Fixed #11340 -- Prevented HttpResponseNotModified to have content/content-type

The HTTP 1.1 spec tells that the 304 response MUST NOT contain a
message body.
Thanks aparajita for the report.

django/http/__init__.py

@@ -744,6 +744,16 @@ class HttpResponsePermanentRedirect(HttpResponseRedirectBase):
 class HttpResponseNotModified(HttpResponse):
     status_code = 304
 
+    def __init__(self, *args, **kwargs):
+        super(HttpResponseNotModified, self).__init__(*args, **kwargs)
+        del self['content-type']
+
+    @HttpResponse.content.setter
+    def content(self, value):
+        if value:
+            raise AttributeError("You cannot set content to a 304 (Not Modified) response")
+        self._container = []
+
 class HttpResponseBadRequest(HttpResponse):
     status_code = 400
 

django/views/static.py

@@ -61,7 +61,7 @@ def serve(request, path, document_root=None, show_indexes=False):
     mimetype = mimetype or 'application/octet-stream'
     if not was_modified_since(request.META.get('HTTP_IF_MODIFIED_SINCE'),
                               statobj.st_mtime, statobj.st_size):
-        return HttpResponseNotModified(content_type=mimetype)
+        return HttpResponseNotModified()
     with open(fullpath, 'rb') as f:
         response = HttpResponse(f.read(), content_type=mimetype)
     response["Last-Modified"] = http_date(statobj.st_mtime)

docs/ref/request-response.txt

@@ -743,8 +743,9 @@ types of HTTP responses. Like ``HttpResponse``, these subclasses live in
 
 .. class:: HttpResponseNotModified
 
-    The constructor doesn't take any arguments. Use this to designate that a
-    page hasn't been modified since the user's last request (status code 304).
+    The constructor doesn't take any arguments and no content should be added
+    to this response. Use this to designate that a page hasn't been modified
+    since the user's last request (status code 304).
 
 .. class:: HttpResponseBadRequest
 

tests/regressiontests/httpwrappers/tests.py

@@ -6,7 +6,7 @@ import pickle
 
 from django.core.exceptions import SuspiciousOperation
 from django.http import (QueryDict, HttpResponse, HttpResponseRedirect,
-                         HttpResponsePermanentRedirect,
+                         HttpResponsePermanentRedirect, HttpResponseNotModified,
                          SimpleCookie, BadHeaderError,
                          parse_cookie)
 from django.utils import six
@@ -330,6 +330,16 @@ class HttpResponseTests(unittest.TestCase):
                               HttpResponsePermanentRedirect, url)
 
 
+class HttpResponseSubclassesTests(unittest.TestCase):
+    def test_not_modified(self):
+        response = HttpResponseNotModified()
+        self.assertEqual(response.status_code, 304)
+        # 304 responses should not have content/content-type
+        with self.assertRaises(AttributeError):
+            response.content = "Hello dear"
+        self.assertNotIn('content-type', response)
+
+
 class CookieTests(unittest.TestCase):
     def test_encode(self):
         """