Fixed #26024 -- Fixed regression in ConditionalGetMiddleware ETag support.

Thanks Denis Cornehl for help with the patch.

django/middleware/common.py

@@ -8,6 +8,7 @@ from django.core.mail import mail_managers
 from django.urls import is_valid_path
 from django.utils.cache import get_conditional_response, set_response_etag
 from django.utils.encoding import force_text
+from django.utils.http import unquote_etag
 from django.utils.six.moves.urllib.parse import urlparse
 
 logger = logging.getLogger('django.request')
@@ -120,9 +121,7 @@ class CommonMiddleware(object):
             if response.has_header('ETag'):
                 return get_conditional_response(
                     request,
-                    # get_conditional_response() requires an unquoted version
+                    etag=unquote_etag(response['ETag']),
-                    # of the response's ETag.
-                    etag=response['ETag'].strip('"'),
                     response=response,
                 )
 

django/middleware/http.py

@@ -1,5 +1,5 @@
 from django.utils.cache import get_conditional_response
-from django.utils.http import http_date, parse_http_date_safe
+from django.utils.http import http_date, parse_http_date_safe, unquote_etag
 
 
 class ConditionalGetMiddleware(object):
@@ -23,7 +23,7 @@ class ConditionalGetMiddleware(object):
         if etag or last_modified:
             return get_conditional_response(
                 request,
-                etag=etag,
+                etag=unquote_etag(etag),
                 last_modified=last_modified,
                 response=response,
             )

django/utils/http.py

@@ -253,6 +253,13 @@ def quote_etag(etag):
     return '"%s"' % etag.replace('\\', '\\\\').replace('"', '\\"')
 
 
+def unquote_etag(etag):
+    """
+    Unquote an ETag string; i.e. revert quote_etag().
+    """
+    return etag.strip('"').replace('\\"', '"').replace('\\\\', '\\') if etag else etag
+
+
 def is_same_domain(host, pattern):
     """
     Return ``True`` if the host is either an exact match or a match

docs/releases/1.9.2.txt

@@ -9,4 +9,5 @@ Django 1.9.2 fixes several bugs in 1.9.1.
 Bugfixes
 ========
 
-* ...
+* Fixed a regression in ``ConditionalGetMiddleware`` causing ``If-None-Match`` checks
+  to always return HTTP 200 (:ticket:`26024`).

tests/middleware/tests.py

@@ -470,6 +470,11 @@ class ConditionalGetMiddlewareTest(SimpleTestCase):
         self.resp = ConditionalGetMiddleware().process_response(self.req, self.resp)
         self.assertEqual(self.resp.status_code, 304)
 
+    def test_if_none_match_and_same_etag_with_quotes(self):
+        self.req.META['HTTP_IF_NONE_MATCH'] = self.resp['ETag'] = '"spam"'
+        self.resp = ConditionalGetMiddleware().process_response(self.req, self.resp)
+        self.assertEqual(self.resp.status_code, 304)
+
     def test_if_none_match_and_different_etag(self):
         self.req.META['HTTP_IF_NONE_MATCH'] = 'spam'
         self.resp['ETag'] = 'eggs'

tests/utils_tests/test_http.py

@@ -158,8 +158,10 @@ class ETagProcessingTests(unittest.TestCase):
         self.assertEqual(etags, ['', 'etag', 'e"t"ag', r'e\tag', 'weak'])
 
     def test_quoting(self):
-        quoted_etag = http.quote_etag(r'e\t"ag')
+        original_etag = r'e\t"ag'
+        quoted_etag = http.quote_etag(original_etag)
         self.assertEqual(quoted_etag, r'"e\\t\"ag"')
+        self.assertEqual(http.unquote_etag(quoted_etag), original_etag)
 
 
 class HttpDateProcessingTests(unittest.TestCase):