Exception parsing HTTP/2 Headers when multiple set-cookie is sent

[st3ffgv4]

Bug Description

By making requests to websites that return multiple set-cookie header with HTTP/2 enabled, an exception is triggered while processing the headers

Reproducible By

https://replit.com/@gava97/H2-headers

Expected Behavior

No exception thrown, headers parsed correctly

Logs & Screenshots

(node:1160) [UNDICI-H2] Warning: H2 support is experimental, expect them to change at any time.
(Use node --trace-warnings ... to show where the warning was created)
TypeError: fetch failed
at fetch (/home/runner/H2-headers/node_modules/undici/index.js:109:13)
at process.processTicksAndRejections (node:internal/process/task_queues:95:5)
at async file:///home/runner/H2-headers/index.js:8:13 {
[cause]: TypeError: Cannot read properties of undefined (reading 'toString')
at Object.onHeaders (/home/runner/H2-headers/node_modules/undici/lib/web/fetch/index.js:2135:97)
at Request.onHeaders (/home/runner/H2-headers/node_modules/undici/lib/core/request.js:243:29)
at ClientHttp2Stream. (/home/runner/H2-headers/node_modules/undici/lib/dispatcher/client-h2.js:420:17)
at Object.onceWrapper (node:events:633:26)
at ClientHttp2Stream.emit (node:events:518:28)
at emit (node:internal/http2/core:331:3)
at process.processTicksAndRejections (node:internal/process/task_queues:85:22)

Environment

Node v20.11.1
undici 6.11.1

Additional context

This bug is present onliy in undici >= 6.11

[metcoder95]

Yes, we changed the way we parse the response headers; it seems the root cause goes to the way we parse the setCookie, as with node:http2, multiple set-cookie headers are consolidated into an array of multiple values, we need to change that so fetch can parse it accordingly to the spec.

Working on a fix