Could margebot squash?

[Ben Gamari]

Joachim Breitner <mail at joachim-breitner.de> writes:

> Hi,
>
> as far as I understand, the expected workflow for MRs is that when they
> are ready, the developer manually squashes the chronological commit
> history of the MR into a logical one with polished commit messages,
> typically consisting of a single commit, but could be multiple ones,
> and then assigns the MR to margebot, which will rebase that sequence of
> commits onto the staging branch and eventually merges that into master.
>
> One downside of this approach is that it requires destructive changes
> to work-in-progres branches: I might think the MR is ready, squash the
> commit sequence into a single commit, but then more work is ready. Now
> it’s hard to revert individual patches, or collaborate with others,
> because the git history was disrupted.
>
> Another is that the commit message itself isn’t very easily visible to
> reviewers.
>
> In other similarly sized projects (e.g. mathlib) I often see a mode
> where the actual commits of the MR are ignored (so they can represent
> the true git history of the branch, including merges and all that grit,
> which is good for collaboration and for reviewers to understand what
> has happened, without requiring developers to spend cosmetics effort on
> them), and upon merging by margebot/bors/mergify/whatever, the MR is
> merged as a single commit with the description taken from the MR
> description (which encourages developers to keep the MR description up
> to date as the MR develops, reviewers can easily see that).
>
> A downside of this that you’ll always get one commit on master per MR.
> If you like to submit a curated list of logical commits within one MR,
> then this would not work, and you’d have to use multiple MRs.
>
We would certainly want this to be optional. I, for one, do try when
possible to maintain fine-grained histories. Requiring one MR per commit
would make this significantly more labor-intensive.

> Has this been considered?
>
> (I don’t want to cause unnecessary disruption with a presumptious call
> for action here; take it as a comment to weigh in in if and when this
> part of our infrastructure is about to change anyways, or maybe a
> careful probe if my sentiment may be shared widely.)
>
Indeed we have considered this and, if we didn't need to use marge-bot,
GitLab itself has quite good support for optionally squashing. Sadly
though, we do need to use Marge for the reasons described in #19046. In
the past we have been rather conservative about what features of Marge
we have used since experience has shown it lacking in robustness.

In general I'd very much like to move away from Marge but this will
require help from GitLab upstream. I have made our need of this feature
clear to upstream (and thankfully several other FOSS projects have made
similar requests) but progress has been quite slow. Thankfully there
appears [1] to be a recent uptick in activity; here's to hoping that it
will happen in the next few releases.

Cheers,

- Ben


[1] https://gitlab.com/groups/gitlab-org/-/epics/4911
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