Offloading builds asynchronously for use by low-powered machines (or: can you get a home config derivation without building its packages?)

["Jonathan Frederickson" <jonathan@terracrypt.net>]

Hi - I've been using Guix on a couple slower aarch64 machines for a while. One of the recurring pain points with doing so has been that very often, when I go to build my home environment after a fresh 'guix pull', most of the packages in it do not yet have substitutes built.

I do also have a couple more powerful aarch64 machines available, so build offloading is an option. However, one of the machines I use most frequently is a small laptop (the MNT Pocket Reform), which is meant to be very portable. Even on my more powerful machines, building everything in my home environment when no (or very few) substitutes are available may take multiple hours or sometimes days, and build offloading being synchronous means the machine needs to maintain a constant connection to the build servers during a build. This is very inconvenient for a portable machine!

What I would *like* to be able to do is to trigger a build on another more powerful machine asynchronously, and then later come back and build my home environment using the substitutes from that build. The Guix Build Coordinator seems like it may serve my needs fairly well based on the description in this blog post: https://guix.gnu.org/en/blog/2021/building-derivations-how-complicated-can-it-be/

...but in contrast to building a specific package with 'guix build', it doesn't look like I'm able to get the derivation path for a home configuration without first building the packages! Running 'guix build -d -f /path/to/home/config' attempts to build the packages in my home configuration if they're not yet built, rather than returning a derivation that I could submit to the build coordinator.

I admit that I don't fully grok derivations yet; is this inevitable for derivations, or is it just a limitation of 'guix build' and is there actually a way to get at the home configuration derivation prior to building its packages?

Thanks,
- jfred