[2.0.x] Fixed #29071 -- Fixed contrib.auth.authenticate() crash if a backend doesn't accept a request but a later one does.

Regression in a3ba2662cd.

django/contrib/auth/__init__.py

@@ -82,6 +82,7 @@ def authenticate(request=None, **credentials):
 
 
 def _authenticate_with_backend(backend, backend_path, request, credentials):
+    credentials = credentials.copy()  # Prevent a mutation from propagating.
     args = (request,)
     # Does the backend accept a request argument?
     try:

docs/releases/1.11.10.txt

@@ -11,3 +11,7 @@ Bugfixes
 
 * Fixed incorrect foreign key nullification if a model has two foreign keys to
   the same model and a target model is deleted (:ticket:`29016`).
+
+* Fixed a regression where ``contrib.auth.authenticate()`` crashes if an
+  authentication backend doesn't accept ``request`` and a later one does
+  (:ticket:`29071`).

docs/releases/2.0.2.txt

@@ -20,3 +20,7 @@ Bugfixes
 
 * Fixed a regression where a queryset that annotates with geometry objects
   crashes (:ticket:`29054`).
+
+* Fixed a regression where ``contrib.auth.authenticate()`` crashes if an
+  authentication backend doesn't accept ``request`` and a later one does
+  (:ticket:`29071`).

tests/auth_tests/test_auth_backends_deprecation.py

@@ -13,6 +13,18 @@ class NoRequestBackend:
         pass
 
 
+class NoRequestWithKwargs:
+    def authenticate(self, username=None, password=None, **kwargs):
+        pass
+
+
+class RequestPositionalArg:
+    def authenticate(self, request, username=None, password=None, **kwargs):
+        assert username == 'username'
+        assert password == 'pass'
+        assert request is mock_request
+
+
 class RequestNotPositionArgBackend:
     def authenticate(self, username=None, password=None, request=None):
         assert username == 'username'
@@ -34,6 +46,8 @@ class AcceptsRequestBackendTest(SimpleTestCase):
     method without a request parameter.
     """
     no_request_backend = '%s.NoRequestBackend' % __name__
+    no_request_with_kwargs_backend = '%s.NoRequestWithKwargs' % __name__
+    request_positional_arg_backend = '%s.RequestPositionalArg' % __name__
     request_not_positional_backend = '%s.RequestNotPositionArgBackend' % __name__
     request_not_positional_with_used_kwarg_backend = '%s.RequestNotPositionArgWithUsedKwargBackend' % __name__
 
@@ -79,6 +93,21 @@ class AcceptsRequestBackendTest(SimpleTestCase):
             "argument." % self.no_request_backend
         )
 
+    @override_settings(AUTHENTICATION_BACKENDS=[no_request_with_kwargs_backend, request_positional_arg_backend])
+    def test_credentials_not_mutated(self):
+        """
+        No problem if a backend doesn't accept `request` and a later one does.
+        """
+        with warnings.catch_warnings(record=True) as warns:
+            warnings.simplefilter('always')
+            authenticate(mock_request, username='username', password='pass')
+        self.assertEqual(len(warns), 1)
+        self.assertEqual(
+            str(warns[0].message),
+            "In %s.authenticate(), move the `request` keyword argument to the "
+            "first positional argument." % self.no_request_with_kwargs_backend
+        )
+
     @override_settings(AUTHENTICATION_BACKENDS=[request_not_positional_with_used_kwarg_backend])
     def test_handles_backend_in_kwargs(self):
         with warnings.catch_warnings(record=True) as warns: