[ech] ECH PR split/progression

[Stephen Farrell]

Hiya,

Before the holidays, I submitted a PR for ECH [1]. That's
a whopping 17KLOC big, so understandably daunting in terms
of getting review and working towards a merge.

On the PR discussion, a couple of maintainers suggested a
functional split into smaller PRs might be a more tractable
way to aim for progress.

For me, that implies splitting into 3 PRs, each building
on the eariler, as follows:

- server - code that's needed to operate an OpenSSL server
   that supports ECH
- client - the above plus code that provides client-side
   functionality
- extras - probably most test code and ancillary functions

The logic for the above is mostly that the server code
requires far fewer state machine changes and low level
design decisions, and is more immediately useful (given
that web servers using OpenSSL could make use of the 1st
PR).

Doing that will require a pile of work on my side, so
before going ahead with that, I'd like to get a bit of
reassurance that a) that's seems like a sane plan to more
maintainers, and b) that maintainers are happy to spend
review effort if we follow that plan.

So, I'd love to get that reassurance, whatcha think?

FWIW, I'm unclear if following this plan would mean that
(all going well) each PR would be merged with master before
the next is processed, or if the plan is really to help with
more tractable reviews before ending up again with one PR
that has all the functionality. I'm assuming the former, but
would appreciate confirmation that that's feasible. (E.g.
the 1st "server" PR would have to omit lots of test code
that exists and works today, but that wouldn't work if the
client code is temporarily omitted.)

Thanks,
S.


[1] https://github.com/openssl/openssl/pull/22938
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