bug#63414: Evaluation comparison on cuirass

[Christopher Baines]

Josselin Poiret via Bug reports for GNU Guix <bug-guix@gnu.org> writes:

> Hi Andreas,
>
> Andreas Enge <andreas@enge.fr> writes:
>
>> When working on a branch and deciding whether to merge it, we need a way
>> of comparing its status with that of the master branch. As far as I can see,
>> there is currently no way in cuirass to compare arbitrary evaluations and
>> get a list (or a dashboard) of builds that fail in one, but not the other.
>>
>> Andreas
>
> I guess that this is one of the features that the Build Coordinator was
> built for (and it is pretty damn good at this).  Maybe we could start
> considering whether it makes sense to duplicate effort on Cuirass and
> the Build Coordinator?  I don't know how "production-ready" the build
> coordinator is, compared to Cuirass?  Maybe we could target getting the
> Build Coordinator up to feature parity with Cuirass so that it may be
> used on a wider scale?  If this is something we want to focus on, we
> could create a team around it and set clear goals, which would probably
> lessen the burden that's on Chris currently.
>
> I understand that Cuirass is general enough to support much more than
> Guix, but the coordinator is a wonderful piece of software and our
> workflows might be outgrowing it.

There's some pedantic bits here to bring up. The build coordinator
doesn't have anything to do with comparing revisions (it doesn't even
really know what builds correspond to which revisions), it's just for
performing builds potentially across many machines, and doing something
useful with the results.

The data service however is meant for comparing revisions. There's a
circular relationship between the two as well, since the data service
can provide substitutes for derivations, which enables the build
coordinator to easily build them, and then report the results of those
builds back to the data service. This information about builds is
important since that can then factor in to comparisons between
revisions.

On the bit about "feature parity with Cuirass" though, this is a bit
misleading as the build coordinator exists because I wanted something
with very different design decisions to Cuirass. In terms of core
features, the build coordinator was complete back in late 2020
[1]. There's obviously lots that Cuirass does that the build coordinator
does not, but adding features without looking at the bigger picture can
be detrimental in the long term.

1: https://lists.gnu.org/archive/html/guix-devel/2020-11/msg00417.html

This is not to say there aren't things to work on in the build
coordinator. There are some ideas in the README and I'm more than happy
to try and help people get more involved.

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