Re: [PATCH] BreakingChanges: early adopter option

[Junio C Hamano <gitster@pobox.com>]

Junio C Hamano <gitster@pobox.com> writes:

> Junio C Hamano <gitster@pobox.com> writes:
>
>> Discussing the desire to make breaking changes, declaring that
>> breaking changes are made at a certain version boundary, and
>> recording these decisions in this document, are necessary but not
>> sufficient.  We need to make sure that we can implement, test, and
>> deploy such impactful changes.
>>
>> Formalize the mechanism based on the `feature.*` configuration
>> variable to allow early adopters to opt into the breaking change in
>> a version of Git before the planned version for the breaking change.
>> ...
> ... to see what it involves
> to allow early adopters to experience Git 3.0 features/removals
> before it actually happens.

Sorry for a long monologue on this important topic, while everybody
is away.  Hopefully we'll see more comments when they get back once
the week starts ;-)

> Switching behaviour at runtime with feature.git3 should work well,
> and we can also add tests that checks the new behaviour by doing
> "test_config feature.git3 true".
> ...
> If we are willing to burden early adopters a bit more, we could make
> it a build-time option.  With "make GIT_BUILD_FOR_GIT3=YesPlease",
> binaries will be built for all the then-current Git 3.0 features and
> documentation under development.  It certainly is a simpler-to-build
> option that is easier for us, but I am not sure if that is acceptable
> by those who volunteer to test the upcoming big version.
>
> One thing to note is that depending on the nature of a change, once
> you start using a feature only available in a newer version of Git
> in your repository, the resulting repository may not be understood
> by an older version of Git...

While I still am with the position that we can do this either at
runtime or at build time, with the trade-off being that it is more
costly for developers to do it at runtime and more cumbersome for
early adopters to do it at build time, I realize that the last point
above is unrelated.  If one or some of the features behind either
feature.git3 runtime option or GIT_BUILD_FOR_GIT3 build-time option
makes a repository inaccessible to versions of Git without these
features, we have the extension.* mechanism to make sure nothing
breaks, and testing that such a Git3 feature is properly protected
by the extension.* mechanism is part of the early adopter testing.

How much more costly to do at runtime is still subject to further
analysis, I think.  I know that it means we need to build and
install the docs twice to support "git -c feature.git3=on help", for
example, but I am not sure what the best way to use CI would be
(write tests that check features with different behaviour by
explicitly running them with "git -c feature.git3=on"?  Run the same
set of tests in a separate job that has "[feature] git3" in its
$HOME/.gitconfig?).