jest: Make "modern" fake-timer implementation the default.

Also, remove several now-unnecessary calls of
`jest.useFakeTimers('modern')`, but keep a few assertions that the
"modern" timers are actually being used.

In particular, our `jestSetup` is a central place where we make the
assertion. Not only is it good to check that we still intentionally
set the "modern" implementation, but we want to make sure that the
setting is correctly applied. See the note in fb23341 about it
being silently not applied until we added @jest/source-map as a
direct dependency.

We have an ESLint rule, from 2faad06, preventing imports from
'**/__tests__/**'; the rule is active in all files not matching that
same pattern. Add an additional override so that we can make the
"modern"-timers assertion from within `jest/jestSetup.js`.

.eslintrc.yaml

@@ -339,7 +339,7 @@ overrides:
   #
   # ================================================================
   # Our test suite.
-  - files: ['**/__tests__/**']
+  - files: ['**/__tests__/**', 'jest/jestSetup.js']
     rules:
       no-restricted-imports: off
 

jest.config.js

@@ -50,6 +50,8 @@ const projectForPlatform = platform => {
     // 3) assign `global.Promise` back to what we saved in step 1
     preset: platform === 'ios' ? './jest/presetIos' : './jest/presetAndroid',
 
+    timers: 'modern',
+
     // Finding and transforming source code.
     testPathIgnorePatterns: ['/node_modules/', '/src/__tests__/lib/', '-testlib.js$'],
 

jest/jestSetup.js

@@ -5,6 +5,8 @@ import { polyfillGlobal } from 'react-native/Libraries/Utilities/PolyfillFunctio
 import { URL, URLSearchParams } from 'react-native-url-polyfill';
 import mockAsyncStorage from '@react-native-community/async-storage/jest/async-storage-mock';
 
+import { assertUsingModernFakeTimers } from '../src/__tests__/lib/fakeTimers';
+
 // Use the same `URL` polyfill we do in the app.
 //
 // In the app we let `react-native-url-polyfill` handle doing this, by
@@ -18,6 +20,11 @@ import mockAsyncStorage from '@react-native-community/async-storage/jest/async-s
 polyfillGlobal('URL', () => URL);
 polyfillGlobal('URLSearchParams', () => URLSearchParams);
 
+/**
+ * Default should be set with `timers: 'modern'` in Jest config.
+ */
+assertUsingModernFakeTimers();
+
 // Mock `react-native` ourselves, following upstream advice [1] [2].
 //
 // Note that React Native's Jest setup (in their jest/setup.js)

src/__tests__/lib/fakeTimers.js

@@ -1,14 +1,37 @@
 /* @flow strict-local */
 import { sleep } from '../../utils/async';
 
+/**
+ * Ensure the "modern" fake-timer implementation is used.
+ *
+ * By setting `timers: 'modern'` in our Jest config, the modern
+ * implementation is the default. May be used to double-check that
+ * this default is in fact set, in our Jest setup file.
+ *
+ * Also, in one or two files, we switch over to using real timers,
+ * with `jest.useRealTimers()`. May be used in those files to make
+ * sure this setting doesn't linger where we don't want it to.
+ */
+export const assertUsingModernFakeTimers = () => {
+  // "Note: This function is only available when using modern fake
+  // timers implementation"
+  //
+  // -- https://jestjs.io/docs/en/jest-object#jestgetrealsystemtime
+  jest.getRealSystemTime();
+};
+
 /**
  * Return a promise to sleep `ms` after advancing fake timers by `ms`.
  */
 export const fakeSleep = async (ms: number): Promise<void> => {
-  // Only available if using the "modern" implementation
-  if (typeof jest.getRealSystemTime !== 'function') {
-    throw new Error("Tried to call `fakeSleep` without `jest.useFakeTimers('modern')` in effect.");
+  try {
+    assertUsingModernFakeTimers();
+  } catch (e) {
+    throw new Error(
+      'Tried to call `fakeSleep` without "modern" fake-timer implementation enabled.',
+    );
   }
+
   const sleepPromise = sleep(ms);
   jest.advanceTimersByTime(ms);
   return sleepPromise;

src/__tests__/metatests.js

@@ -1,6 +1,8 @@
 /** @jest-environment jest-environment-jsdom-global */
 // @flow strict-local
 
+import { assertUsingModernFakeTimers } from './lib/fakeTimers';
+
 /**
  * This file should not test any part of the application. It exists to test that
  * certain functionality is present in the development environment used to run
@@ -75,9 +77,7 @@ describe('jsdom-global', () => {
 });
 
 describe('Jest modern fake timers', () => {
-  jest.useFakeTimers('modern');
-  // Will throw if not actually using the "modern" implementation
-  jest.getRealSystemTime();
+  assertUsingModernFakeTimers();
 
   afterEach(() => {
     // clear any unset timers

src/api/__tests__/queueMarkAsRead-test.js

@@ -12,10 +12,6 @@ messagesFlags.default = jest.fn(
 );
 
 describe('queueMarkAsRead', () => {
-  jest.useFakeTimers('modern');
-  // Will throw if not actually using the "modern" implementation
-  jest.getRealSystemTime();
-
   afterEach(() => {
     jest.clearAllMocks();
     jest.clearAllTimers();

src/message/__tests__/fetchActions-test.js

@@ -85,8 +85,6 @@ describe('fetchActions', () => {
 
   describe('tryFetch', () => {
     beforeAll(() => {
-      jest.useFakeTimers('modern');
-
       // So we don't have to think about the (random, with jitter)
       // duration of these waits in these tests. `BackoffMachine` has
       // its own unit tests already, so we don't have to test that it

src/presence/__tests__/heartbeat-test.js

@@ -93,11 +93,6 @@ describe('Heartbeat', () => {
   // ===================================================================
   // Jest hooks
 
-  // before running tests: set up fake timer API
-  beforeAll(() => {
-    jest.useFakeTimers('modern');
-  });
-
   afterAll(() => {
     JestHeartbeatHelper.clearExtant();
   });

src/utils/__tests__/async-test.js

@@ -1,5 +1,6 @@
 /* @flow strict-local */
 import { sleep } from '../async';
+import { assertUsingModernFakeTimers } from '../../__tests__/lib/fakeTimers';
 
 const sleepMeasure = async (expectedMs: number) => {
   const start = Date.now();
@@ -11,7 +12,7 @@ const sleepMeasure = async (expectedMs: number) => {
 
 describe('sleep (ideal)', () => {
   beforeAll(() => {
-    jest.useFakeTimers('modern');
+    assertUsingModernFakeTimers();
   });
 
   afterEach(() => {

src/utils/__tests__/backoffMachine-test.js

@@ -9,8 +9,6 @@ import { BackoffMachine } from '../async';
 // (https://github.com/facebook/jest/pull/8897). But as of 2020-03, putting them
 // in a separate file is our workaround.
 
-jest.useFakeTimers('modern');
-
 describe('BackoffMachine', () => {
   afterEach(() => {
     expect(jest.getTimerCount()).toBe(0);

src/utils/__tests__/promiseTimeout-test.js

@@ -4,10 +4,6 @@ import { fakeSleep } from '../../__tests__/lib/fakeTimers';
 
 const ONE_MINUTE_MS: number = 1000 * 60;
 
-jest.useFakeTimers('modern');
-// Will throw if not actually using the "modern" implementation
-jest.getRealSystemTime();
-
 describe('promiseTimeout', () => {
   afterEach(() => {
     // clear any unset timers