Parse tar data backed up via stdin

[dionorgua]

Hi,

It would be cool to have a way to 'import' tar archive as stdin backup.

Something like

tar c <....> | rustic backup -

Why? Sometimes it's too hard to get access to actual data that needs to be saved.
For example proxmox have vzdump command that can save backups to .tar.gz/zstd format on disk or can output tar stream to stdout. Surely we can 'unpack' that tar temporary but this extra I/O that can be avoided

Same as restic/restic#2226

[aawsome]

Thanks for opening this issue @dionorgua!

I also want to add this feature but I must say that some more groundwork is needed. I'll describe a bit the problem:

• rustic backup is optimized to read files in parallel (which is an already implemented performance tuning)
• To allow this, the scanning of files (i.e. get the information which files are to backup) and the actual reading is separated and may occur in multiple thread
• This is incompatible with reading tar data from a stream, as here we get the information about the contained file directly followed by the content before we get the information of the next file.

Besides solving this problem we also have to enhance the possibility to config backup sources, e.g. how to differentiate whether to interpret the stdin stream as tar or just as content. But this is the smaller part of the problem...