file_system.rb

require 'uri'

module Store
  class FileSystem
    CHUNKSIZE = 65536

    def initialize(uri)
      @uri  = uri
      @path = uri.path
    end

    def get
      open(@path, "rb") do |file|
        yield file.read(CHUNKSIZE) until file.eof?
      end
    end
  end
end

server.rb

    class GetImage < Goliath::API
      use Goliath::Rack::Render, 'json'

      def response(env)
        begin
          meta = db.get_image(params[:id])
        rescue MyExceptions::NotFound => e
          return [404, {}, {code: 404, message: e.message}]
        end

        uri     = meta[:location]
        headers = push_meta_into_headers(meta)

        # duhh, this was obviously blocking the reactor with big files (now testing with 700mb iso)...
        # EM.next_tick do
        #  Store::FileSystem.new(uri).get { |chunk| env.stream_send(chunk) }
        #  env.stream_close
        # end 

        # this doesn't block, but probably there is a better way?
        operation = proc do
          Store::FileSystem.new(uri).get { |chunk| env.stream_send(chunk) }
        end

        callback = proc do |result|
          env.stream_close
        end

        EM.defer operation, callback

        headers.merge!('Content-Type' => 'application/octet-stream', 'X-Stream' => 'Goliath')
        [200, headers, Goliath::Response::STREAMING]
      end
    end

    class Server < Goliath::API
      use Goliath::Rack::Heartbeat
      use Goliath::Rack::Params

      get '/images/:id', GetImage
    end