refactor: put PackageCache into an enum in preparation for a multi-layered package cache

[kelvinou01]

Description

As per https://github.com/conda/rattler/issues/43, we want to implement layered package caches for Rattler. Putting PacakgeCache into an enum allows switching between implementations (SingletonPackageCache and LayeredPackageCache)

I'm considering both traits and enums. Leaning towards enum because:

• My impression is that we don't allow downstream users to add their own PackageCache types, thus a closed set of types (enum) is more suitable.
• Enums are more performant

Stuff to note

• This will be a breaking change
• To let PackageCache be public, I had to make PackageCacheInner public as well (even though inner is a private struct field), not sure if there's a way not to.

[baszalmstra]

Hey! Thanks for making this PR! Im wondering what your design considerations are. Why have two separate implementations? Isnt a non-layered cache just cache with one layer?

[kelvinou01]

Isnt a non-layered cache just cache with one layer?

Hey @baszalmstra! I was actually referencing this

We could introduce a trait for PackageCache that is implemented for both the current implementation as well as a layered version.

Maybe we can allow users to pick between the singleton and layered implementations, while we polish the latter? Or should we just create a layered implementation and have it be shipped to users (with the default being a cache with one layer)

[baszalmstra]

Maybe we can allow users to pick between the singleton and layered implementations, while we polish the latter? Or should we just create a layered implementation and have it be shipped to users (with the default being a cache with one layer)

Yeah, I think (in hindsight) that makes the most sense.

FYI I'm also about to merge a relatively large refactor of the PackageCache in https://github.com/conda/rattler/pull/837.

[kelvinou01]

Thanks for the heads up on the PR!

Also just to be sure, which idea did you think made the most sense, using the enum or not using it?

[baszalmstra]

Also just to be sure, which idea did you think made the most sense, using the enum or not using it?

I think a single implementation makes more sense. But if you feel uncertain about the implementation using an enum is fine for me as well.

[baszalmstra]

#837 was just merged, you might have a few conflicts.