Resolve thin backend server port from Rails controller

backend_server_port.rb

  # request.port is unfortunately useless from within the app, as it
  # does not reflect the backend bind port, but the frontend proxy.
  # So we have to reverse lookup the PID to figure out our port.
  def backend_server_port
    Thread.current[:backend_server_port] ||= begin
      logger.debug "Attempting to reverse-lookup server port from PID file"
      port = nil
      Dir["#{RAILS_ROOT}/tmp/pids/thin.*.pid"].each do |f|
        pid = File.read(f).chomp.to_i
        if pid == $$
          port = f.gsub(/.*\.([0-9]+)\.pid$/, '\1').to_i
          raise "Malformed PID file does not contain port: #{f}" if port == 0
          break
        end
      end
      port || 3000   # guess 3000 for local dev
    end
  end