`use-case-hack` is a property of a store, not a global setting (except for `libnixfile`-level commands)

[roberth]

Describe the bug

$ nix copy --from ~/tmp-store-normal --to ~/tmp-store-hacked?use-case-hack=true /nix/store/6hypxafvp2fcadphppjcxszx0qcahqbz-dummy
warning: unknown setting 'use-case-hack'

I shouldn't have to apply the case hack (unhack) to the --from store.

Steps To Reproduce

See above.

Expected behavior

• The case hack is a store setting.
• Ideally, need for the case hack is autodetected based on the file system where the store resides
  • Darwin does not imply case-insensitive file system #1446

nix-env --version output

Additional context

• Collect "file system object" parts of libnixutil together #9278

  • libnixfile isn't a thing (yet?), but is a useful indictor of the abstraction level: Nix operations on files without the need for an actual store.

• Darwin does not imply case-insensitive file system #1446

Priorities

Add 👍 to issues you find important.

[edolstra]

Do we even need the case hack anymore? IIRC it was added once upon a time to support NixOps on macOS, where we ran into Linux kernel headers having case conflicts.

Also, since our installer creates a volume for the Nix store these days, I guess we can just enable case sensitivity?

[roberth]

Having a case sensitive store on macOS sounds nice, but it's not free of side effects.

• Build impurity: this difference will be observable in the sandbox, and outputs will differ.
• It doesn't remove the problem, but rather moves it from the remote store <-> local store boundary to the local store <-> rest of system boundary, which seems like a larger surface area in practical use.

If we make such a change we better be sure that it's not a regression in terms of actual use, so I wouldn't block improvements to the case hack functionality on this decision. Note also that we'll have to support the case hack until everyone's had a good chance to migrate, and macOS doesn't have a monopoly on case insensitive file systems.

[j-baker]

I don't believe this hack can be removed. Among other things, the Nix installer on MacOS creates the Nix store on a case-insensitive volume.