[llvm-dev] [RFC] One or many git repositories?

Fri Jul 29 11:50:21 PDT 2016

I don’t know what you mean by dealing with the merging, I don’t expect any
difficulties, you need to elaborate.

What I don’t see you addressing here is why this should be more of a problem in the monorepo case (as it was implied in the email I was answering to).

Your answer made it sound like you thought the monorepo would solve all downstream problems ("I don't know what you mean… I don't expect any difficulties"), which it clearly does not.  Sorry for the misunderstanding.
--paulr

From: mehdi.amini at apple.com [mailto:mehdi.amini at apple.com]
Sent: Friday, July 29, 2016 11:17 AM
To: Robinson, Paul
Cc: David Chisnall; Bruce Hoult; llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] [RFC] One or many git repositories?

On Jul 29, 2016, at 11:07 AM, Robinson, Paul <paul.robinson at sony.com<mailto:paul.robinson at sony.com>> wrote:

-----Original Message-----
From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Mehdi
Amini via llvm-dev
Sent: Friday, July 29, 2016 10:02 AM
To: David Chisnall
Cc: llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>; Bruce Hoult
Subject: Re: [llvm-dev] [RFC] One or many git repositories?

On Jul 29, 2016, at 2:19 AM, David Chisnall
<david.chisnall at cl.cam.ac.uk<mailto:david.chisnall at cl.cam.ac.uk>> wrote:

On 29 Jul 2016, at 05:11, Mehdi Amini via llvm-dev <llvm-
dev at lists.llvm.org<mailto:dev at lists.llvm.org>> wrote:

What I meant by “different problem" is that “downstream users” for
instance don’t need to commit, that makes their problem/workflow quite
different from an upstream developer (for instance it is fairly easy to
maintain a read-only view of the existing individual git repo currently on
llvm.org<http://llvm.org>).

I’m not convinced by this distinction.  A lot of downstream developers
need to patch LLVM and we benefit when they upstream their changes.

I made a difference between downstream users and developers. I.e. someone
that just need to get and build compiler-rt vs someone that want to
*commit* to LLVM. Note that even by getting a single repo you can still
send a patch to the mailing list and someone can commit it for you
(including correct author attribution contrary to SVN).

We should not make it harder for them to do this.  To give a couple of
example downstream projects, both FreeBSD and Swift have patches on LLVM /
Clang in their versions that they gradually filter upstream.  Both
projects have LLVM committers among their members.  If the workflow that
we recommend for them makes upstreaming easy then they benefit
(maintaining a fork is effort) and LLVM benefits (having people provide
bug fixes makes our code better).

The workflow that we want to recommend to these people is:

- Fork the repo that you’re interested in from the LLVM GitHub
organisation

- Make your changes
- Send pull requests for anything that you think is of interest to
upstream

Note that the workflow you describe above still requires to export their
patch and import it in this clone before pushing.
(Note also that we accept patches on the mailing list, so one does not
even need to clone the official repo).

This makes the barrier to entry for sending code back upstream *much*
lower than it currently is,

I don’t understand this statement. As of today you can send a diff to the
mailing list, I don’t see how lower the bar can be.

to the benefit of all.  If the alternative is:

- Fork a read-only repo that you’re interested in from the LLVM GitHub
organisation

- Make your changes

Why? If you know you want to *push* commits upstream, fork the only useful
repo for that in the first place.

- Fork a different repo from the LLVM GitHub organisation
- Run a script to filter some of your changes into that one

I don’t know why you think there is a need for a script, or why it is
different from today.
Let say I’m working on a fork of the compiler-rt read-only repo and I want
to upstream a patch at some point:

Today:

- cd /path/to/compiler_rt-forked
- git format-patch …
- cd /path/to/compiler_rt-upstream
- git am  /path/to/compiler_rt-forked/0001-My-awesome-changes.patch
- git svn dcommit
- done

Tomorrow with a monorepo:

- cd /path/to/compiler_rt-forked
- git format-patch …
- cd /path/to/unifiedrepo-upstream
- git am  /path/to/compiler_rt-forked/0001-My-awesome-changes.patch —
directory=compiler-rt
- git push
- done

Alternatively, if I’m upstream a patch once a year, I don’t really need to
push it myself.

- cd /path/to/compiler_rt-forked
- git format-patch …
- email the patch.

- Send a pull request from that

Note that I think we deferred any change to the workflow for future
discussions (pull-request are not part of our workflow today).

- Deal with merging between the two yourself

I don’t know what you mean by dealing with the merging, I don’t expect any
difficulties, you need to elaborate.

Almost no upstream patch survives contact with the reviewers unscathed
(and quite often it doesn't look exactly like your local patch anyway,
for reasons ranging from expediency to local coding standards).
Therefore the patch that you merge back in to your local repo from
upstream differs from the one you already have locally.  If you are *very*
lucky, the differences can be handled with accept-theirs; in my experience
this is not normal, and in many cases you need to manually revert your
original local patch before merging the patch as received from upstream.
That's what "dealing with merging" means.  I invite you to review
"Living Downstream Without Drowning" if you still don't understand;

Don’t worry, I’m living downstream, and perfectly aware of the situation.
(And I was in the room at the dev meeting).

it describes a number of tactics we've developed over the years to
try to simplify the dealing-with-merging problem.

I had two entire dev-meeting sessions full of people who seemed to be
having this problem.  Maybe you don't; if so, you're lucky, but it
doesn't mean the problem fails to exist.

The “problem” exists, it exists today, and it will tomorrow whatever solution we go with (monorepo or split ones).

What I don’t see you addressing here is why this should be more of a problem in the monorepo case (as it was implied in the email I was answering to).

—
Mehdi

--paulr

I strongly suspect that we’ll get a lot fewer useful contributions from
downstream.  Or downstream people will just work on the monorepo and eat
the cost.

If someone is working on a downstream LLVM project and becoming familiar
with our codebase, then we want them to be subtly nudging their workflow
so that they eventually become LLVM contributors without noticing!

Sure. The distinction between “downstream users” and “developers” was made
in response to “there exists many user that just download and build a
subproject”. These are not people that are *developing* on a downstream
fork.

—
Mehdi

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