Re: [PATCH] Documentation: mark `--object-format=sha256` as experimental

["Martin Ågren" <martin.agren@gmail.com>]

On Fri, 7 Aug 2020 at 22:50, Junio C Hamano <gitster@pobox.com> wrote:
>
> Martin Ågren <martin.agren@gmail.com> writes:
>
> >> I'm fine with marking the functionality experimental for a few releases,
> >> since it is possible we have bugs people haven't found, and adding a
> >> note about interoperability after that point, since I think that's a
> >> fair and valuable issue.  I think if we go a few releases without any
> >> major issues, we can change this to the following:
> >>
> >>   Note that a SHA-256 repository cannot yet share work with "regular"
> >>   SHA-1 repositories.  Many tools do not yet understand SHA-256
> >>   repositories, so users may wish to take this into account when
> >>   creating new repositories.
> >
> > With respect, I think that's too aggressive. By that time, we may
> > conclude that, e.g., the "v2 pack indices with SHA-256" file handling is
> > robust. But I'd be surprised if using `git init --object-format=sha256`
> > in June 2021 won't cause *some* extra work for users or ourselves
> > further down the line compared to using a regular SHA-1 `git init`.
> > Pushing to a SHA-1 hosting service will become *possible* at some point,
> > but maybe it won't be *efficient enough to be practical in the real
> > world* until some time after that.
>
> IOW, you question "if we go a few releases without any major issues"
> part?  I tend to agree that for a large change like this, a few
> releases may not be sufficiently long time for a feature that is
> marked as experimental in big flashing red letters to get exercised
> enough to get major issues noticed.

Yeah, thanks for summarizing what I failed to express using so many
words.

I'm fully open to the idea that some people want to leave SHA-1 behind
and that they can do it today, in some "local" sense. If those people
are fully aware that they are guinea pigs, it might actually be ok for
us to subject them to a few rounds of "oops, Git v2.32.0 produces data
that v2.34.0 and later will barf on". Or at least it would be on our
table whether we wanted to be that cavalier.

Once SHA-256 repos as such are no longer experimental, I fear that we
can only buy ourselves that leeway by introducing fiftyeleven different
config flags for "please produce auxiliary files X even if you don't
actually use them", "please do use X, and I'm fully expecting to trip on
them if you decide to tweak them in backwards-incompatible ways", and so
on. The alternative to buying such leeway might be to establish, pretty
early on, a respectable set of things we support "for compatibility
reasons".

> > Now would probably be a good time to update the hash transition
> > documents, first of all to tick off what we've already done, and second,
> > to reassess the rest.
>
> Yes, it is a good idea to stop and see where in the overall large
> picture we currently are.

Martin