check_shift_log

# File lib/logger.rb, line 655
    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