Ensure conflicting flat headers in HTTP/2 are combined correctly

[pimterry]

This relates to...

When passing a flat header array and using HTTP/2, duplicate keys were discarded and lost unexpectedly.

In addition, the value-combining formatting (effectively join(',')) for explicitly array-valued headers in HTTP/2 didn't quite match the formatting for both Undici HTTP/1 or for Node HTTP/2's value-combining behaviour for headers that differ only by case (both of which do join(', ') - i.e. with whitespace) which was a bit weird.

Rationale

const client = new undici.Client('https://testserver.host', { allowH2: true })

client.request({
  method: 'GET',
  path: '/anything',
  headers: ['a', 'b', 'a', 'c']
}).then(async (response) => {
  console.log(await response.body.text())
})

The response shows what the server receives: headers including a: c but without a: b (all but the last header value is lost).

The header array-to-object logic ignored duplicate keys like this.

Regarding the formatting:

• In Undici HTTP/1, { 'a': ['b', 'c'] } is sent as a: b, c (whitespace)
• In Node built-in HTTP/2, { 'a': 'b', 'A': 'c' } is sent as a: b, c (whitespace)
• In Undici HTTP/2, until now, { 'a': ['b', 'c'] } was sent as a: b,c (no whitespace)

(I'm not claiming this formatting is particularly important, but I would've had to reproduce the inconsistency in the tests, and it's nicer to tidy it up & make these headers a bit more readable en route).

Once nodejs/node#57917 is released (e.g. Node v24) we will be able to skip this entirely, and pass raw headers as-is directly to Node's HTTP/2 APIs, so these headers won't be combined at all - but that will have to wait until later, and will only apply for Node versions including that change anyway.

Changes

N/A

Features

N/A

Bug Fixes

• Duplicate keys in HTTP/2 flat header arrays are now combined correctly, not discarded
• Multi-valued HTTP/2 header values which are combined now include whitespace, for consistency with HTTP/1 and Node itself.

Breaking Changes and Deprecations

N/A

Status

• I have read and agreed to the Developer's Certificate of Origin
• Tested
• Benchmarked (optional)
• Documented
• Review ready
• In review
• Merge ready

[pimterry]

Failing tests (inspection & sqlite-cache-store) seem unrelated to me.