def reset_signal_handlers
      Signal.list_trappable.each_key do |signal|
        begin
          prev_handler = trap(signal, DEFAULT)
          if prev_handler != DEFAULT
            @previous_signal_handlers[signal] = prev_handler
          end
        rescue ArgumentError
          # Signal cannot be trapped; ignore it.
        end
      end
      trap('HUP', IGNORE)
      PhusionPassenger.call_event(:after_installing_signal_handlers)
    end
def reset_signal_handlers
      Signal.list_trappable.each_key do |signal|
        begin
          prev_handler = trap(signal, DEFAULT)
          if prev_handler != DEFAULT
            @previous_signal_handlers[signal] = prev_handler
          end
        rescue ArgumentError
          # Signal cannot be trapped; ignore it.
        end
      end
      trap('HUP', IGNORE)
      PhusionPassenger.call_event(:after_installing_signal_handlers)
    end
Reset signal handlers to their default handler, and install some
 special handlers for a few signals. The previous signal handlers
 will be put back by calling revert_signal_handlers.