snbk: Update the metadata of backed-up snapshots

[jamesljlster]

Currently, if a user modifies the metadata of a snapshot that has already been transferred to the backup target device, the backed-up metadata won't be updated.

I have a few ideas for the design:

• Introduce a new command: snbk update. This command would only check snapshot metadata and update the backed-up metadata if any differences are detected.
• Add an option to the transfer command: snbk transfer --update. When the --update flag is specified, metadata checking and updating would be performed.

The snbk transfer-and-delete command would also require adjustments for automation.
What do you think?

[aschnell]

Since probing target snapshots via ssh is slow the update should be done combined in the transfer command.

An implementation could use sha256sum <target-path>/*/info.xml to avoid executing many ssh commands (which is slow).

[jamesljlster]

It sounds like a good idea.
If the metadata checksum is calculated during the probing phase, would it make sense to introduce a new target state to indicate that the metadata has changed?
For example, adding something like TheBigThing::TargetState::LEGACY. This state would still be considered valid as a Btrfs send parent.

[aschnell]

Yes, a new state seems appropriate.