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

Tue Nov 6 12:03:23 PST 2018

That would be somewhat tricky to implement in the svn conversion pipeline,
as the current process is not based on, and does not interact in any way
with the "split" svn2git-based repository generation process. It would
require linking or merging the two processes.

I'd have to:
1. Ensure I have an updated set of svn2git repositories with the correct
hashes for the newest commit, by either:
 a. Integrating the svn2git generation pipeline and the monorepo generation
pipeline together.
 b. Polling the github/git.llvm.org repositories waiting for the required
commits to appear in all the right projects.
2. After getting the finished svn2git hashes, extract the new mappings, and
pass that into the monorepo conversion script.

On the other hand, modifying migrate-downstream-fork.py to put the original
hash into the description for the git-only (NON-upstream-svn) commits would
be easy. Having the original hash in the description for those git-only
commits might be 95% of the value for people anyways?

On Tue, Nov 6, 2018 at 2:26 PM Chandler Carruth <chandlerc at google.com>
wrote:

>
> On Tue, Nov 6, 2018 at 7:29 AM Joseph Tremoulet via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Would it help to include the llvm-mirror commit hashes in the new commit
>> messages?  Currently the conversion injects “llvm-svn:<revision>” into the
>> commit messages; it seems like with a bit of work it could also inject
>> “llvm-mirror/<subproject>:<hash>”, couldn’t it?  I realize that’s still
>> just providing a way to manually look up new hashes from old hashes, and
>> Justin has explained why that’s still painful, but I’m wondering if it
>> would be more palatable with the mapping a bit more discoverable that way,
>> and consistent across different projects in this boat, and with no more
>> tooling required than `git log --grep <old_hash>` to look up a new hash…
>>
>
> FWIW, I think adding the "official" git mirror hashes along side the svn
> revision to the commit messages would be great if it can be implemented
> reasonably easily.
>
>
>>
>>
>> *From:* llvm-dev <llvm-dev-bounces at lists.llvm.org> *On Behalf Of *James
>> Y Knight via llvm-dev
>> *Sent:* Friday, November 2, 2018 5:56 PM
>> *To:* Justin Bogner <mail at justinbogner.com>
>> *Cc:* llvm-dev <llvm-dev at lists.llvm.org>
>>
>>
>> *Subject:* Re: [llvm-dev] RFC: Dealing with out of tree changes and the
>> LLVM git monorepo
>>
>>
>>
>>
>>
>> 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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>> llvm-dev at lists.llvm.org
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>>
>
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