Reuse serialized server events across SSE connections

tests/unit/telemetry.test.ts

@@ -90,6 +90,45 @@ it('timestamps initial telemetry and subsequent status and telemetry events with
     }
 });
 
+it('serializes each live server event once across connections and releases cancelled subscribers', async () => {
+    const { GET } = await import('../../src/routes/api/events/servers/+server.js');
+    const { emit } = await import('../../src/lib/server/pipeline/bus.js');
+    const readers = await Promise.all([GET(), GET()].map(async response => (await response).body!.getReader()));
+    const stringify = vi.spyOn(JSON, 'stringify');
+    try {
+        for (const reader of readers) {
+            await reader.read(); // Initial comment and per-connection snapshots.
+            await reader.read();
+            await reader.read();
+        }
+        for (const event of [
+            { type: 'server-status' as const, delta: true, entries: [] },
+            { type: 'server-telemetry' as const, entries: [{ ...payload, receivedAt: new Date().toISOString() }] }
+        ]) {
+            stringify.mockClear();
+            emit(event);
+            expect(stringify).toHaveBeenCalledTimes(1);
+            const [first, second] = await Promise.all(readers.map(reader => reader.read()));
+            expect(first.value).toBe(second.value);
+            const frame = new TextDecoder().decode(first.value);
+            expect(frame.startsWith(`event: ${event.type}\ndata: `)).toBe(true);
+            expect(JSON.parse(frame.split('\n')[1].slice(6))).toEqual({ ...event, serverNow: expect.any(Number) });
+        }
+        await readers[0].cancel();
+        stringify.mockClear();
+        emit({ type: 'server-status', entries: [] });
+        expect(stringify).toHaveBeenCalledTimes(1);
+        expect((await readers[1].read()).done).toBe(false);
+        await readers[1].cancel();
+        stringify.mockClear();
+        emit({ type: 'server-telemetry', entries: [] });
+        expect(stringify).not.toHaveBeenCalled();
+    } finally {
+        stringify.mockRestore();
+        await Promise.all(readers.map(reader => reader.cancel()));
+    }
+});
+
 it.each(['round_change', 'heartbeat'] as const)('persists %s stats, broadcasts updates, and leaves RCON safety data untouched', async (event) => {
     const payload = gameEvent(event, 17);
     const { subscribe } = await import('../../src/lib/server/pipeline/bus.js');