Measure multiplayer relay ping

server/src/netplay.rs

@@ -19,7 +19,7 @@
 //   {"t":"set_ai","side":2,"ai":true}              -> the host marks an empty seat as AI, or clears it again
 //   {"t":"leave"}                                  -> a player leaves; a started human seat becomes AI-controlled while anyone is connected
 //
-// Server messages include: hello, rooms, room, start, turn, sum, chat, closed, error.
+// Server messages include: hello, rooms, room, start, turn, sum, chat, closed, error, pong.
 
 use crate::AppState;
 use axum::{
@@ -276,6 +276,9 @@ impl Netplay {
                     self.leave_room(conn, &code, false);
                 }
             }
+            "ping" => {
+                self.send(conn, json!({"t":"pong","c":msg.get("c").cloned().unwrap_or(Value::Null)}));
+            }
             _ => self.error(conn, "unknown message"),
         }
         Vec::new()
@@ -988,4 +991,15 @@ mod tests {
         assert_eq!(team_red["side"].as_u64(), Some(0));
         assert!(rx2.try_recv().is_err());
     }
+
+    #[test]
+    fn the_relay_answers_a_ping_with_the_same_correlation_number() {
+        let mut net = Netplay::new();
+        let (tx, mut rx) = mpsc::unbounded_channel();
+        let red = net.connect(tx, Some("red".into()), None);
+        net.handle(red, json!({"t":"ping","c":7}));
+
+        let pong = until(&mut rx, "pong");
+        assert_eq!(pong["c"].as_u64(), Some(7));
+    }
 }