Mocked stream response with multiple enqueues hangs process

[aduth]

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

20.11.1

Reproduction repository

https://github.com/aduth/msw-repro

Reproduction steps

1. Run npm install
2. Run node index.js

Current behavior

Output logs "text helloworld" and the process hangs indefinitely.

Expected behavior

Output logs "text helloworld" and the process ends cleanly.

[aduth]

With the reproduction example, I'm largely copying directly from the documented example: https://mswjs.io/docs/recipes/streaming

Strangely, calling enqueue once seems to work fine.

Using why-is-node-running, there's a few pointers referencing the clone happening in the fetch interceptor.

[kettanaito]

Hi, @aduth. Thanks for reporting this!

Interesting. Your use case looks almost identical to this test we have passing:

msw/test/node/rest-api/response/body-stream.node.test.ts

Lines 23 to 50 in 313691b

	test('responds with a ReadableStream', async () => {
	server.use(
	http.get('https://api.example.com/stream', () => {
	const stream = new ReadableStream({
	start(controller) {
	controller.enqueue(encoder.encode('hello'))
	controller.enqueue(encoder.encode('world'))
	controller.close()
	},
	})
	return new HttpResponse(stream, {
	headers: {
	'Content-Type': 'text/event-stream',
	},
	})
	}),
	)
	const response = await fetch('https://api.example.com/stream')
	expect(response.status).toBe(200)
	expect(response.statusText).toBe('OK')
	expect(response.body).toBeInstanceOf(ReadableStream)
	expect(response.body!.locked).toBe(false)
	expect(await response.text()).toBe('helloworld')
	})

Does anything different catch your eye between these two? We should certainly support pushing as many chunks as you need.

[kettanaito]

Can you try this on the latest minor version of Node.js v18?

I could reproduce your issue on v18.14.2 but the issue is gone on v18.19.0. Node.js can have issues too! In fact, there have been a few in the past that broke FormData. Changing the Node.js version makes your reproduction repository pass (the process exists properly).

[aduth]

I just re-tested on latest versions of Node.js 18 and 20 (18.19.1 and 20.11.1) and unfortunately still seems to hang in both versions.

Regarding your comment on the similarities, yes, I tried code adapted both from documentation and from those exact tests to try to use in my project, which is why I was surprised when I found it wasn't working.

[kettanaito]

@aduth, quite strange. Your repo passes for me on v18.19.

This is very unlikely an issue with MSW. I encourage you to look at Node.js/Unidici issues, especially related to request body cloning. For example, I've reported one today to Node.js (nodejs/node#52167), which is actually an active issue in Undici (nodejs/undici#2413).

[aduth]

That makes sense. If I find time, I'll try testing on another device to see if it's specific to my machine. In the meantime, fair to assume it's something outside MSW.