Bypassed requests miss cookie headers with axios + axios-cookiejar-support

[aristofun]

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

20.17.0

Reproduction repository

https://github.com/aristofun/bugs-js-msw

Reproduction steps

Minimal example: https://github.com/aristofun/bugs-js-msw

Change the msw version to 2.4.3 reinstall and yarn start to see the difference.

Current behavior

After mswServer.listen({ onUnhandledRequest: "bypass" }); all localhost requests issued by axios in the same process are stripped off all cookie headers.

Reproduced on all msw versions starting with 2.4.4

Explicitly adding cookie header like this:

 const result2 = await axios.post("/mypath", {}, { headers: { 'Cookie': 'key=value' } });

Works. That means the problem is with interference with axios-cookiejar-support

Expected behavior

mswServer.listen({ onUnhandledRequest: "bypass" }); should not interfere with axios-cookiejar-support

[kettanaito]

I suspect this has to do with how axios-cookiejar-support works. If it hides the request headers, or adds them somehow at the wrong time, there will be little MSW should do about it. Given our XHR interception has been stable for years, I suspect that would be the case, sadly. But I will provide you with more details once I take a look at this properly.

[kettanaito]

Root cause

This was a fun exercise to untangle.

First, axios-cookiejar-support uses a custom HTTP agent:

https://github.com/3846masa/axios-cookiejar-support/blob/a42f8a61de4fbf6af3be195633f94ded3837ff9e/src/index.ts#L36

Then, the custom HttpCookieAgent extends the default HTTP agent and overrides the addRequest method to provision its logic related to cookies:

https://github.com/3846masa/http-cookie-agent/blob/783654f06b54d285d13150c0a8e79c0380c27990/src/http/create_cookie_agent.ts#L126

There was the culprit. Interceptors tap into the createConnection() method of the mock agent but disregarded the addRequest() method on the custom agent.

I've added the fix in mswjs/interceptors#666, alongside the test for this use case.

[aristofun]

Wow, amazing job! You're saving all those like me who want to have just 1 client (axios) evertywhere - server, client, tests

[aristofun]

Just as a side note - curious what whas different in msw <2.4.4 in that area

[kettanaito]

@aristofun, somewhere along those versions we've migrated to a new interception algorithm that's socket-based. That's the main difference.

The issue is non-trivial though. To fix it, we need to call .addRequest of custom agents. But that method likely calls super.addRequest of the default http.Agent, which, in turn, triggers the actual request connection all the time.

Obviously, that won't work when you want to mock a request.

I'm exploring what we can do to allow the .addRequest method of custom agents to run. For now, having thoughts of patching the default http.Agent to make its methods do nothing so that custom agents extending them can safely tap into super without triggering the request. That responsibility will be delegated to the MockAgent that's explicitly set on each request.

Any thoughts are welcome on this!

[aristofun]

Hmm, what do you think might be downsides of patching default http.Agent?

Sounds to me like you have to do it one way or another, if addRequest is a legitimate part of its API - to catch all possible surfaces of intercepting requests. Unless I miss something.