text: # Copyright (c) 2012 The Chromium Authors. All rights reserved.
  # Use of this source code is governed by a BSD-style license that can be
  # found in the LICENSE file.
  
  import base64
  import posixpath
  
  from appengine_wrappers import GetAppVersion, urlfetch
  from future import Future
  
  
  class _AsyncFetchDelegate(object):
    def __init__(self, rpc):
      self._rpc = rpc
  
    def Get(self):
      return self._rpc.get_result()
  
  
  def _MakeHeaders(username, password):
    headers = {
      'User-Agent': 'Chromium docserver %s' % GetAppVersion(),
      'Cache-Control': 'max-age=0',
    }
    if username is not None and password is not None:
      headers['Authorization'] = 'Basic %s' % base64.b64encode(
          '%s:%s' % (username, password))
    return headers
  
  
  class AppEngineUrlFetcher(object):
    """A wrapper around the App Engine urlfetch module that allows for easy
    async fetches.
    """
    def __init__(self, base_path=None):
      assert base_path is None or not base_path.endswith('/'), base_path
      self._base_path = base_path
  
    def Fetch(self, url, username=None, password=None):
      """Fetches a file synchronously.
      """
      return urlfetch.fetch(self._FromBasePath(url),
                            headers=_MakeHeaders(username, password))
  
    def FetchAsync(self, url, username=None, password=None):
      """Fetches a file asynchronously, and returns a Future with the result.
      """
      rpc = urlfetch.create_rpc()
      urlfetch.make_fetch_call(rpc,
                               self._FromBasePath(url),
                               headers=_MakeHeaders(username, password))
      return Future(delegate=_AsyncFetchDelegate(rpc))
  
    def _FromBasePath(self, url):
      assert not url.startswith('/'), url
      if self._base_path is not None:
        url = posixpath.join(self._base_path, url) if url else self._base_path
      return url
  