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
-                    # of the response's ETag.
-                    etag=response['ETag'].strip('"'),
+                    etag=unquote_etag(response['ETag']),
                     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