Fixed #27211 -- Made UpdateCacheMiddleware include caching headers for "304 Not Modified" responses.

django/middleware/cache.py

@@ -77,7 +77,7 @@ class UpdateCacheMiddleware(MiddlewareMixin):
             # We don't need to update the cache, just return.
             return response
 
-        if response.streaming or response.status_code != 200:
+        if response.streaming or response.status_code not in (200, 304):
             return response
 
         # Don't cache responses that set a user-specific (and maybe security
@@ -95,7 +95,7 @@ class UpdateCacheMiddleware(MiddlewareMixin):
             # max-age was set to 0, don't bother caching.
             return response
         patch_response_headers(response, timeout)
-        if timeout:
+        if timeout and response.status_code == 200:
             cache_key = learn_cache_key(request, response, timeout, self.key_prefix, cache=self.cache)
             if hasattr(response, 'render') and callable(response.render):
                 response.add_post_render_callback(

tests/cache/tests.py

@@ -22,7 +22,9 @@ from django.core.cache import (
 )
 from django.core.cache.utils import make_template_fragment_key
 from django.db import connection, connections
-from django.http import HttpRequest, HttpResponse, StreamingHttpResponse
+from django.http import (
+    HttpRequest, HttpResponse, HttpResponseNotModified, StreamingHttpResponse,
+)
 from django.middleware.cache import (
     CacheMiddleware, FetchFromCacheMiddleware, UpdateCacheMiddleware,
 )
@@ -2147,6 +2149,19 @@ class CacheMiddlewareTest(SimpleTestCase):
         # Inserting a CSRF cookie in a cookie-less request prevented caching.
         self.assertIsNone(cache_middleware.process_request(request))
 
+    def test_304_response_has_http_caching_headers_but_not_cached(self):
+        original_view = mock.Mock(return_value=HttpResponseNotModified())
+        view = cache_page(2)(original_view)
+        request = self.factory.get('/view/')
+        # The view shouldn't be cached on the second call.
+        view(request).close()
+        response = view(request)
+        response.close()
+        self.assertEqual(original_view.call_count, 2)
+        self.assertIsInstance(response, HttpResponseNotModified)
+        self.assertIn('Cache-Control', response)
+        self.assertIn('Expires', response)
+
 
 @override_settings(
     CACHE_MIDDLEWARE_KEY_PREFIX='settingsprefix',