Fixed #18171 -- Checked signature of authenticate() to avoid supressing TypeErrors.

The current auth backend code catches TypeError to detect backends that
do not support specified argumetnts. As a result, any TypeErrors raised
within the actual backend code are silenced.

In Python 2.7+ and 3.2+ this can be avoided by using inspect.getcallargs().
With this method, we can test whether arguments match the signature without
actually calling the function.

Thanks David Eyk for the report.

django/contrib/auth/__init__.py

@@ -1,3 +1,4 @@
+import inspect
 import re
 
 from django.conf import settings
@@ -46,10 +47,13 @@ def authenticate(**credentials):
     """
     for backend in get_backends():
         try:
-            user = backend.authenticate(**credentials)
+            inspect.getcallargs(backend.authenticate, **credentials)
         except TypeError:
             # This backend doesn't accept these credentials as arguments. Try the next one.
             continue
+
+        try:
+            user = backend.authenticate(**credentials)
         except PermissionDenied:
             # This backend says to stop in our tracks - this user should not be allowed in at all.
             return None

django/contrib/auth/tests/test_auth_backends.py

@@ -480,3 +480,32 @@ class ChangedBackendSettingsTest(TestCase):
             # anonymous as the backend is not longer available.
             self.assertIsNotNone(user)
             self.assertTrue(user.is_anonymous())
+
+
+class TypeErrorBackend(object):
+    """
+    Always raises TypeError.
+    """
+    supports_object_permissions = True
+    supports_anonymous_user = True
+    supports_inactive_user = True
+
+    def authenticate(self, username=None, password=None):
+        raise TypeError
+
+
+@skipIfCustomUser
+class TypeErrorBackendTest(TestCase):
+    """
+    Tests that a TypeError within a backend is propagated properly.
+
+    Regression test for ticket #18171
+    """
+    backend = 'django.contrib.auth.tests.test_auth_backends.TypeErrorBackend'
+
+    def setUp(self):
+        self.user1 = User.objects.create_user('test', 'test@example.com', 'test')
+
+    @override_settings(AUTHENTICATION_BACKENDS=(backend, ))
+    def test_type_error_raised(self):
+        self.assertRaises(TypeError, authenticate, username='test', password='test')