee that data will arrive to the remote. To
    /// ensure that, you should call `flush_substream`.
    ///
    /// It is incorrect to call this method on a substream if you called `shutdown_substream` on
    /// this substream earlier.
    fn write_substream(
        &self,
        cx: &mut Context<'_>,
        s: &mut Self::Substream,
        buf: &[u8],
    ) -> Poll<Result<usize, Self::Error>>;

    /// Flushes a substream. The behaviour is the same as `futures::AsyncWrite::poll_flush`.
    ///
    /// After this method has been called, data written earlier on the substream is guaranteed to
    /// be received by the remote.
    ///
    /// If `Pending` is returned, then the current task will be notified once the substream
    /// is ready to be read. For each individual substream, only the latest task that was used to
    /// call this method may be notified.
    ///
    /// > **Note**: This method may be implemented as a call to `flush_all`.
    fn flush_substream(
        &self,
        cx: &mut Context<'_>,
        s: &mut Self::Substream,
    ) -> Poll<Result<(), Self::Error>>;

    /// Attempts to shut down the writing side of a substream. The behaviour is similar to
    /// `AsyncWrite::poll_close`.
    ///
    /// Contrary to `AsyncWrite::poll_close`, shutting down a substream does not imply
    /// `flush_substream`. If you want to make sure that the remote is immediately informed about
    /// the shutdown, use `flush_substream` or `flush_all`.
    ///
    /// After this method has been called, you should no longer attempt to write to this substream.
    ///
    /// An error can be generated if the connection has been closed, or if a protocol misbehaviour
    /// happened.
    fn shutdown_substream(
        &self,
        cx: &mut Context<'_>,
        s: &mut Self::Substream,
    ) -> Poll<Result<(), Self::Error>>;

    /// Destroys a substream.
    fn destroy_substream(&self, s: Self::Substream);

    /// Closes this `StreamMuxer`.
    ///
    /// After this has returned `Poll::Ready(Ok(()))`, the muxer has become useless. All
    /// subsequent reads must return either `EOF` or an error. All subsequent writes, shutdowns,
    /// or polls must generate an error or be ignored.
    ///
    /// Calling this method implies `flush_all`.
    ///
    /// > **Note**: You are encouraged to call this method and wait for it to return `Ready`, so
    /// >           that the remote is properly informed of the shutdown. However, apart from
    /// >           properly informing the remote, there is no difference between this and
    /// >           immediately dropping the muxer.
    fn poll_close(&self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>>;
}

/// Event about a connection, reported by an implementation of [`StreamMuxer`].
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum StreamMuxerEvent<T> {
    /// Remote has opened a new substream. Contains the substream in question.
    InboundSubstream(T),

    /// Address to the remote has changed. The previous one is now obsolete.
    ///
    /// > **Note**: This can for example happen when using the QUIC protocol, where the two nodes
    /// >           can change their IP address while retaining the same QUIC connection.
    AddressChange(Multiaddr),
}

impl<T> StreamMuxerEvent<T> {
    /// If `self` is a [`StreamMuxerEvent::InboundSubstream`], returns the content. Otherwise
    /// returns `None`.
    pub fn into_inbound_substream(self) -> Option<T> {
        if let StreamMuxerEvent::InboundSubstream(s) = self {
            Some(s)
        } else {
            None
        }
    }

    /// Map the stream within [`StreamMuxerEvent::InboundSubstream`] to a new type.
    pub fn map_inbound_stream<O>(self, map: impl FnOnce(T) -> O) -> StreamMuxerEvent<O> {
        match self {
            StreamMuxerEvent::InboundSubstream(stream) => {
                StreamMuxerEvent::InboundSubstream(map(stream))
            }
            StreamMuxerEvent::AddressChange(addr) => StreamMuxerEvent::AddressChange(addr),
        }
    }
}

/// Polls for an event from the muxer and, if an inbound substream, wraps this substream in an
/// object that implements `Read`/`Write`/`AsyncRead`/`AsyncWrite`.
pub fn event_from_ref_and_wrap<P>(
    muxer: P,
) -> impl Future<Output = Result<StreamMuxerEvent<SubstreamRef<P>>, <P::Target as StreamMuxer>::Error>>
where
    P: Deref + Clone,
    P::Target: StreamMuxer,
{
    let muxer2 = muxer.clone();
    future::poll_fn(move |cx| muxer.poll_event(cx)).map_ok(|event| match event {
        StreamMuxerEvent::InboundSubstream(substream) => {
            StreamMuxerEvent::InboundSubstream(substream_from_ref(muxer2, substream))
        }
        StreamMuxerEvent::AddressChange(addr) => StreamMuxerEvent::AddressChange(addr),
    })
}

/// Same as `outbound_from_ref`, but wraps the output in an object that
/// implements `Read`/`Write`/`AsyncRead`/`AsyncWrite`.
pub fn outbound_from_ref_and_wrap<P>(muxer: P) -> OutboundSubstreamRefWrapFuture<P>
where
    P: Deref + Clone,
    P::Target: StreamMuxer,
{
    let inner = outbound_from_ref(muxer);
    OutboundSubstreamRefWrapFuture { inner }
}

/// Future returned by `outbound_from_ref_and_wrap`.
pub struct OutboundSubstreamRefWrapFuture<P>
where
    P: Deref + Clone,
    P::Target: StreamMuxer,
{
    inner: OutboundSubstreamRefFuture<P>,
}

impl<P> Future for OutboundSubstreamRefWrapFuture<P>
where
    P: Deref + Clone,
    P::Target: StreamMuxer,
{
    type Output = Result<SubstreamRef<P>, <P::Target as StreamMuxer>::Error>;

    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        match Future::poll(Pin::new(&mut self.inner), cx) {
            Poll::Ready(Ok(substream)) => {
                let out = substream_from_ref(self.inner.muxer.clone(), substream);
                Poll::Ready(Ok(out))
            }
            Poll::Pending => Poll::Pending,
            Poll::Ready(Err(err)) => Poll::Ready(Err(err)),
        }
    }
}

/// Builds a new future for an outbound substream, where the muxer is a reference.
pub fn outbound_from_ref<P>(muxer: P) -> OutboundSubstreamRefFuture<P>
where
    P: Deref,
    P::Target: StreamMuxer,
{
    let outbound = muxer.open_outbound();
    OutboundSubstreamRefFuture {
        muxer,
        outbound: Some(outbound),
    }
}

/// Future returned by `outbound_from_ref`.
pub struct OutboundSubstreamRefFuture<P>
where
    P: Deref,
    P::Target: StreamMuxer,
{
    muxer: P,
    outbound: Option<<P::Target as StreamMuxer>::OutboundSubstream>,
}

impl<P> Unpin for OutboundSubstreamRefFuture<P>
where
    P: Deref,
    P::Target: StreamMuxer,
{
}

impl<P> Future for OutboundSubstreamRefFuture<P>
where
    P: Deref,
    P::Target: StreamMuxer,
{
    type Output = Result<<P::Target as StreamMuxer>::Substream, <P::Target as StreamMuxer>::Error>;

    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        // We use a `this` because the compiler isn't smart enough to allow mutably borrowing
        // multiple different fields from the `Pin` at the same time.
        let this = &mut *self;
        this.muxer
            .poll_outbound(cx, this.outbound.as_mut().expect("outbound was empty"))
    }
}

impl<P> Drop for OutboundSubstreamRefFuture<P>
where
    P: Deref,
    P::Target: StreamMuxer,
{
    fn drop(&mut self) {
        self.muxer
            .destroy_outbound(self.outbound.take().expect("outbound was empty"))
    }
}

/// Builds an implementation of `Read`/`Write`/`AsyncRead`/`AsyncWrite` from an `Arc` to the
/// muxer and a substream.
pub fn substream_from_ref<P>(
    muxer: P,
    substream: <P::Target as StreamMuxer>::Substream,
) -> SubstreamRef<P>
where
    P: Deref,
    P::Target: StreamMuxer,
{
    SubstreamRef {
        muxer,
        substream: Some(substream),
        shutdown_state: ShutdownState::Shutdown,
    }
}

/// Stream returned by `substream_from_ref`.
pub struct SubstreamRef<P>
where
    P: Deref,
    P::Target: StreamMuxer,
{
    muxer: P,
    substream: Option<<P::Target as StreamMuxer>::Substream>,
    shutdown_state: ShutdownState,
}

enum ShutdownState {
    Shutdown,
    Flush,
    Done,
}

impl<P> fmt::Debug for SubstreamRef<P>
where
    P: Deref,
    P::Target: StreamMuxer,
    <P::Target as StreamMuxer>::Substream: fmt::Debug,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> Result<(), fmt::Error> {
        write!(f, "Substream({:?})", self.substream)
    }
}

impl<P> Unpin for SubstreamRef<P>
where
    P: Deref,
    P::Target: StreamMuxer,
{
}

impl<P> AsyncRead for SubstreamRef<P>
where
    P: Deref,
    P::Target: StreamMuxer,
{
    fn poll_read(
        mut self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        buf: &mut [u8],
    ) -> Poll<Result<usize, io::Error>> {
        // We use a `this` because the compiler isn't smart enough to allow mutably borrowing
        // multiple different fields from the `Pin` at the same time.
        let this = &mut *self;

        let s = this.substream.as_mut().expect("substream was empty");
        this.muxer.read_substream(cx, s, buf).map_err(|e| e.into())
    }
}

impl<P> AsyncWrite for SubstreamRef<P>
where
    P: Deref,
    P::Target: StreamMuxer,
{
    fn poll_write(
        mut self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        buf: &[u8],
    ) -> Poll<Result<usize, io::Error>> {
        // We use a `this` because the compiler isn't smart enough to allow mutably borrowing
        // multiple different fields from the `Pin` at the same time.
        let this = &mut *self;

        let s = this.substream.as_mut().expect("substream was empty");
        this.muxer.write_substream(cx, s, buf).map_err(|e| e.into())
    }

    fn poll_close(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), io::Error>> {
        // We use a `this` because the compiler isn't smart enough to allow mutably borrowing
        // multiple different fields from the `Pin` at the same time.
        let this = &mut *self;

        let s = this.substream.as_mut().expect("substream was empty");
        loop {
            match this.shutdown_state {
                ShutdownState::Shutdown => match this.muxer.shutdown_substream(cx, s) {
                    Poll::Ready(Ok(())) => this.shutdown_state = ShutdownState::Flush,
                    Poll::Ready(Err(err)) => return Poll::Ready(Err(err.into())),
                    Poll::Pending => return Poll::Pending,
                },
                ShutdownState::Flush => match this.muxer.flush_substream(cx, s) {
                    Poll::Ready(Ok(())) => this.shutdown_state = ShutdownState::Done,
                    Poll::Ready(Err(err)) => return Poll::Ready(Err(err.into())),
                    Poll::Pending => return Poll::Pending,
                },
                ShutdownState::Done => {
                    return Poll::Ready(Ok(()));
                }
            }
        }
    }

    fn poll_flush(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), io::Error>> {
        // We use a `this` because the compiler isn't smart enough to allow mutably borrowing
        // multiple different fields from the `Pin` at the same time.
        let this = &mut *self;

        let s = this.substream.as_mut().expect("substream was empty");
        this.muxer.flush_substream(cx, s).map_err(|e| e.into())
    }
}

impl<P> Drop for SubstreamRef<P>
where
    P: Deref,
    P::Target: StreamMuxer,
{
    fn drop(&mut self) {
        self.muxer
            .destroy_substream(self.substream.take().expect("substream was empty"))
    }
}