If /var/lib/machines is mounted as btrfs loopback file system in
/var/lib/machines.raw with this change we automatically grow the file
system as it fills up. After each 10M we write to it during imports, we
check the free disk space, and if the fill level grows beyond 66% we
increase the size of the file system to 3x the fill level (thus lowering
it to 33%).
With this change the pull protocol implementation processes will pass
progress data to importd which then passes this information on via the
bus. We use sd_notify() as generic transport for this communication,
making importd listen to them, while matching the incoming messages to
the right transfer.
Lennart Poettering