Show authenticated unique online player count on home screen

src/server/presence.ts

@@ -0,0 +1,33 @@
+import type { Server } from 'socket.io';
+import type { GameSessionStore } from './game-session-store.js';
+import { PRESENCE_AUTH_REFRESH_MS } from '../../constants/presence.js';
+
+// The application is deployed as one process, as is the ranked match service.
+export function registerPresence(io: Server, sessions: GameSessionStore) {
+  const namespace = io.of('/presence');
+  let count = 0;
+  function publish() {
+    const next = new Set([...namespace.sockets.values()].filter(socket => socket.connected).map(socket => socket.data.playerId)).size;
+    if (next !== count) { count = next; namespace.emit('presence:count', count); }
+  }
+  namespace.use((socket, next) => {
+    const { gameSessionId, token } = socket.handshake.auth;
+    void sessions.get({ gameSessionId, token }).then(session => {
+      socket.data.playerId = session.playerId ?? session.gameSessionId;
+      next();
+    }, () => next(new Error('INVALID_SESSION')));
+  });
+  namespace.on('connection', socket => {
+    const before = count; publish();
+    if (before === count) socket.emit('presence:count', count);
+    let checking = false;
+    const timer = setInterval(() => {
+      if (checking) return;
+      checking = true;
+      const { gameSessionId, token } = socket.handshake.auth;
+      void sessions.get({ gameSessionId, token }).catch(() => socket.disconnect(true)).finally(() => { checking = false; });
+    }, PRESENCE_AUTH_REFRESH_MS);
+    timer.unref();
+    socket.on('disconnect', () => { clearInterval(timer); publish(); });
+  });
+}