Node's fetch implementation does not allow changing headers on request object

[evert]

Version

v18.1.0

Platform

Linux evertbook6 5.15.0-27-generic nodejs/node#28-Ubuntu SMP Thu Apr 14 04:55:28 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux

Subsystem

No response

What steps will reproduce the bug?

Node will lose headers that were added to the request after constructing the Request object, but_only_ if a custom Headers object was also passed in. This reproduces the issue

const headers = new Headers();
const request = new Request('http://localhost:8080', { headers });
request.headers.set('X-Foo', 'bar');

await fetch(request);

This does not happen when a headers object was not passed in:

const request = new Request('http://localhost:8080');
request.headers.set('X-Foo', 'bar');

await fetch(request);

Oof this took a lot of experimentation to get to :S

I think this was introduced with Node 18.1.0, because I have a feeling my my tests passed with 18.0.0

How often does it reproduce? Is there a required condition?

No response

What is the expected behavior?

The following headers on the server:

{
  host: 'localhost:8080',
  connection: 'keep-alive',
  'x-foo': 'bar',
  accept: '*/*',
  'accept-language': '*',
  'sec-fetch-mode': 'cors',
  'user-agent': 'undici',
  'accept-encoding': 'gzip, deflate'
}

What do you see instead?

{
  host: 'localhost:8080',
  connection: 'keep-alive',
  accept: '*/*',
  'accept-language': '*',
  'sec-fetch-mode': 'cors',
  'user-agent': 'undici',
  'accept-encoding': 'gzip, deflate'
}

@jasnell

[ronag]

I think this was fixed in #1401

[evert]

I'm not fully certain if #1401 tests for the the exact same issue. One thing that's at least different is that I'm changing a header after constructing the Request object. I don't think there's a test there that covers that case, but it does seem related to very possible that this is also fixed.

[mcollina]

Could you verify if this still applies to undici v5.2.0?

[evert]

I imported Undici in my project directly, overwrite global.fetch() and family and can confirm it's fixed!