[WIP] remote: use explicit tree parameter for cache operations

[pmrowla]

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• remote.save() now takes an explicit tree parameter
  • When tree is remote.tree, save is done via move() + link()
  • For all other trees, save is done by copying object from tree into remote.tree

[efiop]

So we still don't have a cache/remote separation?

[pmrowla]

@efiop Not yet. I'm still thinking about the best way to go about that particular separation.

The "remote" functions like push/pull/status (which are currently in LocalRemote) are really functions for syncing the contents of two caches. So I've been wondering if they should just be abstracted into functions for syncing from "this/self" cache to/from any other cache, and then moved into BaseRemote?

And then the remote classes could still be renamed to Cache or RemoteCache (or something else along those lines), but there wouldn't be any further separation into new classes (other than the tree methods).

[pmrowla]

I know that we would still have the external output/dependency use case to consider, but it seems like in practice, that is still just a situation where we would have two "s3 caches"

• one which is used as the typical "dvc remote" for syncing local cache <-> s3
• and the second which is used as the s3 external out cache

[pmrowla]

closed in favor of #4019