Look into better process locking

[bartkl]

Currently, my convenient option of having set max connections = 1 in rsyncd.conf takes care that never two instances of rsync will run concurrently, as I wish.

However, what could happen is that during a multi-module (for instance ALL) backup process, one module finishes and some other external rsync process might hog up the connection, such that the former process fails.

It would be nicer to have backup.py lock all possible rsync syncing.