Feature to re-encrypt/re-chunk data

[nh2]

There are a couple reasons why a user may want to re-encrypt and re-chunk data:

• Changed chunk size
  • v0.12.0 writes:

    bupstash is now biased to produce 2 MiB data chunks on average instead of 1 MiB chunks

    Because the chunks generated by bupstash will change [..] your repositories will grow in size if they contain data chunks from previous versions of bupstash, this can be fixed by cycling older data out over time, or recreating your backups. Here the, "recreating your backups" part is what this issue requests.

• Rotating the encryption key
  • This is mainly so that a single bupstash list invocations shows all snapshots. If you have multiple keys, only the snapshots associated with that keys will be listed. --query-encrypted helps with that, but then you cannot read any of the tag metadata.
  • Issue #70 (keychains) could help with it a bit as well.
  • Another motivation is when you literally want to get rid of that key, e.g. because it was compromised somehow.

Unfortunately one cannot use direct bupstash get | bupstash put for this task, because bupstash put docs say:

The difference between piping tar command output into bupstash put, and using bupstash put directly on a directory, is the latter is able to use a send log and avoid reading files that has already been sent to the server, and is able to create a snapshot listing for other commands like bupstash-list-contents.

So one would lose listing functionality.

It would be nice to have either or both of:

• bupstash sync --reencrypt --rechunk
• bupstash put --build-snapshot-listing that can build listing functionality from a tar