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

Thu Mar 5 08:02:20 PST 2020

> On Mar 4, 2020, at 23:18, Mehdi AMINI via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> 
> 
> On Wed, Mar 4, 2020 at 6:18 PM Fangrui Song <maskray at google.com <mailto:maskray at google.com>> wrote:
> 
> On 2020-03-04, Louis Dionne via llvm-dev wrote:
> >
> >
> >> On Mar 4, 2020, at 12:13, Mehdi AMINI <joker.eph at gmail.com <mailto:joker.eph at gmail.com>> wrote:
> >>
> >>
> >>
> >> On Wed, Mar 4, 2020 at 8:14 AM Louis Dionne <ldionne at apple.com <mailto:ldionne at apple.com> <mailto:ldionne at apple.com <mailto:ldionne at apple.com>>> wrote:
> >> Mehdi, Chris & others,
> >>
> >> I guess I did not express the main reasons for wanting to switch over very well in my original message.
> >>
> >> You original message was about “ commit attribution”, but now it is all about testing?
> >
> >It was about allowing GitHub PRs for libc++, and one of the reasons I cited was commit attribution.
> >
> >>
> >> Instead of jumping to a solution (pull-request) why not expressing the actual problem (lack of pre merge testing) and discuss it and explore all the possible solutions?
> >> I think the discussion would be much more productive if we take it from first principles here.
> >
> >That's really what I've tried doing below. Quoting myself:
> >
> >       > 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.
> >
> >Maybe I should have stated that earlier on.
> >
> >>
> >>
> >> 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.
> >>
> >> What changed recently that makes this suddenly critical compared to the previous years?
> >
> >At the risk of oversharing, this has been my biggest source of pain and frustration since I've became a libc++ maintainer. It's always been a problem since I started about 2 years ago. I can still feel the hurt of fixing the ripples of landing aligned allocation in libc++ in 2018, which lasted for weeks and weeks (on and off).
> >
> >Last week, I spent about 4 days fixing the ripples of a high-quality contribution that happened to break some bot configurations (and some botless ones too). That was both very stressful and frustrating, and I swore to myself this was going to be the last week like that.
> >
> >>
> >> Unfortunately, we currently don't have a good way of doing pre-commit testing on Phabricator AFAICT
> >>
> >>
> >> I thought we do now? I got a bunch of libcxx failing on my revision a few weeks ago.
> >
> >My understanding is that Christian was the one to setup the pre-merge testing we have currently (thanks!). And he says:
> >
> >    > This is soooooo much easier to set up and maintain than the Phabricator integration
> >
> >Furthermore, what libc++ needs in this area is not necessarily the same as the rest of LLVM. I don't know what pre-merge bots are setup right now, and it's certainly better than nothing, but we have a lot of test configurations that just don't really make sense for a toolchain:
> >- test suite in c++03/c++11/c++14/etc
> >- exceptions on/off
> >- with per-TU guarantee enabled/disabled
> >- all the macOS back-deployment targets against system dylibs
> >- etc.
> >
> >I'm not expecting anyone else to set these up cause it doesn't make sense. I need to be empowered to do it myself, and right now I'm not (or I'm really not looking in the right spot).
> 
> Looking at the few commits Louis made today:
> 
> commit a27f29c6e498266fa8d8d38c3c3f74305c684f8d
> commit 2b2a1a42c0a24f5a98ce1161e8ea100552126644
> commit 30cbdcb5c3694e2e6c4647ce88df9e3692bf90cf
> 
> They perfectly demonstrate that libc++ indeed requires a wider range of
> configurations that the existing pre-merge testing instructure cannot
> satisfy :/
> 
> I am not sure if you're being sarcastic here? From what I see here this is the kind of things that just requires building with a specific version of the compiler, and all of them seems even testable on Linux (not even some specific MSVC for fun?).
> Isn't this trivially testable in a Docker config on any cloud machine? If so there is no real infrastructure complexity I really see and the pre-merge *infrastructure* can perfectly suit this as well as any other infra as far as I can tell.

I think the point being made is that given the large number of configurations, it's not reasonable to run this pre-merge testing locally (even though the failures above could be reproduced locally). There's just too many bots for that to be practical (47 if we count BuildBot and GreenDragon). I've been doing a subset of that manually with my Mac and docker images, and it's a huge time sink for little benefit.

I don't think you're suggesting to test locally, but instead you're saying that some future pre-merge infrastructure on Phabricator could run these bots -- right? If so, I think you're right, cause at the end of the day these configurations don't require anything very exotic, indeed. But saying that is playing ostrich, cause the problem is that I am not empowered to setup these bots in the first place, the barrier being very high.

Louis

> 
> Compare to for example: bootstrapping a compiler and using it to cross-compile the test-suite that you need then to run on *actual embedded HW devices*, or any use case where you can't use the cloud and have to maintain your own lab with exotic hardware (looking forward to have an FPGA codegen in MLIR and see the kind of runtime test environment we'd need...).
> 
> Best,
> 
> -- 
> Mehdi
> 
> 
>  
> 
> I think libc++abi and libunwind may be in the same boat.
> These subprojects just have lots of variety.
> 
> >> The LLVM-premerge-test project recently added presubmit on Linux.
> >> Windows will hopefully follow soon (in beta right now I believe), and Mac afterward!  (Even though mac is lacking on the cloud provider availability)
> >>
> >>
> >> . 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,
> >>
> >> FYI that can read quite condescending...
> >
> >Sorry, that was really not my intent. The point I tried to make is that we have different priorities because we work on different projects, which have different requirements. So what may seem like just an annoyance to some might be a huge quality of life and productivity issue for someone else.
> >
> >Louis
> >
> >>
> >>
> >> 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/ <https://secure.phabricator.com/book/phabricator/article/harbormaster/> <https://secure.phabricator.com/book/phabricator/article/harbormaster/ <https://secure.phabricator.com/book/phabricator/article/harbormaster/>>
> >>
> >>
> >>> On Feb 29, 2020, at 23:06, Mehdi AMINI <joker.eph at gmail.com <mailto:joker.eph at gmail.com> <mailto:joker.eph at gmail.com <mailto: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> <mailto: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><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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