https_proxy.rb

require 'eventmachine'

# most of code is from [thin-glazed](https://github.com/freelancing-god/thin-glazed)

class HttpClient < EventMachine::Connection
  attr_reader :proxy

  def initialize(proxy)
    @proxy     = proxy
    @connected = EventMachine::DefaultDeferrable.new
  end

  def connection_completed
    @connected.succeed
  end

  def receive_data(data)
    proxy.relay_from_client(data)
  end

  def send(data)
    @connected.callback { send_data data }
  end

  def unbind
    proxy.unbind_client
  end
end

class HttpProxy < EventMachine::Connection
  attr_reader :client_port

  def initialize(client_port)
    @client_port = client_port
  end

  def receive_data(data)
    client.send_data data unless data.nil?
  end

  def relay_from_client(data)
    send_data data unless data.nil?
  end

  def unbind
    client.close_connection
    @client = nil
  end

  def unbind_client
    close_connection_after_writing
    @client = nil
  end

  private

  def client
    @client ||= EventMachine.connect '127.0.0.1', client_port, HttpClient, self
  end
end

class HttpsProxy < HttpProxy
  def post_init
    start_tls
  end

  def receive_data(data)
    super data.gsub(/\r\n\r\n/, "\r\nX_FORWARDED_PROTO: https\r\n\r\n")
  end
end

EventMachine.run do
  host, port = "127.0.0.1", 3793
  EventMachine.start_server(host, port, HttpsProxy, 4567)
  puts "Open https://#{host}:#{port}"
end

# dummy_app.rb
# ------------
#
#     require 'sinatra'
#     
#     get '/foo' do
#       'FOO'
#     end