Initial fetch timeout (with working tests)

.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

@@ -24,34 +24,60 @@ const transformModulesWhitelist = [
 // (This value is correctly a string, not a RegExp.)
 const transformIgnorePattern = `node_modules/(?!${transformModulesWhitelist.join('|')})`;
 
-const common = {
-  // Finding and transforming source code.
-
-  testPathIgnorePatterns: ['/node_modules/', '/src/__tests__/lib/', '-testlib.js$'],
-
-  // When some source file foo.js says `import 'bar'`, Jest looks in the
-  // directories above foo.js for a directory like `node_modules` to find
-  // `bar` in.  If foo.js is behind a `yarn link` symlink and outside our
-  // tree, that won't work; so have it look at our node_modules too.
-  moduleDirectories: ['node_modules', '<rootDir>/node_modules'],
-
-  transform: {
-    '^.+\\.js$': '<rootDir>/node_modules/react-native/jest/preprocessor.js',
-  },
-  transformIgnorePatterns: [transformIgnorePattern],
-
-  // The runtime test environment.
-  globals: {
-    __TEST__: true,
-  },
-  setupFiles: ['./jest/globalFetch.js', './node_modules/react-native-gesture-handler/jestSetup.js'],
-  setupFilesAfterEnv: ['./jest/jestSetup.js', 'jest-extended'],
+const projectForPlatform = platform => {
+  if (!['ios', 'android'].includes(platform)) {
+    throw new Error(`Unsupported platform '${platform}'.`);
+  }
+  return {
+    displayName: platform,
+
+    // Ideally, these would simply be `jest-expo/ios` and
+    // `jest-expo/android`; see
+    // https://github.com/expo/expo/blob/master/packages/jest-expo/README.md#platforms.
+    // These custom presets are a workaround for a bug:
+    //
+    // `jest-expo`'s presets are based on `react-native`'s preset,
+    // which does something messy: it overwrites the global `Promise`.
+    // That's facebook/react-native#29303. Jest doesn't work well with
+    // that; that's facebook/jest#10221.
+    //
+    // So, until one of those is fixed, we use these custom presets to
+    // sandwich the code that replaces `global.Promise` with a fix:
+    //
+    // 1) save `global.Promise` to something else on `global`
+    // 2) let the `react-native` preset do its thing (like mocking RN
+    //    libraries)
+    // 3) assign `global.Promise` back to what we saved in step 1
+    preset: platform === 'ios' ? './jest/presetIos' : './jest/presetAndroid',
+
+    timers: 'modern',
+
+    // Finding and transforming source code.
+    testPathIgnorePatterns: ['/node_modules/', '/src/__tests__/lib/', '-testlib.js$'],
+
+    // When some source file foo.js says `import 'bar'`, Jest looks in the
+    // directories above foo.js for a directory like `node_modules` to find
+    // `bar` in.  If foo.js is behind a `yarn link` symlink and outside our
+    // tree, that won't work; so have it look at our node_modules too.
+    moduleDirectories: ['node_modules', '<rootDir>/node_modules'],
+
+    transform: {
+      '^.+\\.js$': '<rootDir>/node_modules/react-native/jest/preprocessor.js',
+    },
+    transformIgnorePatterns: [transformIgnorePattern],
+
+    // The runtime test environment.
+    globals: {
+      __TEST__: true,
+    },
+    setupFiles: [
+      './jest/globalFetch.js',
+      './node_modules/react-native-gesture-handler/jestSetup.js',
+    ],
+    setupFilesAfterEnv: ['./jest/jestSetup.js', 'jest-extended'],
+  };
 };
 
 module.exports = {
-  // See https://github.com/expo/expo/blob/master/packages/jest-expo/README.md#platforms.
-  projects: [
-    { ...common, displayName: 'ios', preset: 'jest-expo/ios' },
-    { ...common, displayName: 'android', preset: 'jest-expo/android' },
-  ],
+  projects: [projectForPlatform('ios'), projectForPlatform('android')],
 };

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)

jest/presetAndroid.js

@@ -0,0 +1,14 @@
+/**
+ * The preset we're making tweaks to.
+ */
+const basePreset = require('../node_modules/jest-expo/android/jest-preset.js');
+
+// See comment in our Jest config about how this is used.
+module.exports = {
+  ...basePreset,
+  setupFiles: [
+    require.resolve('./savePromise.js'),
+    ...basePreset.setupFiles,
+    require.resolve('./restorePromise.js'),
+  ],
+};

jest/presetIos.js

@@ -0,0 +1,14 @@
+/**
+ * The preset we're making tweaks to.
+ */
+const basePreset = require('../node_modules/jest-expo/ios/jest-preset.js');
+
+// See comment in our Jest config about how this is used.
+module.exports = {
+  ...basePreset,
+  setupFiles: [
+    require.resolve('./savePromise.js'),
+    ...basePreset.setupFiles,
+    require.resolve('./restorePromise.js'),
+  ],
+};

jest/restorePromise.js

@@ -0,0 +1,3 @@
+// For facebook/jest#10221. After savePromise.js and Jest's setup
+// files have run, restore the natural value of `global.Promise`.
+global.Promise = global.originalPromise;

jest/savePromise.js

@@ -0,0 +1,4 @@
+// For facebook/jest#10221. Before Jest's setup files have run, take
+// note of what the natural value of `global.Promise` is, so we can
+// restore it in restorePromise.js.
+global.originalPromise = Promise;

package.json

@@ -119,7 +119,6 @@
     "jest-expo": "^40.0.1",
     "jest-extended": "^0.11.5",
     "jetifier": "^1.6.5",
-    "lolex": "^5.1.1",
     "metro-react-native-babel-preset": "^0.63.0",
     "prettier": "^1.18.2",
     "prettier-eslint": "^12.0.0",

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;