All once handlers are being flagged as used as soon as a request is intercepted, no matter the URL

[acidoxee]

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 14 or higher

Node.js version

v18.18.0

Reproduction repository

https://stackblitz.com/edit/vitest-dev-vitest-xa3evs?file=test%2Fbasic.test.ts

Reproduction steps

Just let the tests run (which should happen as soon as you land on the page), the green test highlights that it shouldn't behave that way.

Current behavior

I've got a detailed answer here already, but for good measure:

• register a once handler, for instance for https://test.local/foo/trpc/getById
• perform a request for another URL, for instance for https://test.local/bar/trpc/getByName
• perform a request for the supposedly mocked URL (https://test.local/foo/trpc/getById), see that it's not properly handled and that the once handler had already been flagged as used

I've seen this behavior in 0.0.0-fetch.rc-23 and 0.0.0-fetch.rc-24.

Expected behavior

The once handler registered for https://test.local/foo/trpc/getById should not have been flagged as used after the first request, because it didn't match. In the reproduction repo, there should even still be one once unused handler in the pool, since only one request to https://test.local/foo/trpc/getById has been triggered, whereas there were two once handlers registered for that URL.

[kettanaito]

Thanks for reporting this, @acidoxee! I'm trying to reproduce this behavior in #1781, so far without luck. The tests are passing, and non-matching handlers are not marked as used.

[acidoxee]

It seems you're using lower-level primitives to test the behavior, whereas in my reproduction I'm using plain fetch requests. Maybe there's something wrong in-between? Sorry if that doesn't make sense, I'm not familiar with MSW's internals at all 🙈

Or maybe you're using a different Node version? I'm having the problem on 18.17.1, 18.18.0 and 18.18.2, I haven't tried with other versions.

I'm also using Vitest, not Jest, both in my own repo and in the reproduction one. I don't have any idea whether that could influence the results.

[joshkel]

I ran into this today.

It seems that the problem is here:

msw/src/core/handlers/RequestHandler.ts

Lines 193 to 210 in 88c45ea

	// Immediately mark the handler as used.
	// Can't await the resolver to be resolved because it's potentially
	// asynchronous, and there may be multiple requests hitting this handler.
	this.isUsed = true
	const parsedResult = await this.parse({
	request: args.request,
	resolutionContext: args.resolutionContext,
	})
	const shouldInterceptRequest = this.predicate({
	request: args.request,
	parsedResult,
	resolutionContext: args.resolutionContext,
	})
	if (!shouldInterceptRequest) {
	return null
	}

It immediately marks the handler as used, to prevent two back-to-back requests from both going through. However, parsing the request, to determine if the handler should be invoked, is asynchronous (as needed by the GraphQL handler). There appears to be no way to have both "immediately enforce 'once'" and "asynchronously determine whether it's used", unless you do something such as changing isUsed to a promise or serializing access to it via a mutex?

Here's a test in handleRequest.test.ts that should suffice to reproduce the issue.

(collapsed for a shorter issue description)

it('does not mark non-matching one-time handlers as used', async () => {
  const { emitter } = setup()

  const oneTimeHandler = http.get(
    '/resource',
    () => {
      return HttpResponse.text('One-time')
    },
    { once: true },
  )
  const anotherHandler = http.get(
    '/another',
    () => {
      return HttpResponse.text('Another')
    },
    { once: true },
  )
  const handlers: Array<RequestHandler> = [oneTimeHandler, anotherHandler]

  const requestId = uuidv4()
  const firstResult = await handleRequest(
    new Request('http://localhost/another'),
    requestId,
    handlers,
    options,
    emitter,
    callbacks,
  )

  expect(await firstResult?.text()).toBe('Another')
  expect(oneTimeHandler.isUsed).toBe(false)
  expect(anotherHandler.isUsed).toBe(true)

  const secondResult = await handleRequest(
    new Request('http://localhost/resource'),
    requestId,
    handlers,
    options,
    emitter,
    callbacks,
  )

  expect(await secondResult?.text()).toBe('One-time')
  expect(anotherHandler.isUsed).toBe(true)
  expect(oneTimeHandler.isUsed).toBe(true)
})

[kettanaito]

Released: v2.0.2 🎉

This has been released in v2.0.2!

• 📄 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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