feat: send mocked response body as ReadableStream to the worker

[kettanaito]

• Fixes No response when using jest.useFakeTimers() with axios and msw #966 (does not wrap all responses in setTimeout anymore). For Node, awaits timeout conditionally only if response.delay is set.
• Closes fix(rest): support ReadableStream as response body #974
• Fixes ReadableStream object could not be cloned when trying to use stream as a mocked response body #581
• Fixes Binary body of a request gets corrupted in the handler #929
• Related to feat: support multipart binary request bodies #931
• Answers The file msw get is not the original file... Why and How to fix? #1272

Changes

• Transfers mocked response data as a ReadableStream between the client and the worker. This way we support both text and non-text data like Uint8Array to be used as a mocked response body.

Roadmap

• Add an integration test for ctx.delay() in Node. Removing setTimeout in handleRequest breaks response delays for Node but no tests catch that.
• Add an integration test for fake timers that ensures that no timer advance is needed if the mocked response uses no delay.

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[kettanaito]

Delay is performed in the worker in order to affect the response timings.

[kettanaito]

Gotcha

A really important nuance here is to respond with a stream response straight away. This way the client will receive the response immediately and will be able to read the stream as the operation channel keeps pushing the chunks from the response resolver.

[kettanaito]

Since I've removed a hard-coded setTimeout around responses in handleRequest, the Node part now needs to delay mocked responses manually. The worker has been always delaying the responses in the worker thread so that the mocked delay counts for server computing time.

[kettanaito]

No ctx.stream support possible (yet)

There's no ReadableStream in Node.js prior to version 17. We can't support using readable streams as mocked response bodies until version 17 is the minimal version of Node we support. I've tried writing a polyfill for a readable stream but it also depends on the writable stream for pipeTo/pipeThrough methods; the writable stream needs to know about AbortController, and here you are polyfilling half of the browser standard API.

We can still use a readable stream internally to stream mocked responses from the client to the worker. In Node, we'd have to convert mocked responses to Uint8Array (to preserve binary/buffers in mocks) and feed it to the respective patched modules.

[kettanaito]

For Node, we can convert mocked response bodies to Buffer or similar, achieving the unified response body processing as with the ReadableStream in the browser.

[kettanaito]

Released: v0.43.0 🎉

This has been released in v0.43.0!

• 📄 Release notes
• 📦 npm package

Make sure to always update to the latest version (npm i msw@latest) to get the newest features and bug fixes.

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