Jest-worker has no workers by default with one CPU

[biomedia-thomas]

🐛 Bug Report

jest-workers default for numWorkers is the number of CPUs minus 1. When there is only one CPU this results in a default numWorkers of 0 and no workers will be started.

To Reproduce

On a setup with only one CPU, use jest-worker with numWorkers option left to default. No workers are started:

const Worker = require('jest-worker').default;

// Ouputs OS platform:
new Worker(require.resolve('os'), {numWorkers: 1}).platform().then((a) => {
    console.log(a);
});

// Endlessly pending promise:
new Worker(require.resolve('os')).platform().then((a) => {
    console.log(a);
}); 

Expected behavior

Jest-worker using at least one worker in the default case.

Run npx envinfo --preset jest

Paste the results here:

  System:
    OS: Linux 4.9 Debian GNU/Linux 8 (jessie) 8 (jessie)
    CPU: x64 Intel(R) Core(TM) i7-8700K CPU @ 3.70GHz
  Binaries:
    Node: 8.12.0 - /usr/local/bin/node
    Yarn: 1.9.4 - /usr/local/bin/yarn
    npm: 6.4.1 - /usr/local/bin/npm

[thymikee]

Mind sending a fix that falls back the cpus to 1? 😄

[github-actions]

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