Support Remix Single Fetch (v3_unstable_singleFetch)

[kettanaito]

Prerequisites

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

Environment check

• I'm using the latest msw version
• I'm using Node.js version 18 or higher

Node.js version

18/20

Reproduction repository

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Reproduction steps

1. Create a new Remix app today. It will have Single Fetch enabled.
2. Integrate MSW server-side.

Current behavior

MSW doesn't intercept any server-side requests (e.g. requests you make in your loaders) because they are invisible to MSW. Remix has to implement a single connection that routes multiple requests, and as such they are not individual requests that MSW can recognize and handle (this is my wild guess).

Expected behavior

MSW handles server-side requests when Single Fetch is enabled in Remix.

Important

This works fine with Single Fetch disabled.

[kettanaito]

Overall, the fact that requests aren't being made is concerning. MSW doesn't ship any framework-specific logic, operating on the sensible assumption that when you make a request, you trigger the network code (e.g. fetch or http.ClientRequest in Node.js). If you don't, you aren't really making any requests and MSW cannot do much about that.

I've skimmed through the Remix and React Router source code relevant for Single Fetch, but didn't find any indication that they would silence or patch request-issuing APIs to achieve network collocation under a single fetch request. It looks like they just flush all requests for a single route from all the affected loaders/actions, then wait for them in parallel, then feed that data to the loaders.