fix(proto): ensure stats reported in Discarded events are updated

noq-proto/src/connection/mod.rs

@@ -3388,8 +3388,9 @@ impl Connection {
             );
         }
         // Before removing the path, we fetch the final path stats via `Self::path_stats`.
-        // This updates some values for the last time.
-        let path_stats = self.path_stats.discard(&path_id);
+        // This ensures snapshot values (like rtt) are properly updated.
+        let path_stats = self.path_stats(path_id).unwrap_or_default();
+        self.path_stats.discard(&path_id);
         self.partial_stats += path_stats;
         self.paths.remove(&path_id);
         self.spaces[SpaceId::Data].number_spaces.remove(&path_id);

noq-proto/src/connection/stats.rs

@@ -361,7 +361,7 @@ impl PathStatsMap {
     /// Removes the stats for a given path.
     ///
     /// Only do this once you are discarding the path.
-    pub(super) fn discard(&mut self, path_id: &PathId) -> PathStats {
-        self.0.remove(path_id).unwrap_or_default()
+    pub(super) fn discard(&mut self, path_id: &PathId) {
+        self.0.remove(path_id);
     }
 }

noq-proto/src/tests/multipath.rs

@@ -2228,3 +2228,48 @@ fn regression_delayed_path_cids_blocked() -> TestResult {
     }
     Ok(())
 }
+
+#[test]
+fn regression_discarded_path_stats_are_up_to_date() -> TestResult {
+    let _guard = subscribe();
+    let mut pair = ConnPair::builder().enable_multipath().connect();
+
+    let server_addr = pair.routes.public_server_addr();
+    let path_id = pair.open_path(
+        Client,
+        FourTuple::from_remote(server_addr),
+        PathStatus::Available,
+    )?;
+    pair.drive();
+
+    // Drain establishment events.
+    while pair.poll(Client).is_some() {}
+    while pair.poll(Server).is_some() {}
+
+    // Close the path and drive until both sides discard it.
+    pair.close_path(Client, path_id, 0u8.into())?;
+    pair.drive();
+
+    assert_matches!(
+        pair.poll(Client),
+        Some(Event::Path(PathEvent::Abandoned { id, .. })) if id == path_id
+    );
+    assert_matches!(
+        pair.poll(Server),
+        Some(Event::Path(PathEvent::Abandoned { id, .. })) if id == path_id
+    );
+
+    pair.drive();
+
+    let discarded_stats = assert_matches!(
+        pair.poll(Client),
+        Some(Event::Path(PathEvent::Discarded { id, path_stats })) if id == path_id
+        => *path_stats
+    );
+
+    // After a full handshake + MTU probing on the second path, these must be non-zero.
+    assert_ne!(discarded_stats.cwnd, 0);
+    assert_ne!(discarded_stats.current_mtu, 0);
+
+    Ok(())
+}