[llvm-dev] Allowing PRs on GitHub for some subprojects

Wed Mar 4 08:13:48 PST 2020

Mehdi, Chris & others,

I guess I did not express the main reasons for wanting to switch over very well in my original message. Like Christian talked about, for me it's all about pre-commit testing. I believe pre-commit testing is a widely shared desire among this community. However, how badly it is missed depends on sub-projects, because they have different realities. For example, in libc++:

1. We have a lot of first-time contributors, which means that the maintainers end up shepherding many contributions. In particular, this often means fixing small breakage following their changes, which can be difficult for them because they can't reproduce the failures locally, and they might not even know where to look. While these contributors can submit valuable improvements and bug fixes, we can't expect them to fix every last platform that we support in the current state of things -- it's hard, it's boring, and it's stressful.

2. Our testing matrix is very large, and interactions between different configurations (usually #ifs/#elses) is very subtle. This means the rate of mistake-on-first-try is, I think, higher in libc++ than in most other LLVM projects. Even with careful review, I find that a large percentage of changes end up breaking something somewhere, and I have to fix it (usually quickly enough to avoid reverting).

As a result, the lack of pre-commit testing is actively harming the health of libc++ as a project. It might be true for other projects as well, but I can only speak for libc++ because that's where I have first hand experience. Unfortunately, we currently don't have a good way of doing pre-commit testing on Phabricator AFAICT. From the Harbormaster documentation [1]:

    You'll need to write a nontrivial amount of code to get this working today. In the future, Harbormaster will become more powerful and have more builtin support for interacting with build systems.

So while I appreciate all the efforts being made in this area, I still don't even know where to start if I want to setup pre-commit testing for libc++ today. However, the path is very clear with GitHub PRs and there are many options available.

Whenever I hear arguments of dividing the community, not being able to share infrastructure, the lack of Herald -- those all make a lot of sense to me and I think they're good arguments. However, it is clear that folks who even think about these arguments are not paying the same cost for the lack of good pre-commit testing that I'm paying on a weekly basis, because for me that outweighs everything else.

I don't know how to come to a decision here, all I know is that libc++ needs to get out of the status quo soon. And if the solution is that Harbormaster suddenly becomes usable without an unreasonable time investment from me, then I'm fine with that too. I'm not looking to switch to GitHub PRs for the sake of it, I'm looking to solve problems that are harming libc++ in the current system.

Cheers,
Louis

[1]: https://secure.phabricator.com/book/phabricator/article/harbormaster/

> On Feb 29, 2020, at 23:06, Mehdi AMINI <joker.eph at gmail.com> wrote:
> 
> 
> 
> On Tue, Feb 25, 2020 at 4:19 AM Christian Kühnel via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
> Hi Louis,
> 
> I think this is a good idea. We should start with some local experiments where people are willing to try it and figure out how well that works and what does not. Why not allow this for "not significant" changes? They are merged without review today, so we could do them with reviews (and automated tests) via pull requests instead.
> 
> I still feel this is only a recipe for confusion if "some" pull-requests are accepted on Github but not all. So -1 from me on this.
>  
> 
> @Mehdi
> - it does not favor to build common tooling: the recent work on enabling pre-submit CI tests on Phabricator is valuable and I'm looking forward to get this extended. But splitting the various ways of contributing to the repo just means more infrastructure to build to sustain this kind of efforts. (the infrastructure is easier built on GitHub by the way, but that is an argument in favor of migrating from Phab to GH for the full-project).
> 
> Oh I'm happy to add Github support as soon as someone switches on PRs. This is soooooo much easier to set up and maintain than the Phabricator integration. And we already have builds for the release branch (https://buildkite.com/llvm-project/llvm-release-builds <https://buildkite.com/llvm-project/llvm-release-builds>) anyway. So we could easily scale that up. And we can only get pre-merge testing on Phabricator to a certain point, as it's not triggering builds for ~50% of the code reviews.
> 
> @Chris Lattner
> Although I am one of the (many) people who would love to see us move from Phabricator to GitHub PRs, I think it is super important that we do the transition all at once to keep the LLVM community together.  I’m already concerned about the fragmentation the discourse server is causing, e.g. MLIR not using a -dev list.  I’d rather the community processes stay consistent.
> 
> Please allow me to disagree there. IMHO we're way too large and diverse of a project to do binary, overnight transitions.
> 
> You seem to be arguing the "how to transition" while there is no agreement on a transition happening in the first place.
>  
> We're also too large to follow a one-size-fits-all approach. If we agree,
> 
> I don't: we went with a monorepo because we believed that the one-size-fits-all would be more beneficial than splitting, both in terms of infrastructure, but also in terms of the practices of the community, etc.
> 
> Github PRs are the right glow, why take this step-by-step. We should have something like a list of important and supported use cases/interactions for the infrastructure. Then we could start working on them one-by-one and figure out if/how they could be implemented on Github and how we could do a smooth transition between these.
> 
> If Herald rules are important: Find a way to implement something similar for Github. Maybe there is even a market for such a tool.
> If transparency is the problem: Find a way to mirror PRs into Phabricator, so people can at least see them there. 
> We're not restricted to community contributions there. We can also pay someone to build the things we need.
> 
> One aspect here though is that we can pay someone to build the things we need in Phabricator, we can't change GitHub though.
> It was mentioned in the past that we should engage with GitHub and see if they would add the feature we're missing to their roadmap, if it hasn't been done I'd start there: building up this list of things that need to happens before we can agree towards a transition, and engaging with GitHub to have these. 
> 
> -- 
> Mehdi
> 

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