Allow asynchronous mocking replies

[LuudJanssen]

This would solve...

When trying to create a Undici adapter for Polly.JS I tried to use the mocking capabilities from Undici to record network requests and replay them later using stored HAR files. However, the MockInterceptor.reply method doesn't support asynchronous callbacks. So receiving the request, loading a file from the filesystem (asynchronously) (or passing through the request) and resolving with a promise doesn't work.

Sidenote: nock supports asynchronous callbacks: https://github.com/nock/nock#specifying-replies

The implementation should look like...

The MockInterceptor.reply method could accept asynchronous callbacks:

mockPool
  .intercept({
    path: () => true
  })
  .reply(async (options) => {
    const response = await fetchFromFileSystem(options)
    return mapping(response)
  })

I have also considered...

Creating my own version of MockAgent that supports async replies, but that just seemed too much work for a possibly temporary problem.

Additional context

Feature request for Undici adapter for Polly.JS Feature request for Undici support for Nock

[mcollina]

This would be an amazing feature to add? Would you like to send a Pull Request to address this issue? Remember to add unit tests.

[austinkelleher]

Worked on this over the weekend and have a WIP branch. I'm supposed to leave for vacation tomorrow, so I'll try to at least get a draft PR up today 😅.

Our team is heavily invested in Polly.JS and definitely interested in pursuing a custom Undici adapter.

[austinkelleher]

Here is a WIP branch: https://github.com/austinkelleher/undici/commit/5e331e8c036cc37ed659aa8ec850a48ae0f42040

Here is the remaining work:

• Consider additional edge error cases
• Add more tests
• Update TypeScript typings
• Add documentation

I'm leaving early tomorrow morning for about a week. I'll pick this back up when I get back unless someone else does first! 😅

[kibertoad]

@austinkelleher Could you use any help with this?

[jimmywarting]

think it should rather be something like fetch event and using respondWith