pre-unlock — Notification of a path unlock attempt.
The pre-unlock hook runs whenever
          someone attempts to remove a lock on a file.  It can be used
          to create policies that specify which users are allowed to
          unlock particular paths.  It's particularly important for
          determining policies about lock breakage.  If user A locks a
          file, is user B allowed to break the lock?  What if the lock
          is more than a week old?  These sorts of things can be
          decided and enforced by the hook.
If the pre-unlock hook program
          returns a nonzero exit value, the unlock action is aborted
          and anything printed to stderr is marshalled back to the
          client.