DNS interceptor breaks headers of type `[string, string][]`

[peterhirn]

Bug Description

Unable to use headers of type [string, string][], using Record<string, string> works (see below).

Reproducible By

import http from "node:http";
import { Agent, interceptors, request } from "undici";

const port = process.env.PORT ?? 3001;

const { cache, dns } = interceptors;

const good = new Agent().compose(cache());

const fail = new Agent().compose(cache(), dns());

http
  .createServer((req, res) => {
    console.log(req.headers);
    res.end();
  })
  .listen(port, async () => {
    const headers = [["foo", "bar"]];

    await request(`http://localhost:${port}`, {
      dispatcher: good,
      headers,
    });

    await request(`http://localhost:${port}`, {
      dispatcher: fail,
      headers,
    });

    const headersAsRecord = { foo: "bar" };
    await request(`http://localhost:${port}`, {
      dispatcher: fail,
      headers: headersAsRecord,
    });
  });

node index.js
{ host: 'localhost:3001', connection: 'keep-alive', foo: 'bar' }
{ '0': 'foo, bar', host: 'localhost:3001', connection: 'keep-alive' }
{ host: 'localhost:3001', connection: 'keep-alive', foo: 'bar' }

[peterhirn]

The problem seems to be here: https://github.com/nodejs/undici/blob/main/lib/interceptor/dns.js#L416

Maybe I should switch all my code to use IncomingHttpHeaders (which still allows multiple values)

export type IncomingHttpHeaders = Record<string, string | string[] | undefined>

Which type do you recommend for headers?

[Uzlopak]

I agree with you.. there should be proper handling of headers. I think we have some functions which takes care of headers being processed properly. we just need to use them in the interceptor.

[PandaWorker]

I agree with you.. there should be proper handling of headers. I think we have some functions which takes care of headers being processed properly. we just need to use them in the interceptor.

Or just normalize the headers into one normal view and use this structure everywhere after the request, in each interceptor to check the type and format of the headers so-so

https://github.com/nodejs/undici/issues/4336

[Uzlopak]

We dont normalize headers for performance reasons.

[PandaWorker]

In any case, the headers are normalized when sending requests.

undici/lib/core/request.js

Line 165 in aec609b

if (Array.isArray(headers)) {

[FelixVaughan]

@Uzlopak could you elaborate? If the overhead isnt too bad I would be +1 for closing #4336 as well. If not we can just make changes to the dns interceptor