Cache can add duplicate if-none-match, if-modified-since & varied headers (case-sensitivity)

[alxndrsn]

Bug Description

The code at

undici/lib/interceptor/cache.js

Lines 235 to 249 in e43d015

	let headers = {
	...opts.headers,
	'if-modified-since': new Date(result.cachedAt).toUTCString()
	}
	if (result.etag) {
	headers['if-none-match'] = result.etag
	}
	if (result.vary) {
	headers = {
	...headers,
	...result.vary
	}
	}

will overwrite lower-cased user-defined headers, but duplicate non-lower-cased headers.

Reproducible By

import { Agent, fetch, interceptors } from 'undici';
import express from 'express';

const app = express();
app.get('/', (req, res) => {
  res.set('Cache-Control', 'max-age=1');
  res.set('ETag', '"asdf1234"');
  res.send({ requestHeaders:req.headers });
});

const dispatcher = new Agent().compose(interceptors.cache({
  cacheByDefault: Number.MAX_SAFE_INTEGER,
  type: 'private',
}));

let prevEtag;

const s = app.listen(4444, async () => {
  for(let i=1; i<5; ++i) {
    const headers = {};
    if(i == 2) headers['If-None-Match'] = prevEtag;
    if(i == 3) headers['if-none-match'] = prevEtag;

    const res = await fetch('http://localhost:4444', { dispatcher, headers });
  
    prevEtag = res.headers.get('etag');
  
    console.log(i, 'res.body:', (await res.json()).requestHeaders['if-none-match']);

    await new Promise(resolve => setTimeout(resolve, 2000));
  }

  s.close();
});

Output:

1 res.body: undefined
2 res.body: "asdf1234", "asdf1234"
3 res.body: "asdf1234"
4 res.body: "asdf1234"

Expected Behavior

Either:

1. cache code defers to user-defined header values, or
2. cache code consistently overwrites user-defined header values

Logs & Screenshots

Environment

$ echo "node: $(node --version)"; echo "undici: $(jq -r .version ./node_modules/undici/package.json)"; echo "express: $(jq -r .version ./node_modules/express/package.json)"
node: v22.14.0
undici: 7.5.0
express: 4.21.2

Additional context

There are some other issues relating to cache & case-sensitivity of headers:

• Cache interceptor does not process capitalized Cache-Control header in request #3904
• cache: make vary headers case-insensitive #3990
• fix: handle missing vary header values #4031

[metcoder95]

Thanks for the report!

cache code consistently overwrites user-defined header values

I believe this should be the way to move forward, as these are handled by the interceptor itself and overwrite them can lead to undefined behaviour.

Would you like to send a PR to address that?

[alxndrsn]

Would you like to send a PR to address that?

I'll take a look 👍