bug#35621: Simplify comparing guix challenge results

["Ludovic Courtès" <ludo@gnu.org>]

Hello!

Vagrant Cascadian <vagrant@reproducible-builds.org> skribis:

> I'd like to see a simpler process for comparing challenge results
> producing differences.
>
> While the manual documents how to compare a failing challenge result
>
>   $ wget -q -O - https://ci.guix.info/nar/…-git-2.5.0 \
>      | guix archive -x /tmp/git
>   $ diff -ur --no-dereference /gnu/store/…-git.2.5.0 /tmp/git
>
> To check "git", you need to run "guix challenge git" and if the results
> are inconclusive, manually cut-and-paste the correct URL(s) from the
> challenge output and manually download it and unpack with guix archive,
> and then run a comparison utility...
>
> What about an argument to "guix challenge" that handles the downloading
> and unpacking to a temporary directory, and possibly another that also
> runs a comparison tool against the results. Maybe something like:
>
>   $ guix challenge --download-differences
>   /tmp/git-XXXXX/SUBSTITTE-X/...-git-2.5.0
>   /tmp/git-XXXXX/SUBSTITUTE-Y/...-git-2.5.0
>   /gnu/store/...-git-2.5.0
>
> Downloads and unpacks the substitutes, and outputs the resulting
> directories.
>
>
>   $ guix challenge --download-differences --compare-with="diffoscope ..."

I had forgotten about this bug report (thanks for the reminder on IRC!)
and came up with something similar to/different from it:

  https://issues.guix.gnu.org/issue/38518

Let’s see whether we should adapt it!

Ludo’.