[llvm-dev] Enable Contributions Through Pull-request For LLVM

Jameson Nash via llvm-dev llvm-dev at lists.llvm.org
Thu Nov 7 11:16:38 PST 2019

I'm not sure I understand the need for that, although that's perhaps just
my lack of experience with certain types of workflows. And seems somewhat
more relevant to the choice to switch to git than whether to use GitHub PR
workflow. As that page says, each fork supports an unlimited number of
branches, each of which is essentially equivalent to a full fork. So you
can have multiple clones locally each working from a different branch of
the same upstream fork. That's less true if you are using the issue tracker
or PR features, but that seems like it'd be a relatively small set of teams
that wanted those extra features.

What would this look like in Phabricator today? My impression has been that
the approximate equivalent was manually copying around patch files. Or
perhaps setting up a new svn remote copy?

Thinking back on my own transition from svn to git, perhaps one of the
points of confusion is in the way to use the respective tools. In svn, it's
usually necessary to do most work directly on `trunk`. In git, it's much
better to always create feature branches. The git tool won't stop you from
committing to the master branch directly (it's the default), but I'd say
that git will keep trying to make life hard for you if you do. Since git
will keep trying to synchronize the meaning of `master` across all remotes,
it can keep creating conflicts with work-in-progress or old work. I don't
know if there's any resources warning you not to make a GitHub PR from
master on your fork (or between any other two branches with the same name);
this is just something I have learned from experience to avoid.

On Thu, Nov 7, 2019 at 12:32 PM Finkel, Hal J. <hfinkel at anl.gov> wrote:

> On 11/7/19 10:13 AM, Jameson Nash wrote:
> I don't intend to weigh in on either side, but just give a perspective on
> a few questions asked.
> From an outsiders perspective, that list isn't what I'd typically describe
> as the workflow, since it makes "fork" and "branch" sound like difficult
> operations. This sounds akin to thinking that someone would reclone the svn
> repo before working on a new commit—possible, but rather extreme. The
> distributed nature of git means that the fork is pretty simple for github
> (essentially creates a new branch in their database, but that is just
> defining a new name for an existing commit).
> FWIW, I've not found it to be so straightforward in practice. If you have
> multiple LLVM forks, representing different distinct projects, because
> GitHub only lets you have one fork per account, the process via which you
> can work with multiple derived projects is actually quite annoying.
> GitHub has a page on this (
> https://github.community/t5/Support-Protips/Alternatives-to-forking-into-the-same-account/ba-p/7428),
> but all of the work-arounds are pretty bad.
> In short, only the first fork is easy. After that, it gets harder.
>  -Hal
> I've noticed GitHub recently made it easier to delete the fork, but I'd
> argue most users shouldn't care. What you'd typically do instead is to do
> `git remote add <name> <url>` and/or use their `hub` CLI wrapper (
> https://hub.github.com) to synchronize work between multiple remote and
> local locations.
> > On the note of branches for PRs, don't they require users to push their
> local branches to the remote repo to create? That means we'll end up
> thousands of branches in git.
> Yes, but a branch is just a name for a particular list of commits. Unlike
> svn, it is not the commits themselves, so they have almost no performance
> implications. I don't think LLVM would want everyone pushing to their work
> in progress to the central repo, so the "thousands of branches" would be
> scattered across each individual contributor's repo, and would represent an
> likely inconsequential fraction of the number of branches already on GitHub.
> > Impossible to chain reviews - a PR diff can only be made on top of git
> master branch
> It is possible to make a PR against someone else's PR on their fork,
> although I hesitate to compare this to the far more usable phabricator
> capability, so perhaps "impossible" is the right term anyways.
> By contrast though, GitHub has no intrinsic concept of a "master" branch.
> Even aside from the question of using GitHub PRs, you can ask GitHub to
> make a diff between any two (or three) points in time in a few ways, such
> as you could also do locally with git. For example, this'll show you the
> last two commits:
> https://github.com/llvm/llvm-project/compare/HEAD^^...master
> <https://github.com/llvm/llvm-project/compare/HEAD%5E...master> (two dots
> for a direct diff, three for showing only changes since the branch point).
> There's also some convenient additional tooling such as adding ".patch"
> will download it as a raw patch file instead showing in the rich editor.
> > There is no way to see previous version of the patch.
> This is a fairly new feature, but the PR itself now lists these as events
> (e.g. "user force-pushed from abcdefg to gfedcba", where each of those are
> hyperlinks) in the comment feed.
> > how do i go back to previous revision
> Answering a slightly different question in the hopes someone finds this
> information useful: locally you can do `git reflog <branchname>` to get a
> history of the state of the HEAD of any tag on that particular machine for
> the last 90 days.
> On Thu, Nov 7, 2019 at 8:10 AM Nemanja Ivanovic via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>> I find Hal's analysis of this subject to be the most pragmatic one in the
>> thread and really want to chime in with a big +1 for the balanced approach.
>> TL; DR;
>> As it was outlined by previous comments in the thread, the GitHub PR
>> review tool is inferior to Phabricator (and I would argue that this is an
>> objective statement). Those that are only familiar with GitHub may not
>> ascribe much value to capabilities such as commenting on unchanged lines,
>> expanding whatever context you're interested in, seeing the diff of two
>> revisions, etc. But for a good portion of the LLVM community, these things
>> are important.
>> I think a well integrated solution would not place undue burden on the
>> author or the reviewer. Allowing pull requests seems to be desired from the
>> perspective of making it easy for authors. Allowing the review to happen on
>> Phabricator seems to be desired from the perspective of making it easy for
>> the reviewers. So a solution that can do both is the best of both worlds.
>> One comment on the initial proposal is that it seems like what is
>> proposed may take away the supposed benefit of moving to PR's in the first
>> place - making it easy for the author. I fail to see how the workflow*:
>> - Fork
>> - Clone
>> - Create branch and modify code
>> - Push
>> - Create PR
>> - Squash+merge after approval
>> - Delete the fork
>> Is easier from the current:
>> - Clone
>> - Create branch and modify code
>> - git diff -U99999
>> - Post through web UI (or use arcanist for these two steps)
>> - Apply the approved version and git push
>> * There is a distinct possibility that I am misunderstanding how the new
>> workflow would work and it would be easier than what I outlined above, but
>> that's just the way I understand it.
>> On Thu, Nov 7, 2019 at 6:42 AM Finkel, Hal J. via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>> I think that it's really important that we try to strike some balance
>>> here. Based on my experience, this thread, and offline conversations, two
>>> things seem clear to me:
>>>  1. Overall, Phabricator is a superior tool for managing code reviews
>>> and some related processes (although GitHub's tools certainly have some
>>> benefits, and both are getting better over time).
>>>  2. Not accepting GitHub PRs forms a barrier to entry for casual
>>> contributors, and perhaps, causes workflow-integration inefficiencies for
>>> some others.
>>> I'm also deeply concerned about having another place to search for
>>> historical data, track conversations (including the ability to cross-link),
>>> and so on.
>>> I think that we should first explore the solution that Facebook uses or
>>> used, see: https://github.com/facebook/hhvm/wiki/What-is-Phabricator
>>> They had an import system that, when a PR was submitted, would import it
>>> into Phabriactor and post a reply providing an explanation and a link to
>>> the imported differential. This might be the right balance between ease of
>>> use for casual contributors and the needs of the more-regular contributors
>>> providing the code review (and, likely, those actually committing changes).
>>> Other projects have worked on various kinds of integration schemes
>>> (e.g., https://github.com/framawiki/Github-notif-bot) that we should
>>> investigate. It seems that the Phabricator developers are also working on
>>> some more-general API which better supports this kind of use case (e.g.,
>>> https://secure.phabricator.com/T12739), although the status seems
>>> unclear to me.
>>>  -Hal
>>> On 11/7/19 4:30 AM, James Henderson via llvm-dev wrote:
>>> Having been using Github internally for code reviews of private patches
>>> on LLVM, and Phabricator for upstream ones, I've found the latter to be far
>>> easier to use. Prior to working with LLVM, I had basically no experience
>>> with either, so I'd say I'm coming from a fairly neutral starting point.
>>> Here are some of my observations (I want to highlight 9 as a particular
>>> issue, due to other recent discussions I've seen on the mailing list):
>>> 1) I don't know of a natural way to chain related patches together on
>>> Github (aside from explicitly mentioning them), but as separate reviews.
>>> This is useful for bigger features/refactorings where each individual step
>>> provides some benefit, but seeing the bigger picture is easy. Phabricator
>>> has the child/parent objects option.
>>> 2) Phabricator allows placing comments anywhere in the diff context,
>>> which is useful when the commit affects lines that haven't actually been
>>> changed, and those lines need addressing/referencing in some way. Github
>>> doesn't allow comments outside the immediate context around changed files
>>> 3) As a +1 to Github on the other hand, commits always come with full
>>> context, so you don't have to remind people to include it.
>>> 4) Phabricator's ability to see what has changed since the previous time
>>> you commented seems to be much more reliable than what Github provides.
>>> 5) With a Github PR, if you want to include a minor change to the patch
>>> prior to committing, you have to commit it to the PR, if you wish to use
>>> the UI to do the merging. Of course, you could just not push the patch via
>>> Github.
>>> 6) I myself have on numerous occasions messed up my commit message when
>>> committing via the Github UI, because it's not obvious unless you are a
>>> seasoned user that the title of the commit appears as the first line of the
>>> commit message. This is important when doing a squash and merge.
>>> 7) The PR approach does at least allow committing via the UI, which is
>>> perhaps a little less fiddly in some cases.
>>> 8) I rarely bother creating a branch for Phabricator reviews, because I
>>> don't need it (I'm often not working on multiple things at once), so the
>>> extra hassle of creating a branch/checking it out etc that Github requires
>>> is annoying. On the other hand, Github PRs are generally quite easy to
>>> create once you have pushed a branch.
>>> 9) On the note of branches for PRs, don't they require users to push
>>> their local branches to the remote repo to create? That means we'll end up
>>> thousands of branches in git. Not sure that this will do performance any
>>> good, and I seem to remember there was general agreement that we didn't
>>> want people to push their branches generally. Yes, in theory branches
>>> should be deleted after they're merged, but I've seen that locally not
>>> happen regularly, and that's even assuming that all PRs get merged in (they
>>> won't).
>>> 10) Expanding context on Github is a pain: there is no option to just
>>> expand the whole context in a block, which means that if you need to see
>>> something much earlier in a large file, you have to click a LOT. Also, the
>>> browser view often doesn't then go where you expect it to from my
>>> experience. Phabricator has a "expand all N lines" option.
>>> 11) Small one this one, but missing new lines at end of file are much
>>> more obvious on Phabricator than Github.
>>> 12) Not sure if this is a real issue, but Github reviewers are limited
>>> in number (I think it's 15?). To my knowledge, there is no such limit with
>>> Phabricator (but then how often do you end up with 15 people marked as
>>> reviewers?).
>>> I'm sure I could come up with other points for/against Github PRs. On
>>> balance I definitely prefer Phabricator.
>>> James
>>> On Thu, 7 Nov 2019 at 09:53, Aaron Ballman via llvm-dev <
>>> llvm-dev at lists.llvm.org> wrote:
>>>> On Thu, Nov 7, 2019 at 3:09 AM Roman Lebedev via llvm-dev
>>>> <llvm-dev at lists.llvm.org> wrote:
>>>> >
>>>> > Strong -1 personally.
>>>> Likewise, for many of the same reasons detailed below.
>>>> ~Aaron
>>>> > * What is the endgoal? To fully kill phab and move to github
>>>> pullrequests?
>>>> >   it might be best to discuss *that* first. (did i miss an RFC?)
>>>> > * Separation of attention - does everyone who cares
>>>> >   now has to also look at pull requests for reviews;
>>>> >   or should they be exempt from general review attention?
>>>> > * For me personally after using phabricator, github now seems
>>>> >   extremely crude, laggy, limited. To name a few:
>>>> >   * There is no way to see previous version of the patch.
>>>> >     I don't think there is any way to disable force-push for PR's.
>>>> >     While this is only 'slightly' limiting for the reviewer,
>>>> >     this can be more limiting for the author - how do i go back
>>>> >     to previous revision? I can't, i need to maintain a copy
>>>> >     of every branch i pushed manually.
>>>> >   * Impossible to chain reviews - a PR diff can only be made
>>>> >     on top of git master branch. Any non-trivial change consists of
>>>> >     separable PR's. Now either one will only be able to submit
>>>> >     dependent PR after the prereqs are merged, or the diff will be
>>>> >     impossible to read.
>>>> >   * Does not load large diffs by default.
>>>> >     That caught me by surprise several times
>>>> >     when i was searching for something.
>>>> >   * No diffs in mail - *super* inconvenient.
>>>> >     One has to open each pr in browser (or fetch via git etc etc)
>>>> >   * Github is an official US-based commercial organisation.
>>>> >     It *has* to follow U.S. export law. In particular i'm thinking of
>>>> >
>>>> https://techcrunch.com/2019/07/29/github-ban-sanctioned-countries/
>>>> >       https://github.com/tkashkin/GameHub/issues/289
>>>> >     Does phabricator already have such restrictions, blocks?
>>>> >     If not, wouldn't you say adding such restrictions is not being
>>>> >     more open for contributors?
>>>> >     What happens when, in not so long in the future, the entirety of,
>>>> say,
>>>> >     china or russian federation is blocked as such?
>>>> >   * Same question can be asked about internet "iron" curtains
>>>> >     certain (*cough*) countries are raising. That also has already
>>>> happened
>>>> >     before (and *will* happen again), and i was personally affected:
>>>> >       https://en.wikipedia.org/wiki/Censorship_of_GitHub#Russia
>>>> >     I don't recall that happening to phabricator yet.
>>>> >     I fail to see how that is more contributor-friendly.
>>>> >   * Not sure anyone cares, but while using github as main git
>>>> >     repository "mirror" is perfectly fine - git is distributed, only
>>>> canonical
>>>> >     write-repo would be affected anything bad happen. But that isn't
>>>> the case
>>>> >     for reviews, issues; as it has been discussed in the "let's
>>>> migrate bugzilla
>>>> >     to github issues", it is far more involved.
>>>> >   * What about DMCA? Not sure how this is currently handled.
>>>> >   * UI feels laggy. Not much to add here, pretty subjective.
>>>> >   * I'm sure i'm missing a few.
>>>> >
>>>> > The github does come with it's benefits, sure:
>>>> > * It is *simpler* to preserve git commit author.
>>>> >   Currently one has to ask the author for the "Author: e at ma.il" line,
>>>> >   and do `git commit --amend --author="<>"`.
>>>> > * @mention is wide-r-reaching - whole github, not just llvm
>>>> phabricator
>>>> > * No more "phabricator disk full" issues
>>>> > * ???
>>>> >
>>>> > TLDR: such migration lowers the bar for new, first time,
>>>> > unestabilished contributors, but i personally feel it *significantly*
>>>> > raises the bar for the existing contributors, reviewers.
>>>> > We don't live in perfect world. Aspirational goals are aspirational.
>>>> > They should be attempted to be reached, but they shouldn't shadow and
>>>> > overweight, take over the main goal of the LLVM project.
>>>> >
>>>> > Personally, i don't see that benefits out-/over- weight the drawbacks.
>>>> >
>>>> > Roman.
>>>> >
>>>> > On Thu, Nov 7, 2019 at 8:32 AM Mehdi AMINI via llvm-dev
>>>> > <llvm-dev at lists.llvm.org> wrote:
>>>> > >
>>>> > > Hi all,
>>>> > >
>>>> > > Now that we're on GitHub, we can discuss about pull-requests.
>>>> > > I'd like to propose to enable pull-request on GitHub, as a first
>>>> step as an experimental channel alongside the existing methods for
>>>> contributing to LLVM.
>>>> > > This would allow to find workflow issues and address them, and also
>>>> LLVM contributors in general to start getting familiar with pull-requests
>>>> without committing to switching to pull-requests immediately. The community
>>>> should evaluate after a few months what would the next steps be.
>>>> > >
>>>> > > GitHub pull-requests is the natural way to contribute to project
>>>> hosted on GitHub: this feature is so core to GitHub that there is no option
>>>> to disable it!
>>>> > >
>>>> > > The current proposal is to allow to integrate contributions to the
>>>> LLVM project directly from pull-requests. In particular the exact setup
>>>> would be the following:
>>>> > >
>>>> > >   - Contributors should use their own fork and push a branch in
>>>> their fork.
>>>> > >   - Reviews can be performed on GitHub. The canonical tools are
>>>> still the mailing-list and Phabricator: a reviewer can request the review
>>>> to move to Phabricator.
>>>> > >   - The only option available will be to “squash and merge”. This
>>>> mode of review matches the closest our current workflow (both phabricator
>>>> and mailing-list): conceptually independent contributions belongs to
>>>> separate review threads, and thus separate pull-requests.
>>>> > > This also allow the round of reviews to not force-push the original
>>>> branch and accumulate commits: this keeps the contextual history of
>>>> comments and allow to visualize the incremental diff across revision of the
>>>> pull-request.
>>>> > >   - Upon “merge” of a pull-request: history is linear and a single
>>>> commit lands in master after review is completed.
>>>> > >
>>>> > > As an alternative staging proposal: we could enable pull-requests
>>>> only for a small subset of sub-projects in LLVM (i.e. not LLVM/clang to
>>>> begin with for example) in the repo. In this case, we would propose to
>>>> begin with the MLIR project (as soon as it gets integrated in the
>>>> monorepo). This would be a good candidate to be the guinea pig for this
>>>> process since it does not yet have a wide established community of
>>>> contributors, and the current contributors are already exclusively using
>>>> pull-requests.
>>>> > >
>>>> > > Here is a more complete doc on the topic:
>>>> https://docs.google.com/document/d/1DSHQrfydSjoqU9zEnj3rIcds6YN59Jxc37MdiggOyaI
>>>> > >
>>>> > > Cheers,
>>>> > >
>>>> > > --
>>>> > > Mehdi
>>>> > >
>>>> > > _______________________________________________
>>>> > > LLVM Developers mailing list
>>>> > > llvm-dev at lists.llvm.org
>>>> > > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>> > _______________________________________________
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>>>> > llvm-dev at lists.llvm.org
>>>> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>> _______________________________________________
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>>> _______________________________________________
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>>> --
>>> Hal Finkel
>>> Lead, Compiler Technology and Programming Languages
>>> Leadership Computing Facility
>>> Argonne National Laboratory
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>> _______________________________________________
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> --
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
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