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List:       webkit-dev
Subject:    Re: [webkit-dev] WebKit Transition to Git
From:       Ryosuke Niwa <rniwa () webkit ! org>
Date:       2020-10-13 23:32:07
Message-ID: CABNRm60T--QET_FDjxy_2CzuGc8giNP83PmD9esciMpR0y0E_A () mail ! gmail ! com
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On Tue, Oct 13, 2020 at 4:12 PM Konstantin Tokarev <annulen@yandex.ru> wrote:
> 
> 
> 
> 14.10.2020, 02:01, "Ryosuke Niwa" <rniwa@webkit.org>:
> > On Tue, Oct 13, 2020 at 3:53 PM Konstantin Tokarev <annulen@yandex.ru> wrote:
> > > 14.10.2020, 01:45, "Ryosuke Niwa" <rniwa@webkit.org>:
> > > > On Tue, Oct 13, 2020 at 3:40 PM Konstantin Tokarev <annulen@yandex.ru> wrote:
> > > > > 14.10.2020, 01:30, "Ryosuke Niwa" <rniwa@webkit.org>:
> > > > > > On Tue, Oct 13, 2020 at 2:37 PM Konstantin Tokarev <annulen@yandex.ru> \
> > > > > > wrote:
> > > > > > > 13.10.2020, 22:33, "Maciej Stachowiak" <mjs@apple.com>:
> > > > > > > > > On Oct 2, 2020, at 10:59 AM, Michael Catanzaro \
> > > > > > > > > <mcatanzaro@gnome.org> wrote: 
> > > > > > > > > On Fri, Oct 2, 2020 at 6:36 pm, Philippe Normand <philn@igalia.com> \
> > > > > > > > > wrote:
> > > > > > > > > > Would you also consider preventing merge commits in order to keep \
> > > > > > > > > > a clean mainline branch?
> > > > > > > > > 
> > > > > > > > > Big +1 to blocking merge commits. Merge commits in a huge project \
> > > > > > > > > like WebKit would make commit archaeology very frustrating. (I \
> > > > > > > > > assume this is implied by the monotonic commit identifiers \
> > > > > > > > > proposal, but it doesn't exactly say that.)
> > > > > > > > 
> > > > > > > > I'm assuming your objection is to regular merges, but how do you feel \
> > > > > > > > about squash merges? Or do you think all PRs should be landed by \
> > > > > > > > rebasing?
> > > > > > > 
> > > > > > > I'm not Michael but will add my 2 dollars anyway :)
> > > > > > > 
> > > > > > > In these two approaches commits inside PR have different meaning, and \
> > > > > > > workflow is different. 
> > > > > > > Below I use a term "atomic change" to describe minimal code change \
> > > > > > >                 which is a self-contained work unit with following \
> > > > > > >                 properties:
> > > > > > > * It implements well-defined task which can be summarized as a short \
> > > > > > >                 English sentence (typical soft limit is 60 characters)
> > > > > > > * It doesn't introduce defects (e.g. bugs, compilation breakages, style \
> > > > > > >                 errors, typos) which were discovered during review \
> > > > > > >                 process
> > > > > > > * It doesn't include any code changes unrelated to main topic. This \
> > > > > > > separation is sometimes subjective, but it's usually recommended to \
> > > > > > > split refactoring and implementation of feature based on that, bug fix \
> > > > > > > and new feature, big style change and fix or feature. 
> > > > > > > AFAIU our current review process has similar requirements to patches \
> > > > > > > submitted to Bugzilla, though sometimes patches include unrelated \
> > > > > > > changes. This can be justified by weakness of webkit-patch/Bugzilla \
> > > > > > > tooling which has no support for patch series, and by fact that SVN \
> > > > > > > doesn't support keeping local patch series at all. 
> > > > > > > 1. Workflow 1 - "Squash merge" policy
> > > > > > > 
> > > > > > > * Whole PR is considered to be a single atomic change of WebKit source \
> > > > > > > tree. If work is supposed to be landed as a series of changes which \
> > > > > > > depend on each other (e.g. refactoring and feature based on it, or \
> > > > > > > individual separate features touching same parts of code), each change \
> > > > > > > needs a separate PR, and, as a consequence, only one of them can be \
> > > > > > >                 efficiently reviewed at the moment of time
> > > > > > > * Commits in PR represent review iterations or intermediate \
> > > > > > >                 implementation progress
> > > > > > > * Reviewers' comments are addressed by pushing new commits without \
> > > > > > > rewriting history, which works around GitHub's lack of "commit \
> > > > > > > revisions". Also this workflow has lower entry barrier for people who \
> > > > > > > haven't mastered git yet, as it requires only "git commit" and "git \
> > > > > > > push" without rebases. 
> > > > > > > 2. Workflow 2 - "Rebase" ("cherry-pick")) or "Merge" policy
> > > > > > > 
> > > > > > > * PR is considered to be a series of atomic changes. If work consists \
> > > > > > >                 of several atomic changes, each commit represent an \
> > > > > > >                 atomic change
> > > > > > > * Review iterations are done by fixing commits in place and reuploading \
> > > > > > > entire series using force push (of course if review discovers that \
> > > > > > > substantial part of work is missing it can be added as a new atomic \
> > > > > > >                 commit to the series)
> > > > > > > * It's possible to review each commit in the series separately
> > > > > > > * Workflow requires developers to have more discipline and experience \
> > > > > > > with using git rebase for history rewriting. Entry barrier can be \
> > > > > > > lowered by providing step by step instructions like e.g. [1].
> > > > > > 
> > > > > > I really dislike this workflow due to its inherent complexity. Having
> > > > > > to use Git is enough of a burden already. I don't want to deal with an
> > > > > > extra layer of complexity to deal with.
> > > > > 
> > > > > There is simplified version of workflow 2 when you have only one commit in \
> > > > > PR. In this case you can easily edit this single commit with gic commit \
> > > > > --amend or GUI tools to address review comments. At the same time those who \
> > > > > are more comfortable with git can use longer patch series.
> > > > 
> > > > Except that reviewers would still have to review each commit
> > > > separately, and the time comes to revert someone's patch, we still
> > > > need to remember how to revert a sequence of commits that belong to a
> > > > single PR.
> > > 
> > > Workflow 2 assumes that you forget about PR after it was merged and operate
> > > on its commits as equal parts of history.
> > > 
> > > In this sequence of commits each one can be reverted on their own merits,
> > > like separate (but consequential) Bugzilla patches in current workflow.
> > > Sometimes it's not possible to revert one patch without reverting a few others
> > > or solving conflicts, but you rarely think about reverting a whole range of
> > > patches unless it becomes really necessary.
> > 
> > Currently, when we revert a patch, we reopen the bug. If we're
> > reverting individual commits and they don't all correspond to a single
> > PR, then we would need a new system for tracking the partial(?)
> > introduction of the original issue that PR fixed. This is extremely
> > confusing because a single PR may have many to many relationships with
> > Bugzilla bugs / GitHub issues. In which case, there isn't a clear
> > communication of what got reverted and what needs to happen other than
> > the history in Git.
> 
> Each commit could have a reference to issue it solves, which could be set up
> to be reopened automatically after revert. I guess webkitbot could do that.

So now we're talking about each PR containing multiple commits each of
which fixes some distinct issue?

I don't think we should have a single PR which fixes multiple distinct
issues like that.

> > Again, I dislike all these complexities that come with workflow 2.
> > Contributing to WebKit is already too damn complicated. Please don't
> > make it even more complicated.
> 
> FWIW, having to create individual PR for every patch in a series (and wait before
> previous PR is merged to avoid confusion, because of git branch containing
> previous commit reviewed elsewhere) is also a complication which decrease
> developers' productivity.

I've contributed to WebKit for 11 years and this has never been an
issue because I've never found a need to upload a patch up review
before the previous patch had been landed.

This is only an issue if you're working on a large feature and trying
to upload a series of patches for a review. I do get that something
like that might be useful when reviewing a large WIP feature but then
you can just point to a branch in your fork for the entire patch /
diff. There isn't really a need to have a PR for it.

- R. Niwa
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