fs-atomic-storage: Integrate fs-atomic-versions with fs-storage

[kirillt]

• Move atomic files abstraction from arklib to ark-rust, as separate module with its own Cargo.toml

• Use it in the storage component and in ark-cli project.

Most likely, it won't need JNI later.

[kirillt]

Crate fs-atomic-versions shouldn't be used directly by data-link and other crates (although we leave this possibility to users/apps). Crate fs-atomic-versions should be used by fs-storage.

Basically, any dependency on fs-atomic-versions will require to create initialization methods.

[kirillt]

This thing should be done after these 3 are completed:

• fs-storage: Implement storages in Rust #1
• fs-storage: Port BaseStorage abstraction from Kotlin #18

JNI probably be done after atomics, because I'd expect some API change:

• JNI for fs-storage ark-android#90

[kirillt]

We can also look at the atomic versioning not only as a mechanism preventing dirty writes, but also as an update discovery mechanism.

• We don't allow writing storages without versions
• We maintain N latest versions for each storage
• If we attempt to write storage with version N, while N+1 is already on disk, we merge
• If we attempt to read storage with version N, while N+1 is already in memory, we merge

Monoid or CRDT can help us here. It seems that we're going to use them anyway.

[kirillt]

One more consideration: we want to use storages for caching, too. I don't think it makes sense to version cache items⏤we just refer to them by immutable key and overwrite any with new version when needed.

How to implement caching nicely if we adopt atomic versioning for storages?

Options:

1. Switch on/off versioning for this particular kind of storage.
2. N=0 effectively disables versioning, where N is the number of versions to maintain during writing/reading.

[twitu]

The ark-rust repo already has a crate called fs-atomic-versions which has the AtomicFile implementation. Is there anything else this issue needs to do?

[kirillt]

Here, we need to create experimental version of fs-storage that uses fs-atomic-versions inside for each file it writes.

Let's call it fs-atomic-storage.