[1.5.x] Fixed #19779 -- Checked contrib.sites presence in RedirectFallbackMiddleware

Thanks Aymeric Augustin for the report and directions for the patch.
Backport of 2ed90eac from master.

2 files changed, 19 insertions, 1 deletions

diff --git a/django/contrib/redirects/middleware.py b/django/contrib/redirects/middleware.py
index f5d35946bc..03c9d97c0d 100644
--- a/django/contrib/redirects/middleware.py
+++ b/django/contrib/redirects/middleware.py
@@ -1,11 +1,20 @@
 from __future__ import unicode_literals
 
+from django.conf import settings
 from django.contrib.redirects.models import Redirect
 from django.contrib.sites.models import get_current_site
+from django.core.exceptions import ImproperlyConfigured
 from django import http
-from django.conf import settings
+
 
 class RedirectFallbackMiddleware(object):
+    def __init__(self):
+        if 'django.contrib.sites' not in settings.INSTALLED_APPS:
+            raise ImproperlyConfigured(
+                "You cannot use RedirectFallbackMiddleware when "
+                "django.contrib.sites is not installed."
+            )
+
     def process_response(self, request, response):
         if response.status_code != 404:
             return response # No need to check for a redirect for non-404 responses.
diff --git a/django/contrib/redirects/tests.py b/django/contrib/redirects/tests.py
index 4ea8523199..bdcdf4a9e1 100644
--- a/django/contrib/redirects/tests.py
+++ b/django/contrib/redirects/tests.py
@@ -1,9 +1,11 @@
 from django.conf import settings
 from django.contrib.sites.models import Site
+from django.core.exceptions import ImproperlyConfigured
 from django.test import TestCase
 from django.test.utils import override_settings
 from django.utils import six
 
+from .middleware import RedirectFallbackMiddleware
 from .models import Redirect
 
 
@@ -52,3 +54,10 @@ class RedirectTests(TestCase):
             site=self.site, old_path='/initial', new_path='')
         response = self.client.get('/initial')
         self.assertEqual(response.status_code, 410)
+
+    @override_settings(
+        INSTALLED_APPS=[app for app in settings.INSTALLED_APPS
+                        if app != 'django.contrib.sites'])
+    def test_sites_not_installed(self):
+        with self.assertRaises(ImproperlyConfigured):
+            RedirectFallbackMiddleware()