server.rb

require 'socket'

class Server
  
  def initialize( port )
    @server = TCPServer.new( port )
    @threads = []
  end
  
  def run( &block )
    request = nil
    trap ( 'INT' ) { shutdown }
    loop do
    	begin
    		request = @server.accept_nonblock
    	rescue Errno::EAGAIN, Errno::ECONNABORTED, Errno::EPROTO, Errno::EINTR
    	  @threads.delete_if { |t| !t.alive? && t.join }
    		IO.select([@server])
    		retry
    	end
    	process( request, block )
    end
  end
  
  private
  
  def process( request, block )
  	@threads << Thread.new do
  	  IO.select([request])
  	  data = ''; chunk = nil
  	  data << chunk until (( chunk = request.gets).chomp.empty? ) 
      request.close_read
      IO.select([], [request])
  		request.print( block[ data ] )
  	  request.close
    end
  end

  def shutdown
    exit
  end
  
end

test.rb

load 'server.rb'

servlet = lambda do |data|
  puts data
  "HTTP/1.0 200 OK\r\nContent-Type: text/plain; charset=UTF-8\r\n\r\nYes.\n\r\n"
end
server = Server.new( 2201 )
server.run( &servlet )