If mkdir(queue.lock) failed for a reason other than "already exists"
(e.g. a permissions or disk problem on the data directory), is_dir()
was false, so the loop `continue`d before ever reaching the tries>=40
retry-limit check or the usleep(), spinning at full CPU forever
instead of failing with a clear error.
Move the staleness check inside an is_dir() guard instead of using
continue to skip past the retry-limit logic, so the bounded-retry
behavior applies regardless of why mkdir failed.
Marek Miklewicz