Junio C Hamano <gitster@xxxxxxxxx> writes: > Junio C Hamano <gitster@xxxxxxxxx> 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?).
