Fix SSE keep-alive timer lifecycle in Streamable HTTP server transport (v1.x) (#2547)

test/server/sseKeepAlive.test.ts

@@ -0,0 +1,29 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+
+import { armSseKeepAlive } from '../../src/server/sseKeepAlive.js';
+
+describe('armSseKeepAlive', () => {
+    beforeEach(() => vi.useFakeTimers());
+    afterEach(() => vi.useRealTimers());
+
+    it.each([0, -1, 0.5, Number.NaN, Number.POSITIVE_INFINITY])('disables invalid delay %s', delay => {
+        expect(armSseKeepAlive(delay, () => {})).toBeUndefined();
+        expect(vi.getTimerCount()).toBe(0);
+    });
+
+    it('ticks at the configured interval', async () => {
+        const tick = vi.fn();
+        const timer = armSseKeepAlive(1_000, tick)!;
+        await vi.advanceTimersByTimeAsync(3_000);
+        expect(tick).toHaveBeenCalledTimes(3);
+        clearInterval(timer);
+    });
+
+    it('clamps overflowing delays instead of creating a 1ms timer', async () => {
+        const tick = vi.fn();
+        const timer = armSseKeepAlive(2 ** 31, tick)!;
+        await vi.advanceTimersByTimeAsync(60_000);
+        expect(tick).not.toHaveBeenCalled();
+        clearInterval(timer);
+    });
+});