hyperium · benjaminp · Aug 27, 2025
diff --git a/src/proto/streams/prioritize.rs b/src/proto/streams/prioritize.rs
@@ -186,13 +186,7 @@ impl Prioritize {
 
             // `try_assign_capacity` will queue the stream to `pending_capacity` if the capcaity
             // cannot be assigned at the time it is called.
-            //
-            // Streams over the max concurrent count will still call `send_data` so we should be
-            // careful not to put it into `pending_capacity` as it will starve the connection
-            // capacity for other streams
-            if !stream.is_pending_open {
-                self.try_assign_capacity(stream);
-            }
+            self.try_assign_capacity(stream);
         }
 
         if frame.is_end_stream() {
@@ -414,6 +408,12 @@ impl Prioritize {
 
     /// Request capacity to send data
     fn try_assign_capacity(&mut self, stream: &mut store::Ptr) {
+        // Streams over the max concurrent count should not have capacity assign to avoid starving the connection
+        // capacity for open streams
+        if stream.is_pending_open {
+            return;
+        }
+
         let total_requested = stream.requested_send_capacity;
 
         // Total requested should never go below actual assigned

diff --git a/tests/h2-tests/tests/flow_control.rs b/tests/h2-tests/tests/flow_control.rs
@@ -391,7 +391,7 @@ async fn stream_close_by_data_frame_releases_capacity() {
 
         // The capacity should be immediately available as nothing else is
         // happening on the stream.
-        assert_eq!(s1.capacity(), window_size);
+        let mut s1 = h2.drive(util::wait_for_capacity(s1, window_size)).await;
 
         let request = Request::builder()
             .method(Method::POST)
@@ -414,7 +414,7 @@ async fn stream_close_by_data_frame_releases_capacity() {
         s1.send_data("".into(), true).unwrap();
 
         // The capacity should be available
-        assert_eq!(s2.capacity(), 5);
+        let mut s2 = h2.drive(util::wait_for_capacity(s2, 5)).await;
 
         // Send the frame
         s2.send_data("hello".into(), true).unwrap();
@@ -461,9 +461,7 @@ async fn stream_close_by_trailers_frame_releases_capacity() {
         // This effectively reserves the entire connection window
         s1.reserve_capacity(window_size);
 
-        // The capacity should be immediately available as nothing else is
-        // happening on the stream.
-        assert_eq!(s1.capacity(), window_size);
+        let mut s1 = h2.drive(util::wait_for_capacity(s1, window_size)).await;
 
         let request = Request::builder()
             .method(Method::POST)
@@ -486,7 +484,7 @@ async fn stream_close_by_trailers_frame_releases_capacity() {
         s1.send_trailers(Default::default()).unwrap();
 
         // The capacity should be available
-        assert_eq!(s2.capacity(), 5);
+        let mut s2 = h2.drive(util::wait_for_capacity(s2, 5)).await;
 
         // Send the frame
         s2.send_data("hello".into(), true).unwrap();
@@ -919,10 +917,10 @@ async fn recv_no_init_window_then_receive_some_init_window() {
 
         let (response, mut stream) = client.send_request(request, false).unwrap();
 
-        stream.reserve_capacity(11);
+        stream.reserve_capacity(10);
 
-        let mut stream = h2.drive(util::wait_for_capacity(stream, 11)).await;
-        assert_eq!(stream.capacity(), 11);
+        let mut stream = h2.drive(util::wait_for_capacity(stream, 10)).await;
+        assert_eq!(stream.capacity(), 10);
 
         stream.send_data("hello world".into(), true).unwrap();
 
@@ -1990,6 +1988,120 @@ async fn reclaim_reserved_capacity() {
     join(mock, h2).await;
 }
 
+#[tokio::test]
+async fn capacity_not_assigned_to_unopened_streams() {
+    h2_support::trace_init!();
+
+    let (io, mut srv) = mock::new();
+
+    let mock = async move {
+        let mut settings = frame::Settings::default();
+        settings.set_max_concurrent_streams(Some(1));
+        let settings = srv.assert_client_handshake_with_settings(settings).await;
+        assert_default_settings!(settings);
+
+        srv.recv_frame(frames::headers(1).request("POST", "https://www.example.com/"))
+            .await;
+        srv.recv_frame(frames::data(1, "hello")).await;
+        srv.recv_frame(frames::data(1, "world").eos()).await;
+        srv.send_frame(frames::headers(1).response(200).eos()).await;
+
+        srv.recv_frame(frames::headers(3).request("POST", "https://www.example.com/"))
+            .await;
+        srv.send_frame(frames::window_update(
+            0,
+            frame::DEFAULT_INITIAL_WINDOW_SIZE + 10,
+        ))
+        .await;
+        srv.recv_frame(frames::reset(3).cancel()).await;
+    };
+
+    let h2 = async move {
+        let (mut client, mut h2) = client::handshake(io).await.unwrap();
+        let request = Request::builder()
+            .method(Method::POST)
+            .uri("https://www.example.com/")
+            .body(())
+            .unwrap();
+
+        let (response1, mut stream1) = client.send_request(request.clone(), false).unwrap();
+        stream1.send_data("hello".into(), false).unwrap();
+        let (_, mut stream2) = client.send_request(request.clone(), false).unwrap();
+        stream2.reserve_capacity(frame::DEFAULT_INITIAL_WINDOW_SIZE as usize);
+        stream1.send_data("world".into(), true).unwrap();
+        h2.drive(response1).await.unwrap();
+        let stream2 = h2
+            .drive(util::wait_for_capacity(
+                stream2,
+                frame::DEFAULT_INITIAL_WINDOW_SIZE as usize,
+            ))
+            .await;
+        drop(stream2);
+        h2.await.unwrap();
+    };
+
+    join(mock, h2).await;
+}
+
+#[tokio::test]
+async fn new_initial_window_size_capacity_not_assigned_to_unopened_streams() {
+    h2_support::trace_init!();
+
+    let (io, mut srv) = mock::new();
+
+    let mock = async move {
+        let mut settings = frame::Settings::default();
+        settings.set_max_concurrent_streams(Some(1));
+        settings.set_initial_window_size(Some(10));
+        let settings = srv.assert_client_handshake_with_settings(settings).await;
+        assert_default_settings!(settings);
+
+        srv.recv_frame(frames::headers(1).request("POST", "https://www.example.com/"))
+            .await;
+        srv.recv_frame(frames::data(1, "hello")).await;
+        srv.send_frame(frames::settings().initial_window_size(frame::DEFAULT_INITIAL_WINDOW_SIZE))
+            .await;
+        srv.recv_frame(frames::settings_ack()).await;
+        srv.send_frame(frames::headers(1).response(200).eos()).await;
+        srv.recv_frame(frames::data(1, "world").eos()).await;
+
+        srv.recv_frame(frames::headers(3).request("POST", "https://www.example.com/"))
+            .await;
+        srv.send_frame(frames::window_update(
+            0,
+            frame::DEFAULT_INITIAL_WINDOW_SIZE + 10,
+        ))
+        .await;
+        srv.recv_frame(frames::reset(3).cancel()).await;
+    };
+
+    let h2 = async move {
+        let (mut client, mut h2) = client::handshake(io).await.unwrap();
+        let request = Request::builder()
+            .method(Method::POST)
+            .uri("https://www.example.com/")
+            .body(())
+            .unwrap();
+
+        let (response1, mut stream1) = client.send_request(request.clone(), false).unwrap();
+        stream1.send_data("hello".into(), false).unwrap();
+        let (_, mut stream2) = client.send_request(request.clone(), false).unwrap();
+        stream2.reserve_capacity(frame::DEFAULT_INITIAL_WINDOW_SIZE as usize);
+        h2.drive(response1).await.unwrap();
+        stream1.send_data("world".into(), true).unwrap();
+        let stream2 = h2
+            .drive(util::wait_for_capacity(
+                stream2,
+                frame::DEFAULT_INITIAL_WINDOW_SIZE as usize,
+            ))
+            .await;
+        drop(stream2);
+        h2.await.unwrap();
+    };
+
+    join(mock, h2).await;
+}
+
 // ==== abusive window updates ====
 
 #[tokio::test]

diff --git a/tests/h2-tests/tests/prioritization.rs b/tests/h2-tests/tests/prioritization.rs
@@ -52,7 +52,9 @@ async fn single_stream_send_large_body() {
     stream.reserve_capacity(payload.len());
 
     // The capacity should be immediately allocated
-    assert_eq!(stream.capacity(), payload.len());
+    let mut stream = h2
+        .drive(util::wait_for_capacity(stream, payload.len()))
+        .await;
 
     // Send the data
     stream.send_data(payload.into(), true).unwrap();
@@ -108,7 +110,9 @@ async fn multiple_streams_with_payload_greater_than_default_window() {
         // The capacity should be immediately
         // allocated to default window size (smaller than payload)
         stream1.reserve_capacity(payload_clone.len());
-        assert_eq!(stream1.capacity(), DEFAULT_WINDOW_SIZE);
+        let mut stream1 = conn
+            .drive(util::wait_for_capacity(stream1, DEFAULT_WINDOW_SIZE))
+            .await;
 
         stream2.reserve_capacity(payload_clone.len());
         assert_eq!(stream2.capacity(), 0);
@@ -179,7 +183,9 @@ async fn single_stream_send_extra_large_body_multi_frames_one_buffer() {
     stream.reserve_capacity(payload.len());
 
     // The capacity should be immediately allocated
-    assert_eq!(stream.capacity(), payload.len());
+    let mut stream = h2
+        .drive(util::wait_for_capacity(stream, payload.len()))
+        .await;
 
     // Send the data
     stream.send_data(payload.into(), true).unwrap();
@@ -296,13 +302,13 @@ async fn single_stream_send_extra_large_body_multi_frames_multi_buffer() {
             0, 0, 16, 1, 4, 0, 0, 0, 1, 131, 135, 65, 139, 157, 41, 172, 75, 143, 168, 233, 25, 151,
             33, 233, 132,
         ])
+        .write(frames::SETTINGS_ACK)
+        .read(frames::SETTINGS_ACK)
         .write(&[
             // DATA
             0, 64, 0, 0, 0, 0, 0, 0, 1,
         ])
         .write(&payload[0..16_384])
-        .write(frames::SETTINGS_ACK)
-        .read(frames::SETTINGS_ACK)
         .wait(Duration::from_millis(10))
         .write(&[
             // DATA
@@ -326,7 +332,9 @@ async fn single_stream_send_extra_large_body_multi_frames_multi_buffer() {
     stream.reserve_capacity(payload.len());
 
     // The capacity should be immediately allocated
-    assert_eq!(stream.capacity(), payload.len());
+    let mut stream = h2
+        .drive(util::wait_for_capacity(stream, payload.len()))
+        .await;
 
     // Send the data
     stream.send_data(payload.into(), true).unwrap();

diff --git a/tests/h2-tests/tests/stream_states.rs b/tests/h2-tests/tests/stream_states.rs
@@ -51,11 +51,11 @@ async fn send_recv_data() {
             0, 0, 16, 1, 4, 0, 0, 0, 1, 131, 135, 65, 139, 157, 41, 172, 75, 143, 168, 233, 25, 151,
             33, 233, 132,
         ])
+        .write(frames::SETTINGS_ACK)
         .write(&[
             // DATA
             0, 0, 5, 0, 1, 0, 0, 0, 1, 104, 101, 108, 108, 111,
         ])
-        .write(frames::SETTINGS_ACK)
         // Read response
         .read(&[
             // HEADERS
@@ -78,10 +78,10 @@ async fn send_recv_data() {
     // Reserve send capacity
     stream.reserve_capacity(5);
 
-    assert_eq!(stream.capacity(), 5);
+    let mut stream = h2.drive(util::wait_for_capacity(stream, 5)).await;
 
     // Send the data
-    stream.send_data("hello".as_bytes(), true).unwrap();
+    stream.send_data("hello".into(), true).unwrap();
 
     // Get the response
     let resp = h2.run(response).await.unwrap();