module Celluloid
  module IO
    class DeadWakerError < StandardError; end # You can't wake the dead

    # Wakes up sleepy threads so that they can check their mailbox
    # Works like a ConditionVariable, except it's implemented as an IO object so
    # that it can be multiplexed alongside other IO objects.
    class Waker
      PAYLOAD = "\0" # the payload doesn't matter. each byte is a signal
      def initialize
        @receiver, @sender = ::IO.pipe
      end

      # Wakes up the thread that is waiting for this Waker
      def signal
        @sender << PAYLOAD
        nil
      rescue IOError, Errno::EPIPE, Errno::EBADF
        raise DeadWakerError, "waker is already dead"
      end

      # Wait for another thread to signal this Waker
      def wait
          byte = @receiver.read(1)
          raise DeadWakerError, "can't wait on a dead waker" unless byte == PAYLOAD
      rescue IOError, RuntimeError
          raise DeadWakerError, "can't wait on a dead waker"
      end

      # Return the IO object which will be readable when this Waker is signaled
      def io
        @receiver
      end

      # Clean up the IO objects associated with this waker
      def cleanup
        @receiver.close rescue nil
        @sender.close rescue nil
        nil
      end
    end
  end
end