Test overriding isolation

[yekver]

I use jest with puppeteer to run my e2e tests. If I run jest --maxWorkers=5 instead for jest --runInBand then I will probably face with a race condition problem when some test use request overriding and will affect all other tests, because it's not scoped only to this test.

Am I right?

[marcosvega91]

Hi @yekver thanks for reaching us.

It could happen.

As we suggest the server instance should be one. If you add handlers in some tests they will be added to the same shared server instance. This will cause this kind of issues

[yekver]

Are there anything we can do to deal with this concurrency and make per-test overriding isolated?

Or this is by design and won't be fixed?

[marcosvega91]

Call of setupServer performs patching of necessary modules (http/XMLHttpRequest). A possible solution is to run multiple instances, but is discouraged at the moment because of patching. I don't know what we can do for now to make it better.

[kettanaito]

As @marcosvega91 has mentioned, request interception in NodeJS is achieved by patching request issuing modules. That patching is done for the entire process.

In that way MSW works almost identical to Nock. Would anybody be interested in looking how Nock handles this scenario? We can learn a thing of two from them.

[marcosvega91]

I think that only http and https modules should have this problem. JSDOM for example should already handle multiple instances of window. What do you think if we try to patch http/XMLHttpRequest one time? If you call server.listen we can increment a counter and decrement on server.close. All mocks are restored only when counter is 0. In this way we can patch everything one time and multiple server instances could work together.

[kettanaito]

Let's discuss this issue in #474. Thanks.