Wharfie

Originally created by: labi

Originally owned by: labi

as we are using tar, we can use the multi level backup and restore concept.
It supports already differential file backups, including deletion of files.
What we need is "*.snar" file, recording the changes. As the snar file will become corrupted when we rebuild older files, we need to track the state of the snar file for each layer.
In practice that means, that every build target has from now on, an additional snar file as its output, as well as an additional snar file as input.

The FROM target doesn't produce a snar file. So it is automatically the level-0 backup.

In fact I believe that packing should not make big trouble, as we can always just check if a a snar is there. if yes, use it. If no, leave it and create a level-0 backup.

The extract of the archives on the other hand is a bit more tricky. We now need the info about all archives, which are involved, starting from the level-0 backup. The safest way will be to track this transparently in the makefile generation process, as we are also managing the archives there.