Run `nh clean`

[GideonBear]

Checklist

• I have searched the issue tracker for relevant or duplicate issues.

I want to suggest some general feature

Topgrade should run nh clean as part of the system and/or home-manager step

Raised in #1712, ping @dtomvan

topgrade/src/steps/os/linux.rs

Line 829 in 3b27933

// TODO: maybe use `nh clean` when available&&wanted ?

More information

• I am able and willing to implement this feature myself

[dtomvan]

We could implement the nh clean in the same way as nh switch:

• If misc.nix_handler == NixHandler::Autodetect -> try nh, then nix-collect-garbage
• If misc.nix_handler == NixHandler::Nh -> force nh
• If misc.nix_handler == NixHandler::Vanilla -> force nix-collect-garbage

There is just one problem: nh clean all also deletes gcroots that match .*/.direnv/.* or .*result.*. This is something we might not want to do OOTB. This however is one of the advantages of nh clean (e.g. for developers), so we might also not want to make this functionality unavailable. We could make a config variable like misc.nh_clean_gcroots: boolean?

Also: deleting all old generations of all profiles, as is currently done, is also not desirable, because you lose the ability to rollback. IMHO we should also make configurable how far back it deletes (using --delete-older-than, or --keep-since for nh), and decide on a reasonable default, like 14 days. nh also has the ability to keep a number of generations, rather than cutting off by age, we could leverage that. I haven't come up yet with a way to ergonomically make that configurable (since using nix-collect-garbage is still also an option).