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

[James Y Knight via llvm-dev]

On Fri, Nov 2, 2018 at 2:11 PM Justin Bogner <mail at justinbogner.com> wrote:

> 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.
>

I note that "we", where "we" = llvm upstream developers, *have* been
developing in a monorepo -- an SVN monorepo, with a linear history. The
llvm-git-prototype repository better matches this actual development
history.

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.
>

You mean "merge commits" here, not "branches", I believe, since your
repository has a single branch, "master".

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.
>

The downside, generally, is that it makes the history _very_ complex. Which
is not necessarily bad, in of it self, but it's not really representative
of the development history of llvm, and it turns out that it causes
problems.

Two concrete disadvantages have been mentioned on this thread, already:
1. gitk cannot be used -- it just falls over when given the history with
300000 merges.
2. git bisect becomes somewhat trickier.

I'd add a couple more to that:

3. "git log -u llvm/" no longer works (for any file/path), because the
commits which *actually* changed the files don't occur at that path, and
the default is to omit diffs arising in a merge commit. (The actual content
change happened at a different path -- the root of the tree, not under
"llvm/", and is just moved under llvm/ in the merge commit.)

You can work around this, via "git log -u -m --first-parent llvm/" to get
the diffs from the merge commit itself. But this is a large annoyance --
looking at path histories is a very common task. Making matters worse right
now is that the zipper merge-commit doesn't have the full commit message,
only the first line. That, at least, can be fixed.

4. Other commands like "git log -u -S CFGStackify" become trickier -- it
returns the individual project commit, not the merge commit, and gives the
"wrong" pathnames (without the subproject prefix). So every time you look
at one of these, you need to map it back yourself.

Those are just the first things I tried -- I think there will be *more*
variants of these sorts of issues which will show up with further attempts
to use a repository built in this style. Certainly none of this is FATAL
problems, but will be a constant irritation.

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


I can't speak to your exact problem, not knowing anything about your
infrastructure or workflows. But, if you were to want to keep using your
old separated repositories for your old branches, and to switch only master
and future branches over to the new monorepo, "git format-patch" and "git
apply" do make copying commits or stacks of commits between completely
different repositories (even between split and mono repositories)
relatively straightforward.


[...]



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 ;)
>

The point here is not to offer "competing" versions -- the
llvm-git-prototype monorepo (with the linear history) would be the official
repository, recommended by default for everyone.

The technique outlined above simply offers a solution for those who may
desire to have their historical commits appear in the zipper-merge fashion,
to do so up to the point in history where they last pulled from the split
git repositories into their private forks, and to then switch to the
official monorepo afterward.

I'd still recommend avoiding that, generally, because of the issues caused
by the more complex structure of the zipper-repository. But if that's the
path that is best for your repository and infrastructure, I believe it's
feasible to do so without needing to impact others.
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