feat!(H2): enable H2 by default

[metcoder95]

This relates to...

Enable HTTP/2 support by default.

Rationale

After a chat with @mcollina, we have noticed that the most important pieces for HTTP/2 to work within Undici has been already implemented.
The number of issues and bugs has been reduced since the implementation of the feature, and now the HTTP/2 client is fully onboarded into the Undici's queue system.

Given that the adoption seems good and stable, I propose to enable the H2 by default for the upcoming major release.

Changes

Features

Bug Fixes

Breaking Changes and Deprecations

Status

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

[ronag]

I'm ok with it being enabled by default for fetch. Though I don't think it provides any meaningful advantage outside of fetch.

[metcoder95]

My thoughts on this is that it benefits mostly the use-cases that requires usually network calls to the internet (basically in the wild), as its where most of the HTTP/2 interactions happen.

Fetch sadly does not really benefits from it as is not totally capable of handling H2 headers accordingly (and based on its semantics it seems mostly modeled around H1 in mind).

The benefit I see on enabling it is for the uses cases previously mentioned, but it would beup to the server to decide which protocol to use.

On top, this will only have visible effects on TLS connections, as we do not support h2c; for instance in a good majority of cases H1 will prevail.

Maybe worth to document more of its intricacies just to put users aware?

[mcollina]

Fetch sadly does not really benefits from it as is not totally capable of handling H2 headers accordingly (and based on its semantics it seems mostly modeled around H1 in mind).

How? Are you saying that a fetch() call will fail if we enable this?

[metcoder95]

Fetch sadly does not really benefits from it as is not totally capable of handling H2 headers accordingly (and based on its semantics it seems mostly modeled around H1 in mind).

How? Are you saying that a fetch() call will fail if we enable this?

Nop, fetch works with H2 regardless. What I am referring to with incompatibilities is that fetch does not handle several concepts of H2 (e.g. pseudo headers handling, does not fail but we do not parse these headers for the Headers map)