knox: handle race condition on concurrent logout calls

With AUTO_REFRESH enabled in case of multiple logout calls when
one has removed the token while the other is still authenticating
we may get a DatabaseError on TokenAuthentication.renew_token
while updating the expiry of a now removed auth_token.
Fix that by ignoring the token that returned a database error so that
in case of double logout the last request will get an
AuthenticationFailed exception instead of a 500 error.

knox/auth.py

@@ -7,6 +7,7 @@ def compare_digest(a, b):
 import binascii
 
 from django.contrib.auth import get_user_model
+from django.db import DatabaseError
 from django.utils import timezone
 from django.utils.translation import ugettext_lazy as _
 from rest_framework import exceptions
@@ -73,7 +74,11 @@ def authenticate_credentials(self, token):
                 raise exceptions.AuthenticationFailed(msg)
             if compare_digest(digest, auth_token.digest):
                 if knox_settings.AUTO_REFRESH and auth_token.expiry:
-                    self.renew_token(auth_token)
+                    try:
+                        self.renew_token(auth_token)
+                    except DatabaseError:
+                        # avoid race condition with concurrent logout calls
+                        continue
                 return self.validate_user(auth_token)
         raise exceptions.AuthenticationFailed(msg)
 

tests/tests.py

@@ -2,9 +2,11 @@
 from datetime import datetime, timedelta
 
 from django.contrib.auth import get_user_model
+from django.db import DatabaseError
 from django.test import override_settings
 from django.utils.six.moves import reload_module
 from freezegun import freeze_time
+from rest_framework.exceptions import AuthenticationFailed
 from rest_framework.serializers import DateTimeField
 from rest_framework.test import APIRequestFactory, APITestCase as TestCase
 
@@ -15,6 +17,13 @@
 from knox.settings import CONSTANTS, knox_settings
 from knox.signals import token_expired
 
+try:
+    # Python 3
+    from unittest import mock
+except ImportError:
+    # Python 2
+    import mock
+
 try:
     # For django >= 2.0
     from django.urls import reverse
@@ -396,3 +405,13 @@ def test_expiry_is_present(self):
             response.data['expiry'],
             DateTimeField().to_representation(AuthToken.objects.first().expiry)
         )
+
+    def test_authenticate_credentials_handle_database_error_on_renew_token(self):
+        instance, token = AuthToken.objects.create(user=self.user)
+        with override_settings(REST_KNOX=auto_refresh_knox):
+            reload_module(auth)
+            token_auth = TokenAuthentication()
+            with mock.patch.object(token_auth, 'renew_token') as m:
+                m.side_effect = DatabaseError()
+                with self.assertRaises(AuthenticationFailed):
+                    token_auth.authenticate_credentials(token.encode('utf-8'))

tox.ini

@@ -35,6 +35,7 @@ deps =
     django22: Django>=2.2,<2.3
     django-nose
     markdown<3.0
+    mock
     isort
     djangorestframework
     freezegun