Rack middleware for process X-Reproxy-URL.

x_reproxy_url_filter.rb

=begin
require 'grape'
require 'x_reproxy_url_filter'

class Hello < Grape::API
  get 'static' do
    status 200
    %w{The quick brown fox jumps over the lazy dog}
  end
  get 'download' do
    status 204
    header XReproxyUrlFilter::X_REPROXY_URL, 'http://localhost/static'
  end
end

use XReproxyUrlFilter
run Hello
=end

require 'open-uri'
require 'rest-client'

class XReproxyUrlFilter
  BYPASS_KEYS = %w{content-type content-length}
  X_REPROXY_URL = 'x_reproxy_url'

  class Adapter
    def initialize(uri)
      @uri = uri
      RestClient.head(uri){|response, request, result, &block|
        @res = result
        response.return!(request, result, &block)
      }
    end

    def each(&block)
      Kernel.open(@uri){|res|
        res.each(&block)
      }
    end

    def to_rack_response
      headers = @res.each_header.select{|k,v| BYPASS_KEYS.include?(k)}.to_h
      [@res.code.to_i, headers, self]
    end
  end

  def initialize(app)
    @app = app
  end
  def call(env)
    res = @app.call(env)
    url = res[1][X_REPROXY_URL]
    return res unless url
    Adapter.new(url).to_rack_response
  end
end