add: support rollback/recovery from partial/failed dvc add

If the user does a long running dvc add directory that dies somewhere in the middle, there is no obvious (to the user) way to recover from the state where

1. DVC has not generated a .dvc file
2. half of the users files are still in the workspace (and have not been moved into cache)
3. half of the user’s files have been moved into cache, but have not been checked out/linked (and now appear to be lost)

Currently it’s possible to recover from this state manually as long as we have the .dir file for the original complete directory, but not in a straightforward way

discord context https://discord.com/channels/485586884165107732/485596304961962003/872423982463340555

[skshetry]

I’m bumping the priority here, we already have a guideline against breaking the user’s workspace, so we need to realize the correct implementation.

Another user nearly lost their ~140GB of data. We should take data loss seriously. Even if we don’t add a support for transactions (rollback/commit), we should not break user’s workspace by moving files to the cache.

[emremrah]

Suggestion: at least prioritize to implement a progress bar to show the ETA. Lack of pb was the reason I was interupted the add process. Thanks again for the support for getting the data back.