From e9efababc041d061c52255acbbc9eff19c2e8758 Mon Sep 17 00:00:00 2001
From: Arni Dagur <adg@adg.is>
Date: Tue, 28 Apr 2026 00:19:13 +0100
Subject: [PATCH] fix: Avoid panic or capacity leak when a stream is cancelled
 after reserve_capacity

---
 src/proto/streams/prioritize.rs      | 12 ++---
 tests/h2-tests/tests/flow_control.rs | 74 ++++++++++++++++++++++++++++
 2 files changed, 78 insertions(+), 8 deletions(-)

diff --git a/src/proto/streams/prioritize.rs b/src/proto/streams/prioritize.rs
index 75358d47..f7ec0fb1 100644
--- a/src/proto/streams/prioritize.rs
+++ b/src/proto/streams/prioritize.rs
@@ -442,14 +442,10 @@ impl Prioritize {
             return;
         }
 
-        // If the stream has requested capacity, then it must be in the
-        // streaming state (more data could be sent) or there is buffered data
-        // waiting to be sent.
-        debug_assert!(
-            stream.state.is_send_streaming() || stream.buffered_send_data > 0,
-            "state={:?}",
-            stream.state
-        );
+        // The stream may have been reset or closed since capacity was requested.
+        if !stream.state.is_send_streaming() && stream.buffered_send_data == 0 {
+            return;
+        }
 
         // The amount of currently available capacity on the connection
         let conn_available = self.flow.available().as_size();
diff --git a/tests/h2-tests/tests/flow_control.rs b/tests/h2-tests/tests/flow_control.rs
index 57453e78..cabd8f43 100644
--- a/tests/h2-tests/tests/flow_control.rs
+++ b/tests/h2-tests/tests/flow_control.rs
@@ -1587,6 +1587,80 @@ async fn reset_stream_waiting_for_capacity() {
     join(srv, client).await;
 }
 
+// Regression test for https://github.com/hyperium/h2/pull/893
+#[tokio::test]
+async fn reserve_capacity_then_cancel_does_not_leak() {
+    for explicit_reset in [true, false] {
+        let (io, mut srv) = mock::new();
+
+        let srv = async move {
+            let _ = srv.assert_client_handshake().await;
+            srv.recv_frame(frames::headers(1).request("POST", "https://example.com/"))
+                .await;
+            if !explicit_reset {
+                srv.send_frame(frames::headers(1).response(200)).await;
+            }
+            let mut data_bytes = 0;
+            loop {
+                let frame = srv.next().await.unwrap().unwrap();
+                match frame {
+                    h2::frame::Frame::Reset(_) | h2::frame::Frame::Headers(_) => {}
+                    h2::frame::Frame::Data(d) => {
+                        data_bytes += d.payload().len();
+                        if d.is_end_stream() {
+                            break;
+                        }
+                    }
+                    other => panic!("unexpected: {:?}", other),
+                }
+            }
+            assert_eq!(data_bytes, 65535);
+            srv.send_frame(frames::headers(3).response(200).eos()).await;
+        };
+
+        let client = async move {
+            let (mut client, mut conn) = client::handshake(io).await.expect("handshake");
+            let request = Request::builder()
+                .method(Method::POST)
+                .uri("https://example.com/")
+                .body(())
+                .unwrap();
+            let (response, mut stream) = client.send_request(request, false).unwrap();
+            stream.reserve_capacity(10);
+
+            if explicit_reset {
+                stream.send_reset(Reason::CANCEL);
+                drop(stream);
+                let err = response.await.unwrap_err();
+                assert_eq!(err.reason(), Some(Reason::CANCEL));
+            } else {
+                let resp = conn.drive(response).await.unwrap();
+                assert_eq!(resp.status(), StatusCode::OK);
+                drop(stream);
+                drop(resp);
+            }
+
+            // Open a second stream and send a full window of data. If capacity
+            // leaked, this would stall.
+            let request2 = Request::builder()
+                .method(Method::POST)
+                .uri("https://example.com/")
+                .body(())
+                .unwrap();
+            let (response2, mut stream2) = client.send_request(request2, false).unwrap();
+            stream2.send_data(vec![0; 65535].into(), true).unwrap();
+            join(async move { conn.await.expect("conn") }, async move {
+                let resp = response2.await.expect("resp");
+                assert_eq!(resp.status(), StatusCode::OK);
+                drop(client);
+            })
+            .await;
+        };
+
+        join(srv, client).await;
+    }
+}
+
 #[tokio::test]
 async fn data_padding() {
     h2_support::trace_init!();