HTTP basic auth: incorrectly padded base64 string causes an unhandled exception

[glarrain]

The problem is in rest_framework.authentication.BasicAuthentication.authenticate. The call to base64.b64decode does not catch binascii.Error, which can be raised by binascii.a2b_base64

This assumes we want to handle that error and not let it propagate up the stack. I can't think of any reason not to, specially considering the raised exception message:

Invalid basic header. Credentials not correctly base64 encoded.

Stacktrace

Traceback (most recent call last):
  File "/app/.heroku/python/lib/python3.4/site-packages/django/core/handlers/base.py", line 149, in get_response
    response = self.process_exception_by_middleware(e, request)
  File "/app/.heroku/python/lib/python3.4/site-packages/django/core/handlers/base.py", line 147, in get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/app/.heroku/python/lib/python3.4/site-packages/django/views/decorators/csrf.py", line 58, in wrapped_view
    return view_func(*args, **kwargs)
  File "/app/.heroku/python/lib/python3.4/site-packages/django/views/generic/base.py", line 68, in view
    return self.dispatch(request, *args, **kwargs)
  File "/app/.heroku/python/lib/python3.4/site-packages/rest_framework/views.py", line 466, in dispatch
    response = self.handle_exception(exc)
  File "/app/.heroku/python/lib/python3.4/site-packages/rest_framework/views.py", line 454, in dispatch
    self.initial(request, *args, **kwargs)
  File "/app/.heroku/python/lib/python3.4/site-packages/rest_framework/views.py", line 376, in initial
    self.perform_authentication(request)
  File "/app/.heroku/python/lib/python3.4/site-packages/rest_framework/views.py", line 310, in perform_authentication
    request.user
  File "/app/.heroku/python/lib/python3.4/site-packages/rest_framework/request.py", line 353, in __getattribute__
    return super(Request, self).__getattribute__(attr)
  File "/app/.heroku/python/lib/python3.4/site-packages/rest_framework/request.py", line 193, in user
    self._authenticate()
  File "/app/.heroku/python/lib/python3.4/site-packages/rest_framework/request.py", line 316, in _authenticate
    user_auth_tuple = authenticator.authenticate(self)
  File "/app/.heroku/python/lib/python3.4/site-packages/rest_framework/authentication.py", line 78, in authenticate
    auth_parts = base64.b64decode(auth[1]).decode(HTTP_HEADER_ENCODING).partition(':')
  File "/app/.heroku/python/lib/python3.4/base64.py", line 90, in b64decode
    return binascii.a2b_base64(s)
binascii.Error: Incorrect padding

Checklist

• I have verified that that issue exists against the master branch of Django REST framework.
• I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• This is not a usage question. (Those should be directed to the discussion group instead.)
• This cannot be dealt with as a third party library. (We prefer new functionality to be in the form of third party libraries where possible.)
• I have reduced the issue to the simplest possible case.
• I have included a failing test as a pull request. (If you are unable to do so we can still accept the issue.)