taken from #955 where this was suggested:

by @jdchristensen : However, if some kind of automated lock removal was desired, one idea would be for borg to update the timestamp on the lock every 5 minutes. Then a future borg process could break a lock if it found it older than, say, 10 minutes.

by @poelzi : The correct way to handle such locks is with a pid+machine id inside the lock and and timestamp couter that gets updated every minute. If there is no such process on the same machine: stale lock, remove. If the machineid is a foreign machine and the timestamp is not updated some minutes, remove lock. If the lock is updated, wait until cleared.