text: from urllib.parse import unquote
  
  from wptserve.utils import isomorphic_decode, isomorphic_encode
  
  def redirect_response(request, response, visited_count):
    # |visited_count| is used as a unique id to differentiate responses
    # every time.
    location = b'empty.js'
    if b'Redirect' in request.GET:
        location = isomorphic_encode(unquote(isomorphic_decode(request.GET[b'Redirect'])))
    return (301,
    [
      (b'Cache-Control', b'no-cache, must-revalidate'),
      (b'Pragma', b'no-cache'),
      (b'Content-Type', b'application/javascript'),
      (b'Location', location),
    ],
    u'/* %s */' % str(visited_count))
  
  def not_found_response():
    return 404, [(b'Content-Type', b'text/plain')], u"Page not found"
  
  def ok_response(request, response, visited_count,
                  extra_body=u'', mime_type=b'application/javascript'):
    # |visited_count| is used as a unique id to differentiate responses
    # every time.
    return (
      [
        (b'Cache-Control', b'no-cache, must-revalidate'),
        (b'Pragma', b'no-cache'),
        (b'Content-Type', mime_type)
      ],
      u'/* %s */ %s' % (str(visited_count), extra_body))
  
  def main(request, response):
    key = request.GET[b"Key"]
    mode = request.GET[b"Mode"]
  
    visited_count = request.server.stash.take(key)
    if visited_count is None:
      visited_count = 0
  
    # Keep how many times the test requested this resource.
    visited_count += 1
    request.server.stash.put(key, visited_count)
  
    # Return a response based on |mode| only when it's the second time (== update).
    if visited_count == 2:
      if mode == b'normal':
        return ok_response(request, response, visited_count)
      if mode == b'bad_mime_type':
        return ok_response(request, response, visited_count, mime_type=b'text/html')
      if mode == b'not_found':
        return not_found_response()
      if mode == b'redirect':
            return redirect_response(request, response, visited_count)
      if mode == b'syntax_error':
        return ok_response(request, response, visited_count, extra_body=u'badsyntax(isbad;')
      if mode == b'throw_install':
        return ok_response(request, response, visited_count, extra_body=u"addEventListener('install', function(e) { throw new Error('boom'); });")
  
    return ok_response(request, response, visited_count)
  