bug#28310: guix build -n misses package builds

[ludo@gnu.org (Ludovic Courtès)]

Hello,

Andreas Enge <andreas@enge.fr> skribis:

> I am right now in the process of updating pari-gp to version 2.9.3.
> After building it on a git check-out of three days ago, which went smoothly,
> I rebased my patch on today's master and was pleased to see that no rebuild
> was needed:
>
> $ ./pre-inst-env guix build pari-gp -n
> outputs nothing.
>
> However, once the -n dropped, the gd package gets built.
> And then it is starting ruby, which has no connection to pari-gp:
> Downloading https://mirror.hydra.gnu.org/guix/nar/229n3pzp5bdmbdvwslg0dxliysas92k5-ruby-2.4.1.tar.xz...
>    ruby-2.4.1.tar.xz  9.5MiB                                                                                                       22KiB/s 00:06 [                    ]   1.3%^

“-n” now implies “--no-grafts” (commit
fd59105c49965db956fac73c68d8b00d068f5d5c).  This was motivated by the
need to have -n really perform a dry run.

The downside is that with -n we now see only half of the build plan, and
when we remove -n, we start with the other half of the build plan,
grafting.

The “build continuation” idea of ‘wip-gexp-grafts’, discussed in
<https://bugs.gnu.org/22990>, could in theory help with that.

Ludo’.