EADDRINUSE errors when tests run concurrently

We have multiple tests in our test suite that depend on mockttp, making use of its capability to randomly assign a port, and when Jest runs these tests concurrently we occasionally see errors in the form:

listen EADDRINUSE: address already in use :::8000
      at MockttpServer.<anonymous> (node_modules/mockttp/src/server/mockttp-server.ts:138:22)            
      at fulfilled (node_modules/mockttp/dist/server/mockttp-server.js:8:58)

To reproduce this issue we set up the following test:

import { getLocal } from 'mockttp';

describe('mockttp tests running in parallel', () => {
    const mockServer = getLocal();

    beforeAll(async () => {
        await mockServer.start();
    });

    afterAll(async () => await mockServer.stop());

    it('should pass', () => {
        expect(true).toBe(true);
    });
});

and then executed:

printf %s\\n {0..64} | xargs -n 1 -P 16 npx jest <test-name>

to run 16 instances of the test in parallel (on our 16 core machine) 4 times in a row. On most runs we see this issue occurring.

Can we introduce some kind of locking or retry mechanism in mockttp-server.start, or do you think we’re using the library in a way it wasn’t intended to be used?

Tested on version 0.21.2.

[pimterry]

This is now published as v1.0.1 👍

[pimterry]

This has now been released, as part of Mockttp 1.0.0 🎉. Enjoy, let me know if you have any other trouble.