Fix potential hang if extensions contain same StreamRef

If a user stored a `StreamRef` to the same stream in the request or
response extensions, they would be dropped while the internal store lock
was held. That would lead to a deadlock, since dropping a stream ref
will try to take the store lock to clean up.

Clear extensions of Request and Response before locking store, prevent
this.

Fixes hyperium/hyper#2621

tests/h2-tests/tests/server.rs

@@ -1059,3 +1059,37 @@ async fn request_without_authority() {
 
     join(client, srv).await;
 }
+
+#[tokio::test]
+async fn serve_when_request_in_response_extensions() {
+    h2_support::trace_init!();
+    let (io, mut client) = mock::new();
+
+    let client = async move {
+        let settings = client.assert_server_handshake().await;
+        assert_default_settings!(settings);
+        client
+            .send_frame(
+                frames::headers(1)
+                    .request("GET", "https://example.com/")
+                    .eos(),
+            )
+            .await;
+        client
+            .recv_frame(frames::headers(1).response(200).eos())
+            .await;
+    };
+
+    let srv = async move {
+        let mut srv = server::handshake(io).await.expect("handshake");
+        let (req, mut stream) = srv.next().await.unwrap().unwrap();
+
+        let mut rsp = http::Response::new(());
+        rsp.extensions_mut().insert(req);
+        stream.send_response(rsp, true).unwrap();
+
+        assert!(srv.next().await.is_none());
+    };
+
+    join(client, srv).await;
+}