fix: reduce the default value of initial_max_send_streams to 100

src/client.rs

@@ -312,9 +312,19 @@ pub struct Builder {
     reset_stream_duration: Duration,
 
     /// Initial maximum number of locally initiated (send) streams.
+    ///
+    /// The default value is 100; this value is derived from what the HTTP/2
+    /// spec recommends as the minimum value that endpoints advertise to their
+    /// peers, meaning that using this value will minimize the likelihood of the
+    /// failure where the local endpoint attempts to open too many streams
+    /// and gets rejected by the remote peer with the `REFUSED_STREAM` error.
+    /// <https://www.rfc-editor.org/rfc/rfc9113.html#section-6.5.2-2.6.1>
+    ///
     /// After receiving a Settings frame from the remote peer,
     /// the connection will overwrite this value with the
     /// MAX_CONCURRENT_STREAMS specified in the frame.
+    /// If no value is advertised by the remote peer, it will be
+    /// set to usize::MAX.
     initial_max_send_streams: usize,
 
     /// Initial target window size for new connections.
@@ -642,7 +652,7 @@ impl Builder {
             reset_stream_max: proto::DEFAULT_RESET_STREAM_MAX,
             pending_accept_reset_stream_max: proto::DEFAULT_REMOTE_RESET_STREAM_MAX,
             initial_target_connection_window_size: None,
-            initial_max_send_streams: usize::MAX,
+            initial_max_send_streams: 100,
             settings: Default::default(),
             stream_id: 1.into(),
         }
@@ -843,10 +853,21 @@ impl Builder {
 
     /// Sets the initial maximum of locally initiated (send) streams.
     ///
+    /// The default value is 100; this value is derived from what the HTTP/2
+    /// spec recommends as the minimum value that endpoints advertise to their
+    /// peers (see [Section 6.5.2]), meaning that using this value will minimize
+    /// the likelihood of the failure where the local endpoint attempts to open
+    /// too many streams and thus gets rejected by the remote peer with the
+    /// REFUSED_STREAM error.
+    ///
     /// The initial settings will be overwritten by the remote peer when
     /// the Settings frame is received. The new value will be set to the
     /// `max_concurrent_streams()` from the frame.
     ///
+    /// If no value is advertised in the initial SETTINGS sent from the remote
+    /// peer as part of [HTTP/2 Connection Preface], it will be overwritten to
+    /// `usize::MAX`.
+    ///
     /// This setting prevents the caller from exceeding this number of
     /// streams that are counted towards the concurrency limit.
     ///
@@ -855,7 +876,9 @@ impl Builder {
     ///
     /// See [Section 5.1.2] in the HTTP/2 spec for more details.
     ///
-    /// [Section 5.1.2]: https://http2.github.io/http2-spec/#rfc.section.5.1.2
+    /// [Section 6.5.2]: https://www.rfc-editor.org/rfc/rfc9113.html#section-6.5.2-2.6.1
+    /// [HTTP/2 Connection Preface]: https://www.rfc-editor.org/rfc/rfc9113.html#name-http-2-connection-preface
+    /// [Section 5.1.2]: https://www.rfc-editor.org/rfc/rfc9113.html#name-stream-concurrency
     ///
     /// # Examples
     ///

src/proto/settings.rs

@@ -12,6 +12,9 @@ pub(crate) struct Settings {
     /// the socket first then the settings applied **before** receiving any
     /// further frames.
     remote: Option<frame::Settings>,
+    /// Whether the connection has received the initial SETTINGS frame from the
+    /// remote peer.
+    has_received_remote_initial_settings: bool,
 }
 
 #[derive(Debug)]
@@ -32,6 +35,7 @@ impl Settings {
             // the handshake process.
             local: Local::WaitingAck(local),
             remote: None,
+            has_received_remote_initial_settings: false,
         }
     }
 
@@ -96,6 +100,15 @@ impl Settings {
         }
     }
 
+    /// Sets `true` to `self.has_received_remote_initial_settings`.
+    /// Returns `true` if this method is called for the first time.
+    /// (i.e. it is the initial SETTINGS frame from the remote peer)
+    fn mark_remote_initial_settings_as_received(&mut self) -> bool {
+        let has_received = self.has_received_remote_initial_settings;
+        self.has_received_remote_initial_settings = true;
+        !has_received
+    }
+
     pub(crate) fn poll_send<T, B, C, P>(
         &mut self,
         cx: &mut Context,
@@ -108,7 +121,7 @@ impl Settings {
         C: Buf,
         P: Peer,
     {
-        if let Some(settings) = &self.remote {
+        if let Some(settings) = self.remote.clone() {
             if !dst.poll_ready(cx)?.is_ready() {
                 return Poll::Pending;
             }
@@ -121,7 +134,8 @@ impl Settings {
 
             tracing::trace!("ACK sent; applying settings");
 
-            streams.apply_remote_settings(settings)?;
+            let is_initial = self.mark_remote_initial_settings_as_received();
+            streams.apply_remote_settings(&settings, is_initial)?;
 
             if let Some(val) = settings.header_table_size() {
                 dst.set_send_header_table_size(val as usize);

src/proto/streams/counts.rs

@@ -147,9 +147,11 @@ impl Counts {
         self.num_remote_reset_streams -= 1;
     }
 
-    pub fn apply_remote_settings(&mut self, settings: &frame::Settings) {
-        if let Some(val) = settings.max_concurrent_streams() {
-            self.max_send_streams = val as usize;
+    pub fn apply_remote_settings(&mut self, settings: &frame::Settings, is_initial: bool) {
+        match settings.max_concurrent_streams() {
+            Some(val) => self.max_send_streams = val as usize,
+            None if is_initial => self.max_send_streams = usize::MAX,
+            None => {}
         }
     }
 

src/proto/streams/streams.rs

@@ -186,14 +186,18 @@ where
         me.poll_complete(&self.send_buffer, cx, dst)
     }
 
-    pub fn apply_remote_settings(&mut self, frame: &frame::Settings) -> Result<(), Error> {
+    pub fn apply_remote_settings(
+        &mut self,
+        frame: &frame::Settings,
+        is_initial: bool,
+    ) -> Result<(), Error> {
         let mut me = self.inner.lock().unwrap();
         let me = &mut *me;
 
         let mut send_buffer = self.send_buffer.inner.lock().unwrap();
         let send_buffer = &mut *send_buffer;
 
-        me.counts.apply_remote_settings(frame);
+        me.counts.apply_remote_settings(frame, is_initial);
 
         me.actions.send.apply_remote_settings(
             frame,

tests/h2-tests/tests/client_request.rs

@@ -1661,6 +1661,158 @@ async fn client_builder_header_table_size() {
     join(srv, h2).await;
 }
 
+#[tokio::test]
+async fn default_max_concurrent_send_streams_and_update_it_based_on_empty_settings_frame() {
+    h2_support::trace_init!();
+    let (io, mut srv) = mock::new();
+
+    let srv = async move {
+        // Send empty SETTINGS frame (i.e. no MAX_CONCURRENT_STREAMS is set)
+        srv.send_frame(frames::settings()).await;
+    };
+
+    let h2 = async move {
+        let (_client, h2) = client::handshake(io).await.unwrap();
+        let mut h2 = std::pin::pin!(h2);
+        // The default value is 100
+        assert_eq!(h2.max_concurrent_send_streams(), 100);
+        h2.as_mut().await.unwrap();
+        // If the server's initial SETTINGS frame does not include MAX_CONCURRENT_STREAMS,
+        // this should be updated to usize::MAX.
+        assert_eq!(h2.max_concurrent_send_streams(), usize::MAX);
+    };
+
+    join(srv, h2).await;
+}
+
+#[tokio::test]
+async fn configured_max_concurrent_send_streams_and_update_it_based_on_non_empty_settings_frame() {
+    h2_support::trace_init!();
+    let (io, mut srv) = mock::new();
+
+    let srv = async move {
+        // Send SETTINGS frame with MAX_CONCURRENT_STREAMS set to 42
+        srv.send_frame(frames::settings().max_concurrent_streams(42))
+            .await;
+    };
+
+    let h2 = async move {
+        let (_client, h2) = client::Builder::new()
+            // Configure the initial value to 2024
+            .initial_max_send_streams(2024)
+            .handshake::<_, bytes::Bytes>(io)
+            .await
+            .unwrap();
+        let mut h2 = std::pin::pin!(h2);
+        // It should be pre-configured value before it receives the initial
+        // SETTINGS frame from the server
+        assert_eq!(h2.max_concurrent_send_streams(), 2024);
+        h2.as_mut().await.unwrap();
+        // Now the client has received the initial SETTINGS frame from the
+        // server, which should update the value accordingly
+        assert_eq!(h2.max_concurrent_send_streams(), 42);
+    };
+
+    join(srv, h2).await;
+}
+
+#[tokio::test]
+async fn receive_settings_frame_twice_with_second_one_empty() {
+    h2_support::trace_init!();
+    let (io, mut srv) = mock::new();
+
+    let srv = async move {
+        // Send the initial SETTINGS frame with MAX_CONCURRENT_STREAMS set to 42
+        srv.send_frame(frames::settings().max_concurrent_streams(42))
+            .await;
+
+        // Handle the client's connection preface
+        srv.read_preface().await.unwrap();
+        match srv.next().await {
+            Some(frame) => match frame.unwrap() {
+                h2::frame::Frame::Settings(_) => {
+                    let ack = frame::Settings::ack();
+                    srv.send(ack.into()).await.unwrap();
+                }
+                frame => {
+                    panic!("unexpected frame: {:?}", frame);
+                }
+            },
+            None => {
+                panic!("unexpected EOF");
+            }
+        }
+
+        // Should receive the ack for the server's initial SETTINGS frame
+        let frame = assert_settings!(srv.next().await.unwrap().unwrap());
+        assert!(frame.is_ack());
+
+        // Send another SETTINGS frame with no MAX_CONCURRENT_STREAMS
+        // This should not update the max_concurrent_send_streams value that
+        // the client manages.
+        srv.send_frame(frames::settings()).await;
+    };
+
+    let h2 = async move {
+        let (_client, h2) = client::handshake(io).await.unwrap();
+        let mut h2 = std::pin::pin!(h2);
+        assert_eq!(h2.max_concurrent_send_streams(), 100);
+        h2.as_mut().await.unwrap();
+        assert_eq!(h2.max_concurrent_send_streams(), 42);
+    };
+
+    join(srv, h2).await;
+}
+
+#[tokio::test]
+async fn receive_settings_frame_twice_with_second_one_non_empty() {
+    h2_support::trace_init!();
+    let (io, mut srv) = mock::new();
+
+    let srv = async move {
+        // Send the initial SETTINGS frame with MAX_CONCURRENT_STREAMS set to 42
+        srv.send_frame(frames::settings().max_concurrent_streams(42))
+            .await;
+
+        // Handle the client's connection preface
+        srv.read_preface().await.unwrap();
+        match srv.next().await {
+            Some(frame) => match frame.unwrap() {
+                h2::frame::Frame::Settings(_) => {
+                    let ack = frame::Settings::ack();
+                    srv.send(ack.into()).await.unwrap();
+                }
+                frame => {
+                    panic!("unexpected frame: {:?}", frame);
+                }
+            },
+            None => {
+                panic!("unexpected EOF");
+            }
+        }
+
+        // Should receive the ack for the server's initial SETTINGS frame
+        let frame = assert_settings!(srv.next().await.unwrap().unwrap());
+        assert!(frame.is_ack());
+
+        // Send another SETTINGS frame with no MAX_CONCURRENT_STREAMS
+        // This should not update the max_concurrent_send_streams value that
+        // the client manages.
+        srv.send_frame(frames::settings().max_concurrent_streams(2024))
+            .await;
+    };
+
+    let h2 = async move {
+        let (_client, h2) = client::handshake(io).await.unwrap();
+        let mut h2 = std::pin::pin!(h2);
+        assert_eq!(h2.max_concurrent_send_streams(), 100);
+        h2.as_mut().await.unwrap();
+        assert_eq!(h2.max_concurrent_send_streams(), 2024);
+    };
+
+    join(srv, h2).await;
+}
+
 const SETTINGS: &[u8] = &[0, 0, 0, 4, 0, 0, 0, 0, 0];
 const SETTINGS_ACK: &[u8] = &[0, 0, 0, 4, 1, 0, 0, 0, 0];