From 44ccd2ff93e20cfb6561d7a307806bbbedebb4da Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Philipp=20Kr=C3=BCger?= <philipp.krueger1@gmail.com>
Date: Wed, 26 Nov 2025 13:35:10 +0100
Subject: [PATCH] Write failing test case

---
 quinn-proto/src/tests/multipath.rs | 72 ++++++++++++++++++++++++++++++
 1 file changed, 72 insertions(+)

diff --git a/quinn-proto/src/tests/multipath.rs b/quinn-proto/src/tests/multipath.rs
index 5c621ab56..b7bb2b1d2 100644
--- a/quinn-proto/src/tests/multipath.rs
+++ b/quinn-proto/src/tests/multipath.rs
@@ -363,6 +363,78 @@ fn issue_max_path_id() {
     assert_eq!(stats.frame_rx.path_new_connection_id, client_path_new_cids);
 }
 
+/// A copy of [`issue_max_path_id`], but reordering the `MAX_PATH_ID` frame
+/// that's sent from the server to the client, so that some `NEW_CONNECTION_ID`
+/// frames arrive with higher path IDs than the most recently received
+/// `MAX_PATH_ID` frame on the client side.
+#[test]
+fn issue_max_path_id_reordered() {
+    let _guard = subscribe();
+
+    // We enable multipath but initially do not allow any paths to be opened.
+    let multipath_transport_cfg = Arc::new(TransportConfig {
+        max_concurrent_multipath_paths: NonZeroU32::new(1),
+        ..TransportConfig::default()
+    });
+    let server_cfg = Arc::new(ServerConfig {
+        transport: multipath_transport_cfg.clone(),
+        ..server_config()
+    });
+    let server = Endpoint::new(Default::default(), Some(server_cfg), true, None);
+    let client = Endpoint::new(Default::default(), None, true, None);
+
+    let mut pair = Pair::new_from_endpoint(client, server);
+
+    // The client is allowed to create more paths immediately.
+    let client_multipath_transport_cfg = Arc::new(TransportConfig {
+        max_concurrent_multipath_paths: NonZeroU32::new(MAX_PATHS),
+        ..TransportConfig::default()
+    });
+    let client_cfg = ClientConfig {
+        transport: client_multipath_transport_cfg,
+        ..client_config()
+    };
+    let (_client_ch, server_ch) = pair.connect_with(client_cfg);
+    pair.drive();
+    info!("connected");
+
+    // Server should only have sent NEW_CONNECTION_ID frames for now.
+    let server_new_cids = CidQueue::LEN as u64 - 1;
+    let mut server_path_new_cids = 0;
+    let stats = pair.server_conn_mut(server_ch).stats();
+    assert_eq!(stats.frame_tx.max_path_id, 0);
+    assert_eq!(stats.frame_tx.new_connection_id, server_new_cids);
+    assert_eq!(stats.frame_tx.path_new_connection_id, server_path_new_cids);
+
+    // Client should have sent PATH_NEW_CONNECTION_ID frames for PathId::ZERO.
+    let client_new_cids = 0;
+    let mut client_path_new_cids = CidQueue::LEN as u64;
+    assert_eq!(stats.frame_rx.new_connection_id, client_new_cids);
+    assert_eq!(stats.frame_rx.path_new_connection_id, client_path_new_cids);
+
+    // Server increases MAX_PATH_ID, but we reorder the frame
+    pair.server_conn_mut(server_ch)
+        .set_max_concurrent_paths(Instant::now(), NonZeroU32::new(MAX_PATHS).unwrap())
+        .unwrap();
+    pair.drive_server();
+    // reorder the frames on the incoming side
+    let p = pair.client.inbound.pop_front().unwrap();
+    pair.client.inbound.push_back(p);
+    pair.drive();
+    let stats = pair.server_conn_mut(server_ch).stats();
+
+    // Server should have sent MAX_PATH_ID and new CIDs
+    server_path_new_cids += (MAX_PATHS as u64 - 1) * CidQueue::LEN as u64;
+    assert_eq!(stats.frame_tx.max_path_id, 1);
+    assert_eq!(stats.frame_tx.new_connection_id, server_new_cids);
+    assert_eq!(stats.frame_tx.path_new_connection_id, server_path_new_cids);
+
+    // Client should have sent CIDs for new paths
+    client_path_new_cids += (MAX_PATHS as u64 - 1) * CidQueue::LEN as u64;
+    assert_eq!(stats.frame_rx.new_connection_id, client_new_cids);
+    assert_eq!(stats.frame_rx.path_new_connection_id, client_path_new_cids);
+}
+
 #[test]
 fn open_path() {
     let _guard = subscribe();