swarm/connection: Enforce limit on inbound substreams via StreamMuxer

muxers/yamux/CHANGELOG.md

@@ -4,7 +4,11 @@
 
 - Remove `OpenSubstreamToken` as it is dead code. See [PR 2873].
 
+- Drive connection also via `StreamMuxer::poll`. Any received streams will be buffered up to a maximum of 25 streams.
+  See [PR 2861].
+
 [PR 2873]: https://github.com/libp2p/rust-libp2p/pull/2873/
+[PR 2861]: https://github.com/libp2p/rust-libp2p/pull/2861/
 
 # 0.39.0
 

muxers/yamux/Cargo.toml

@@ -16,3 +16,4 @@ libp2p-core = { version = "0.36.0", path = "../../core", default-features = fals
 parking_lot = "0.12"
 thiserror = "1.0"
 yamux = "0.10.0"
+log = "0.4"

muxers/yamux/src/lib.rs

@@ -24,10 +24,12 @@
 use futures::{
     future,
     prelude::*,
+    ready,
     stream::{BoxStream, LocalBoxStream},
 };
 use libp2p_core::muxing::{StreamMuxer, StreamMuxerEvent};
 use libp2p_core::upgrade::{InboundUpgrade, OutboundUpgrade, UpgradeInfo};
+use std::collections::VecDeque;
 use std::{
     fmt, io, iter, mem,
     pin::Pin,
@@ -42,8 +44,20 @@ pub struct Yamux<S> {
     incoming: S,
     /// Handle to control the connection.
     control: yamux::Control,
+    /// Temporarily buffers inbound streams in case our node is performing backpressure on the remote.
+    ///
+    /// The only way how yamux can make progress is by driving the [`Incoming`] stream. However, the
+    /// [`StreamMuxer`] interface is designed to allow a caller to selectively make progress via
+    /// [`StreamMuxer::poll_inbound`] and [`StreamMuxer::poll_outbound`] whilst the more general
+    /// [`StreamMuxer::poll`] is designed to make progress on existing streams etc.
+    ///
+    /// This buffer stores inbound streams that are created whilst [`StreamMuxer::poll`] is called.
+    /// Once the buffer is full, new inbound streams are dropped.
+    inbound_stream_buffer: VecDeque<yamux::Stream>,
 }
 
+const MAX_BUFFERED_INBOUND_STREAMS: usize = 25;
+
 impl<S> fmt::Debug for Yamux<S> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.write_str("Yamux")
@@ -65,6 +79,7 @@ where
                 _marker: std::marker::PhantomData,
             },
             control: ctrl,
+            inbound_stream_buffer: VecDeque::default(),
         }
     }
 }
@@ -84,6 +99,7 @@ where
                 _marker: std::marker::PhantomData,
             },
             control: ctrl,
+            inbound_stream_buffer: VecDeque::default(),
         }
     }
 }
@@ -101,13 +117,11 @@ where
         mut self: Pin<&mut Self>,
         cx: &mut Context<'_>,
     ) -> Poll<Result<Self::Substream, Self::Error>> {
-        self.incoming.poll_next_unpin(cx).map(|maybe_stream| {
-            let stream = maybe_stream
-                .transpose()?
-                .ok_or(YamuxError(ConnectionError::Closed))?;
+        if let Some(stream) = self.inbound_stream_buffer.pop_front() {
+            return Poll::Ready(Ok(stream));
+        }
 
-            Ok(stream)
-        })
+        self.poll_inner(cx)
     }
 
     fn poll_outbound(
@@ -121,9 +135,21 @@ where
 
     fn poll(
         self: Pin<&mut Self>,
-        _: &mut Context<'_>,
+        cx: &mut Context<'_>,
     ) -> Poll<Result<StreamMuxerEvent, Self::Error>> {
-        Poll::Pending
+        let this = self.get_mut();
+
+        loop {
+            let inbound_stream = ready!(this.poll_inner(cx))?;
+
+            if this.inbound_stream_buffer.len() >= MAX_BUFFERED_INBOUND_STREAMS {
+                log::warn!("dropping {inbound_stream} because buffer is full");
+                drop(inbound_stream);
+                continue;
+            }
+
+            this.inbound_stream_buffer.push_back(inbound_stream);
+        }
     }
 
     fn poll_close(mut self: Pin<&mut Self>, c: &mut Context<'_>) -> Poll<YamuxResult<()>> {
@@ -145,6 +171,21 @@ where
     }
 }
 
+impl<S> Yamux<S>
+where
+    S: Stream<Item = Result<yamux::Stream, YamuxError>> + Unpin,
+{
+    fn poll_inner(&mut self, cx: &mut Context<'_>) -> Poll<Result<yamux::Stream, YamuxError>> {
+        self.incoming.poll_next_unpin(cx).map(|maybe_stream| {
+            let stream = maybe_stream
+                .transpose()?
+                .ok_or(YamuxError(ConnectionError::Closed))?;
+
+            Ok(stream)
+        })
+    }
+}
+
 /// The yamux configuration.
 #[derive(Debug, Clone)]
 pub struct YamuxConfig {

swarm/CHANGELOG.md

@@ -5,7 +5,12 @@
 
 - Update to `libp2p-core` `v0.36.0`.
 
+- Enforce backpressure on incoming streams via `StreamMuxer` interface. In case we hit the configured limit of maximum
+  number of inbound streams, we will stop polling the `StreamMuxer` for new inbound streams. Depending on the muxer
+  implementation in use, this may lead to instant dropping of inbound streams. See [PR 2861].
+
 [libp2p-swarm v0.38.0 changelog entry]: https://github.com/libp2p/rust-libp2p/blob/master/swarm/CHANGELOG.md#0380
+[PR 2861]: https://github.com/libp2p/rust-libp2p/pull/2861/
 
 # 0.38.0