class Logger::LogDevice

def check_shift_log
  if @shift_age.is_a?(Integer)
    # Note: always returns false if '0'.
    if @filename && (@shift_age > 0) && (@dev.stat.size > @shift_size)
      lock_shift_log { shift_log_age }
    end
  else
    now = Time.now
    if now >= @next_rotate_time
      @next_rotate_time = next_rotate_time(now, @shift_age)
      lock_shift_log { shift_log_period(previous_period_end(now, @shift_age)) }
    end
  end
end