bug#26608: Provide --only-substitutes flag to "guix package --upgrade"

[ludo@gnu.org (Ludovic Courtès)]

Christopher Allan Webber <cwebber@dustycloud.org> skribis:

> Sometimes I do an upgrade and I want to get the latest security updates,
> but I also am not really available to burn through a lot of cpu time,
> (especially on my x200).
>
> I'd love it if thre were a flag so that I could specify "only bother to
> upgrade the packages where we only upgrade if a substitute is already
> available.

+1

> Note that I looked at the source to see if this would be an easy thing
> to add; I figured that this would be handled in roughly the same place
> as --keep-going or --fallback, but it looks to me like those are
> actually just passed over to the build daemon, so I'm not so sure how
> easy it would be to patch this in while we're still using Nix's C
> daemon?  I'm not sure.

Clients could check, among the packages that are to be installed, which
ones are substitutable (with the ‘substitute-paths’ RPC or similar) and
filter out those that are not.  No C++ involved.

Cheers,
Ludo’.