test: protect durable behavior identified in review

crates/irtt-server/tests/runtime.rs

@@ -155,3 +155,109 @@ async fn wildcard_construction_is_refused_without_reply_source_selection() {
         Err(ServerRuntimeError::WildcardSourceSelectionUnsupported { .. })
     ));
 }
+
+// A current-thread runtime cannot receive while this test holds its thread.
+// Queue real packets before delaying the server: no synthetic ancillary data or
+// transport hooks. A short queue delay selects kernel wall time; a delay beyond
+// the documented one-second sanity bound falls back to userspace. Monotonic and
+// midpoint timestamps must always remain at the later userspace observation.
+#[cfg(target_os = "linux")]
+#[tokio::test(flavor = "current_thread")]
+async fn queued_echoes_select_kernel_receive_wall_only_when_plausible() {
+    use std::time::{SystemTime, UNIX_EPOCH};
+    let ns = || {
+        i64::try_from(
+            SystemTime::now()
+                .duration_since(UNIX_EPOCH)
+                .unwrap()
+                .as_nanos(),
+        )
+        .unwrap()
+    };
+    for (placement, delay, kernel_expected) in [
+        (StampAt::Both, Duration::from_millis(100), true),
+        (StampAt::Both, Duration::from_millis(1100), false),
+        (StampAt::Midpoint, Duration::from_millis(100), false),
+    ] {
+        let mut server = Server::bind("127.0.0.1:0".parse().unwrap(), unthrottled())
+            .await
+            .unwrap();
+        let addr = server.local_addr().unwrap();
+        let client = UdpSocket::bind("127.0.0.1:0").await.unwrap();
+        client.connect(addr).await.unwrap();
+        let (stop_tx, stop_rx) = oneshot::channel();
+        let run = tokio::spawn(async move {
+            server
+                .run(async {
+                    let _ = stop_rx.await;
+                })
+                .await
+        });
+        let params = Params {
+            stamp_at: placement,
+            clock: Clock::Both,
+            received_stats: ReceivedStats::Both,
+            ..Params::with_protocol_defaults()
+        };
+        let open = encode_request(
+            RequestToEncode::Open {
+                params: &params,
+                no_test: false,
+            },
+            None,
+        )
+        .unwrap();
+        client.send(&open).await.unwrap();
+        let mut buffer = [0; 2048];
+        let len = timeout(Duration::from_secs(2), client.recv(&mut buffer))
+            .await
+            .unwrap()
+            .unwrap();
+        let opened = decode_open_reply(&buffer[..len], None).unwrap();
+        let echo = encode_request(
+            RequestToEncode::Echo {
+                token: opened.token,
+                sequence: 0,
+                params: &opened.params,
+                payload: &[],
+            },
+            None,
+        )
+        .unwrap();
+        client.writable().await.unwrap();
+        let before_send = ns();
+        client.try_send(&echo).unwrap(); // No await from submission through resume marker.
+        let queued_by = ns();
+        std::thread::sleep(delay);
+        let resumed_at = ns();
+        let len = timeout(Duration::from_secs(2), client.recv(&mut buffer))
+            .await
+            .unwrap()
+            .unwrap();
+        let reply = decode_echo_reply(&buffer[..len], &opened.params, None).unwrap();
+        assert_eq!(reply.recv_count, Some(1));
+        assert_eq!(reply.recv_window, Some(1));
+        let timestamps = reply.timestamps;
+        if kernel_expected {
+            let receive = timestamps.recv_wall.unwrap();
+            assert!(
+                receive >= before_send && receive <= queued_by,
+                "receive wall must identify the kernel arrival before the queue delay"
+            );
+            let wall_elapsed = timestamps.send_wall.unwrap() - receive;
+            let mono_elapsed = timestamps.send_mono.unwrap() - timestamps.recv_mono.unwrap();
+            assert!(
+                wall_elapsed - mono_elapsed >= 50_000_000,
+                "queue delay must not move the monotonic endpoint"
+            );
+        } else {
+            let receive = timestamps.recv_wall.or(timestamps.midpoint_wall).unwrap();
+            assert!(
+                receive >= resumed_at,
+                "fallback and midpoint must use userspace time"
+            );
+        }
+        stop_tx.send(()).unwrap();
+        run.await.unwrap().unwrap();
+    }
+}