gistfile1.rb

  def render(buffer, algorithm = "dot", format = "png")
    png_buffer = ""
    graph_buffer = buffer.dup.force_encoding("BINARY")

    png_pipe_ruby = nil
    png_pipe_render = nil
    graph_pipe_render = nil
    graph_pipe_ruby = nil
    pid = nil
    status = nil

    begin
      graph_pipe_render, graph_pipe_ruby = IO.pipe
      png_pipe_ruby, png_pipe_render = IO.pipe

      pid = Process.spawn "dot", "-K", algorithm, "-T", format, "/dev/stdin",
        in: graph_pipe_render,
        out: png_pipe_render,
        err: :err,
        close_others: true

      graph_pipe_render.close
      graph_pipe_render = nil
      png_pipe_render.close
      png_pipe_render = nil

      begin
        loop do
          begin
            read_set = [ png_pipe_ruby ]
            write_set = []
            error_set = []

            write_set << graph_pipe_ruby unless graph_buffer.empty?

            read_set, write_set, error_set = IO.select read_set, write_set, error_set, nil

            unless read_set.empty?
              png_buffer << png_pipe_ruby.read_nonblock(8192)
            end

            unless write_set.empty?
              bytes = graph_pipe_ruby.write_nonblock graph_buffer
    
              graph_buffer.slice! 0, bytes

              if graph_buffer.empty?
                graph_pipe_ruby.close
                graph_pipe_ruby = nil
              end
            end

          rescue Errno::EWOULDBLOCK, Errno::EINTR
          end
        end
      rescue EOFError
      end

    ensure
      png_pipe_ruby.close unless png_pipe_ruby.nil?
      png_pipe_render.close unless png_pipe_render.nil?
      graph_pipe_ruby.close unless graph_pipe_ruby.nil?
      graph_pipe_render.close unless graph_pipe_render.nil? 

      unless pid.nil?
        Process.waitpid(pid)
        status = $?
      end
    end

    raise "graphviz failed" unless status.success?

    png_buffer
  end