code: func blocker(c context.Context) fslock.Blocker {
  	return func() error {
  		logging.Debugf(c, "Lock is currently held. Sleeping %v and retrying...", lockHeldDelay)
  		tr := clock.Sleep(c, lockHeldDelay)
  		return tr.Err
  	}
  }
docs: // blocker is an fslock.Blocker implementation that sleeps lockHeldDelay in
  // between attempts.