Consider moving CAS to platform-standard cache directories

[kamalmarhubi]

Ran across this project via pantsbuild.slack.com lurking.

Small suggestion: instead of ~/.nce, store the CAS in $XDG_CACHE_HOME/{scie,nce,whatever} on systems that follow XDG, and in ~/Library/Caches/{scie,nce,whatever} on macos—and probably somewhere under %localappdata% on Windows; less sure about that one.

This makes it clear it's fine to blow away (eg, to free up space), and also removes some clutter from $HOME. It's probably easier to change this before widespread use of scie-packaged programs. :-)

Edit: I just saw #9 which probably conflicts with the idea that the CAS can be blown away. In that case, the equivalent of $XDG_DATA_HOME might be better?

[jsirois]

This should definitely be done. IIUC this crate is the thing to use: https://crates.io/crates/dirs

@kamalmarhubi do you want to take a crack at this?

[jsirois]

To be clear, ~/.nce is just the default. It will be good to get that default conforming to XDG / Mac / Windows standards for user caches, but the author of a lift manifest can also explicitly set scie.lift.base. Since lift manifests are, by definition, platform specific, this works. Its more effort than needed though and defaulting correctly would be great.

[jsirois]

As to #9, the CAS definitely can always be blown away in normal use. The self-modifying idea is aimed at system installers; i.e.: /var/nce or /var/lib/nce or something like that would be used by the system and the installs would run as root. No one could easily blow away that sort of NCE cache.

[jsirois]

This is now released in 0.4.0: https://github.com/a-scie/jump/releases/tag/v0.4.0