Fix getMaxWorkers on termux

CHANGELOG.md

@@ -36,6 +36,7 @@
 - `[jest-circus]` Better error message when a describe block is empty ([#6372](https://github.com/facebook/jest/pull/6372))
 - `[jest-cli]` Fix unhandled error when a bad revision is provided to `changedSince` ([#7115](https://github.com/facebook/jest/pull/7115))
 - `[jest-config]` Moved dynamically assigned `cwd` from `jest-cli` to default configuration in `jest-config` ([#7146](https://github.com/facebook/jest/pull/7146))
+- `[jest-config]` Fix `getMaxWorkers` on termux ([#7154](https://github.com/facebook/jest/pull/7154))
 
 ### Chore & Maintenance
 

packages/jest-config/src/__mocks__/os.js

@@ -0,0 +1,20 @@
+/**
+ * Copyright (c) 2014-present, Facebook, Inc. All rights reserved.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+'use strict';
+
+const os = jest.genMockFromModule('os');
+
+let cpus;
+function __setCpus(newCpus) {
+  cpus = newCpus;
+}
+
+os.__setCpus = __setCpus;
+os.cpus = jest.fn(() => cpus);
+
+module.exports = os;

packages/jest-config/src/__tests__/getMaxWorkers.test.js

@@ -8,16 +8,23 @@
 
 import getMaxWorkers from '../getMaxWorkers';
 
-jest.mock('os', () => ({
-  cpus: () => ({length: 4}),
-}));
+jest.mock('os');
 
 describe('getMaxWorkers', () => {
+  beforeEach(() => {
+    require('os').__setCpus({length: 4});
+  });
+
   it('Returns 1 when runInBand', () => {
     const argv = {runInBand: true};
     expect(getMaxWorkers(argv)).toBe(1);
   });
 
+  it('Returns 1 when the OS CPUs are not available', () => {
+    require('os').__setCpus(undefined);
+    expect(getMaxWorkers({})).toBe(1);
+  });
+
   it('Returns the `maxWorkers` when specified', () => {
     const argv = {maxWorkers: 8};
     expect(getMaxWorkers(argv)).toBe(8);

packages/jest-config/src/getMaxWorkers.js

@@ -17,7 +17,7 @@ export default function getMaxWorkers(argv: Argv): number {
   } else if (argv.maxWorkers) {
     return parseInt(argv.maxWorkers, 10);
   } else {
-    const cpus = os.cpus().length;
+    const cpus = os.cpus() ? os.cpus().length : 1;
     return Math.max(argv.watch ? Math.floor(cpus / 2) : cpus - 1, 1);
   }
 }