[llvm-dev] RFC: Dealing with out of tree changes and the LLVM git monorepo

[Justin Bogner via llvm-dev]

James Y Knight <jyknight at google.com> writes:
> Thanks for writing this up. I think it's a really important point which
> deserves discussion.
>
> Ultimately, I think it is a question as to whether to prioritize the easy
> switchover for existing out of tree forks, or to prioritize having the best
> conversion we can make. I feel very strongly that the latter should be the
> priority for the official repository conversion, and that, therefore, we
> should not use the zipper method for the official repository going forward.

How do you define "best conversion" here? I may be missing something,
but I really don't see any actual advantage to re-writing the git
history from scratch rather than leveraging the existing git mirrors to
build a monorepo.

The re-generated history approach gives us an artificial alternate
history where we developed in a git monorepo from the beginning of time.
It throws away a bunch of information for the sake of making a
"pristine" conversion with fewer branches, even though those branches
have almost zero cost.

The zipper approach gives us the best of both worlds - it provides a
monorepo view for all time for anyone who wants it, but also preserves
the history that people have been using and relying on for a number of
years.

I'd like to hear what you think are the actual disadvantages of the
zipper approach. I've spoken to quite a few people about it in the last
few days and I haven't really found any yet.

> However, it's also worth putting much thought into making switchover as
> easy as possible within the confines of what's possible given that
> prioritization.

While I obviously agree with your intent here, I'm not convinced that
arbitrarily prioritizing a vacuous definition of "the best conversion"
over engineering tradeoffs makes sense. The cost of the switchover is a
real cost that will affect a large portion of the llvm community, and if
the cost is too high it's very likely that it will delay the process of
switching to git even further as various groups aren't able to get
themselves switched over by whatever deadlines we set.

> On Wed, Oct 31, 2018 at 12:22 PM Justin Bogner via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> An arguably cleaner solution would be try to recreate all of my trees'
>> history artificially as if they were based on the monorepo prototype
>> history all along, but this has two problems. First, it's a very
>> significant tooling effort to do this - I'd need to match up several
>> years of merge points to their corresponding spots in the monorepo
>> prototype and somehow redo all of the merges in the same ways. Tools
>> like "rebase --preserve-merges" don't really help here, since they abort
>> on merge conflicts and ask a human to resolve them again.
>
> I realized I had most of the functionality needed for this already written
> for such a conversion tool, so I've written a tool which is able to
> (mostly!) convert a single-project repository (with all its commits and
> merges), into a monorepo repository (with the same commits and merges). The
> transform is conceptually trivial -- take the subproject's tree from the
> old commit, and take the rest of the content from the monorepo parent.
> That's perfect -- no need to deal with any conflict resolution, UNLESS
> there are potential merge conflicts in the parts of the tree OUTSIDE the
> original repository's subproject.
>
> As the original repository will -- by definition -- not touch the other
> directories, such a conflict can only happen if you have merges between
> upstream-svn release branches in your history. E.g., if, in your fork of
> clang.git, you started working from the release_50 branch, then
> (potentially after a bunch of work), merged the release_60 branch. In your
> clang fork, you of course had to resolve any conflicts in clang, but would
> NOT have resolved conflicts between release_50 and release_60 in "llvm" or
> other subprojects. The tool can't necessarily know what to do here either.
>
> Now, in that case, it's pretty likely that you'd want to just take the
> release_60 tree as is, throwing out the changes that happened only on the
> release_50 branch. So, if this seems useful, I can imagine adding some
> heuristics or manual override to support that particular case.
>
> I'll post the tool soon. It could also be extended to support conversions
> from the previous monorepo repositories to make that easier for folks too.

I'm looking forward to trying this out, but I'm still skeptical about
whether it will be enough.

Consider a world where I convert all of my branches as-if they were
based on the monorepo. Now, something comes up and I need to hot fix
last year's branch. I probably can't actually submit this fix from the
monorepo, since it would be too disruptive to also hot fix the
configuration changes to submit from a new layout of repositories. Now I
need to maintain two copies of my code and manage merging between them.

>> Even if I were to come up with tooling that managed this, I'm still
>> left with a completely new set of hashes for commits and no easy way
>> to map them to existing references in emails, bug trackers, and
>> release notes
>
> *Creating* such a commit mapping is certainly easy.

Finding every place to update is much harder, and actually updating them
is probably impractical.

It should be possible to write a tool to find every commit reference in
my bug tracker and add a comment saying where it moved, but everyone
will have to be careful to check for this update if they look at some
particular comment. Emails and release notes are immutable.

I suppose I could provide a tool that mapped an old hash to a new one
and teach everyone to use it every time they looked at a hash related to
my repos. When could we stop using this tool every time we look at a
hash though? 5 years? 10 years?

> [....]
>
>
> One more option -- which I've not yet tried, but seems like it could be
> really promising -- would be to have _your_ repository's history have a
> different shape from everyone else's, but still keep the same commit hashes
> at head, going forward. Of course "That's impossible!" -- editing the
> history will necessarily change the hash! But, actually, you can pull this
> off using "replace" refs (see "git replace").
>
> Start with the git merge you already created (merging all your
> split-repositories into one branch on top of a monorepo-prototype commit).
> Then, "git replace" the monorepo-prototype commit that you merged in with a
> commit that has the same content, but from your "zippered" repository
> history. That won't change the hash (thus, future merging will work
> properly), but it effectively changes the history to be the way you'd like
> to see it.
>
> Thus you'll see the zipped history up until that point, avoiding seeing
> multiple copies of svn commits in your history, and you can use the new
> monorepo commits going forward.
>
> (One note -- users would need to fetch the replace ref after cloning a new
> repo (e.g., with `git fetch origin refs/replace/*:refs/replace/*`), since
> clone won't fetch it automatically. If they forgot to, they'd simply see
> the "normal" history, rather than the zipper history.)

So, we publish two competing versions of the git history and let people
choose? This sounds like a splitting the baby type solution to me ;)