Timeline: repair oldest/newest metrics if we fetch nothing

ts/test-node/util/LatestQueue_test.ts

@@ -0,0 +1,53 @@
+// Copyright 2020 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+import * as sinon from 'sinon';
+
+import { LatestQueue } from '../../util/LatestQueue';
+
+describe('LatestQueue', () => {
+  it('if the queue is empty, new tasks are started immediately', done => {
+    new LatestQueue().add(async () => {
+      done();
+    });
+  });
+
+  it('only enqueues the latest operation', done => {
+    const queue = new LatestQueue();
+
+    const spy = sinon.spy();
+
+    let openFirstTaskGate: undefined | (() => void);
+    const firstTaskGate = new Promise(resolve => {
+      openFirstTaskGate = resolve;
+    });
+    if (!openFirstTaskGate) {
+      throw new Error('Test is misconfigured; cannot grab inner resolve');
+    }
+
+    queue.add(async () => {
+      await firstTaskGate;
+      spy('first');
+    });
+
+    queue.add(async () => {
+      spy('second');
+    });
+
+    queue.add(async () => {
+      spy('third');
+    });
+
+    sinon.assert.notCalled(spy);
+
+    openFirstTaskGate();
+
+    queue.onceEmpty(() => {
+      sinon.assert.calledTwice(spy);
+      sinon.assert.calledWith(spy, 'first');
+      sinon.assert.calledWith(spy, 'third');
+
+      done();
+    });
+  });
+});