facebook authentication: crash on reading "args["access_token"][-1]"

tornado.auth.FacebookGraphMixin crashes when receiving the access token from facebook.

On Ubuntu 17.04, tornado version 4.4.2-1ubuntu1

The problem seems to be at line 983 of /usr/lib/python2.7/dist-packages/tornado/auth.py.

Replacing urlparse.parse_qs with escape.json_decode seems to fix the problem:

- args = urlparse.parse_qs(escape.native_str(response.body))
- session = {
- "access_token": args["access_token"][-1],
- "expires": args.get("expires")
- }
+ args = escape.json_decode(escape.native_str(response.body)) #bug: response is a json string!!
+ session = {
+ "access_token": args["access_token"],
+ "expires": args.get("expires")
+ }

Here's the traceback:

Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/tornado/web.py", line 1469, in _execute
    result = yield result
  File "/usr/lib/python2.7/dist-packages/tornado/gen.py", line 1015, in run
    value = future.result()
  File "/usr/lib/python2.7/dist-packages/tornado/concurrent.py", line 237, in result
    raise_exc_info(self._exc_info)
  File "/usr/lib/python2.7/dist-packages/tornado/gen.py", line 1021, in run
    yielded = self.gen.throw(*exc_info)
  File "webservice.py", line 72, in get
    user = yield self.get_authenticated_user(redirect_uri=self.absolutePathPrefix + '/auth/facebookgraph', client_id=self.settings["facebook_api_key"], client_secret=self.settings["facebook_secret"], code=code)
  File "/usr/lib/python2.7/dist-packages/tornado/gen.py", line 1015, in run
    value = future.result()
  File "/usr/lib/python2.7/dist-packages/tornado/concurrent.py", line 237, in result
    raise_exc_info(self._exc_info)
  File "/usr/lib/python2.7/dist-packages/tornado/stack_context.py", line 314, in wrapped
    ret = fn(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/tornado/auth.py", line 983, in _on_access_token
    "access_token": args["access_token"][-1],
KeyError: 'access_token'