I am wondering if bad things happen in case multiple
instances of sa-update run at the same time, e.g. if a
manual call interferes with a call invoked by cron in

As I understand the code, sa-update downloads any new
rules into a temporary directory where it does some
sanity checks (signature verification, lint). After then
it cleans the channel directory and extracts the downloaded
tar archive again into the channel directory. Basically
the last sa-update run wins, and no data corruption should
occur. But maybe I have missed some point.