llvm dev - Nov 2018 - RFC: Dealing with out of tree changes and the LLVM git monorepo

On 10/31/2018 03:02 PM, Justin Bogner wrote:
> Tom Stellard <tstellar at redhat.com> writes:
>>> On 10/31/2018 10:39 AM, Justin Bogner wrote:
>>>> Tom Stellard <tstellar at redhat.com> writes:
>>>>> On 10/31/2018 09:22 AM, Justin Bogner via llvm-dev wrote:
>>>>>> Hi all,
>>>>>>
>>>>>> I've spent some time in the last couple of days
trying to figure out how
>>>>>> to adopt the [LLVM git monorepo prototype] for an out
of tree backend.
>>>>>> TLDR: I'm not convinced that this prototype is the
right approach to
>>>>>> converting to the monorepo, and I have a possible
alternative.
>>>>>>
>>>>>
>>>>> I think it's too late at this point to start
considering alternative
>>>>> monorepo layouts.  We're already behind in getting the
current monorepo
>>>>> up and running, and I think discussing and implementing an
alternative
>>>>> will take too long and put our goal of moving off SVN by
next year's
>>>>> development meeting at risk.
>>>>
>>>> The layout here is not at all different, only the process by
which the
>>>> repo is generated. I strongly believe that a history preserving
>>>> conversion is very important if we want to avoid making porting
>>>> out-of-tree work horribly disruptive.
>>>
>>> The process is actually what I'm concerned about here, much
more so than
>>> the physical layout of the repo.  It takes time to discuss, develop
>>> and debug a new process for automatically syncing from SVN to a new
git
>>> repository.  We've already gone through all these steps with
the existing
>>> monorepo, so switching to something else at this point would be a
step
>>> backwards in my opinion.
> 
> I appreciate the amount of effort you and others have put in to get us
> this far, but in my opinion these steps are not quite complete. A lot of
> people have just started actually trying to merge with the monorepo
> prototype since it was announced that it's intended to be the official
> one last week. While there's certainly been a lot of discussion about
> the monorepo in general in the last couple of years, I really hadn't
> seen much serious discussion in public about the actual conversion until
> the "New LLVM git repository conversion prototype" thread earlier
this
> month.
> 
> Just to elaborate a bit on why I think this is important, I think the
> difference between the two approaches to conversion have to do with what
> we consider the real source of truth in our repository history. The
> current prototype rebuilds everything with SVN as a source of truth and
> throws out the official git mirrors, which sounds nice in theory, but
> has pragmatic problems. The reality is that a lot of people have been
> basing work off the git mirrors for a number of years now, so throwing
> away that history causes real world problems.
> 
> Mehdi AMINI <joker.eph at gmail.com> writes:
>> At this point we can still consider it, I highly doubt that waiting a
few
>> weeks would jeopardize the one year deadline (that is really not that
>> ambitious).
>>
>> What we should do though in my opinion is go with strict deadlines:
i.e.
>> every stage of discussion should be open for a very limited time.
>> The current linear repo has the edge, but we for example we could leave
>> this "zipper" proposal open for the next 1 week (or 2 if you
want) as an
>> RFC. Unless this alternative gets a high traction then we should close
and
>> move with the linear history repo.
>>
>> After almost two years of more or less stagnation, I feel it'd be
>> unfortunate to rush right now on what I perceive as important design
point
>> (especially for downstream users) like this one.
> 
> I agree with this. I'd appreciate if we give this a bit of time for
> other people to weigh in - I suspect others are hitting the same issues
> as I in trying to integrate with this version of the monorepo.
> 
What is the status with this proposal?  It has been 2 weeks now since
the initial email and it seems like the discussion  is slowing down.  Do
we still want to consider this zippered approach as a possibility for the
official repo?  Is the zippering tool proposed here[1] sufficient for
out-of-tree git users?

I think it's important to make a decision on this soon.  I know a year
seems like a long time to migrate, but with the holiday season approaching in
the US
it's going to be down to 9 months before we know it.

-Tom

[1] http://lists.llvm.org/pipermail/llvm-dev/2018-November/127704.html

Tom Stellard via llvm-dev <llvm-dev at lists.llvm.org> writes:
> What is the status with this proposal?  It has been 2 weeks now since
> the initial email and it seems like the discussion  is slowing down.  Do
> we still want to consider this zippered approach as a possibility for the
> official repo?  
I have very strong feedback from the engineer who does our upstream
merges that he does NOT want to see this zippered repository.  A clean
linear history makes understanding merges much easier.

James made a number of other important points about limitations of the
zippered repository:

http://lists.llvm.org/pipermail/llvm-dev/2018-November/127460.html

git-bisect being more complicated is a deal-breaker for me.  Checking
out a random commit and only getting part of the project is just odd.

                             -David

Based on the feedback so far, I propose that we call this discussion done
-- we will not go with this zippered proposal, but will proceed with
https://github.com/llvm-git-prototype/llvm/.

On Thu, Nov 15, 2018 at 5:06 PM David Greene via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Tom Stellard via llvm-dev <llvm-dev at lists.llvm.org> writes:
>
> > What is the status with this proposal?  It has been 2 weeks now since
> > the initial email and it seems like the discussion  is slowing down. 
Do
> > we still want to consider this zippered approach as a possibility for
the
> > official repo?
>
> I have very strong feedback from the engineer who does our upstream
> merges that he does NOT want to see this zippered repository.  A clean
> linear history makes understanding merges much easier.
>
> James made a number of other important points about limitations of the
> zippered repository:
>
> http://lists.llvm.org/pipermail/llvm-dev/2018-November/127460.html
>
> git-bisect being more complicated is a deal-breaker for me.  Checking
> out a random commit and only getting part of the project is just odd.
>
>                              -David
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