Inherit fetchCache from environment variables / provide a way to opt-out

[mxschmitt]

Goals

1. Server side network request mocking with e.g. MSW is common nowadays
2. This works great besides that Next.js is caching the mocked requests
3. This idea wants to provide a solution for that problem

Non-Goals

1. Expose fine graded control via environment variables (too much API)
2. Be non-testing focused, since usually you want to have caching enabled and disable it only for integration tests

Background

We (Playwright) are exploring ways of providing mocking capabilities of server side fetch() API calls and run into an issue right now that in Next.js the caching is "too good" and its caching our mocked requests.

(The API we are playing with looks currently like this.)

Relates playwright-community/ssr#7

See also here more about the experiment: https://github.com/playwright-community/ssr

Proposal

Either a config option for a global fetchCache with the options from here or an environment, e.g. like DISABLE_NEXTJS_FETCH_CACHE_FOR_TESTING.