Perform worker update in parallel

Prerequisites

[X] I confirm my issue is not in the opened issues
[X] I confirm the Frequently Asked Questions didn't contain the answer to my issue

Environment check

[X] I'm using the latest msw version
[X] I'm using Node.js version 18 or higher

Browsers

No response

Reproduction repository

Any example

Reproduction steps

Have a context that activates MSW.
Have another MSW activation in the same context.

Current behavior

When MSW finds a matching registration and attempts to reuse it, it first updates the worker. That is done to make sure that the existing worker registration is up-to-date. For example, if you are registering a worker that doesn't match the existing worker (i.e. the script has changed due to the MSW version change), the existing worker will be marked as redundant.

https://github.com/mswjs/msw/blob/452686d27cc84c1a18262e9d0b18cf64b13aa71d/src/browser/setupWorker/start/utils/getWorkerInstance.ts#L39-L40

The problem is that .update() takes time. It needs to fetch the worker script, evaluate it byte-by-byte, comparing the old and the new scripts. That's a potential overhead that MSW doesn't have to await.

Expected behavior

Update is scheduled to run in the background. MSW resolves with the existing worker registration immediately.

mswjs / msw