Track changes across multiple branches, c.f. "p4 interchanges" ?

We're moving over from Perforce to git.

In Perforce we can find out definitively if a given commit is on branchA but not on branchB.

It doesn't use textual comparisons (c.f. "git change-id") but tracks which changes are on which branches.

This is invaluable for managing rapidly evolving sets of release branches.

How do I go about doing this in git?

1. Cherry

"git cherry" - but surely once the change gets sufficiently mangled by conflicts, won't it end up being "lost" ? That has certainly been my experience in the past.

2. Merge

I could work on feature branches, and then "git merge --no-ff" the feature branch - but when I do that I end up pulling all the intervening history over. e.g.

                - X -         bugfix
              /       \
    A - B - C - D - E - M     main
     \
      R  relbranch

If I do "git merge bugfix" onto relbranch, then as well as getting X, I also get B and C, which I don't want.

3. Always start from a merge base

I could tell people that if they are making a bugfix that will need to go onto multiple branches, that they need to start from some common merge base, and then merge to the final target branches.

That's possible, but is quite a big change from the current workflow of working on main and then "p4 integrate" individual changes over.

And invariably people will start out thinking their change is not a bugfix, but a new feature, and then find that actually we need the new feature on the release branch.

4. Use gerrit change-ids

We could adopt gerrit change-ids. It feels like this is kind of a kludge, but perhaps it's the only thing that really works?

Is there something better?

Thanks! Luke

Luke Diamand <luke@diamand.org> writes:

This won't work exactly for the reason why you want to do #3 below.

Just to throw another in for completeness (not claiming which one is better and which one is worse):

5. Primarily use #3 to merge, but use "cherry-pick -x" when
   replaying a fix that was built on a wrong base, and tweak the
   procedure to find out "is this fix already on branch X?" to also
   pay attention to it.

It is no worse than #4, I would think, as both approaches would need to scan the commit log messages to find the commits that are not in the ancestry chain that participated to the branch.

On 21/04/2021 19:05, Junio C Hamano wrote:

With the gerrit change-id I can enforce that every commit will always have a change-id - either a brand new one, or one that's preserved from another commit.

If I just rely on people to use "git cherry-pick -x" then they will certainly forget, or do it twice, because people make mistakes.