Storage versioning

A more detailed plan after https://github.com/nextcloud/server/issues/9445 and now that https://github.com/nextcloud/server/pull/12129 is in we can look at this in a bit more depth.

I think it is important to make a distinction here. The implementation in https://github.com/nextcloud/server/pull/12129 is to allow version backends. Which is why we have versions in group folders for example.

This is about adding versioning support on a storage level.

I'd propose a new interface to be implemented by storages. IVersionStorage. This should at least have a method versioningEnabled (or something similar). Because it might not always be available (e.g. S3 versiniong).

Then this would need the following methods I think

• getVersions(string $path): IVersion[]: obtains a list of the current versions
• createVersion(string $path): creates a new version (this can also be a void method if that happens automatically on write?)
• restore(IVersion $version): bool: restores the version

Then IVersion needs

• size
• time
• id
• mime?

That way we could use it in combination with #12129

@icewind1991 for feedback.

Start by reading issue #9445 and pull request #12129 to understand the existing version-backend implementation. Then review the proposed IVersionStorage and IVersion methods and determine the supported storage behaviors and API shape. Done means the storage-level versioning design is agreed and its implementation scope is clearly defined.