When Obnam initializes the chunks
directory, for encryption, it is
done without locking. That's a race condition: two clients started
at the same time against an empty directory may result in both of
them creating the same files.
Normal use of the chunks
directory does not need locking: clients
are free to create new chunks files there, as long as they take care
to not overwrite existing files.
--liw
done in bzr