Get paths faster

[deemp]

Currently, I use nix copy to copy paths from a cached chroot store from and into root store.

• restore code
• save code

Copying takes a lot of time at the start of a job.

Possible solution

• daemon 1 with low priority copies paths that aren't yet in /nix/store.
• daemon 2 with high priority substitutes paths on demand.
• Daemons run during all steps of a job until after saving cache to GitHub Actions cache.
• If a daemon 2 needs to substitute and lock the store, daemon 1 stops for a while, then continues.

Advantages

• Get necessary paths selectively.
• Utilize time of other steps (there are 2+ CPUs - docs).

Disadvantages

• Other steps may require all CPUs

Alternatives

• Cache and restore full store.
  • macOS runners don't support restoring a root store from cache. (I couldn't, TODO docs link)
  • macOS runners don't support chroot stores for building and running (docs)
• Use a local binary cache store via file:// (docs)
  • Triggers a lot of copying from substituters (don't know why)
• Feature request: use a single cache entry DeterminateSystems/magic-nix-cache-action#16

[deemp]

Solved by https://github.com/deemp/cache-nix-too/tree/use-nix-quick-install-action as it becomes possible to restore and save store paths directly without copying from an intermediate store.