From ad332e5ca9503bba2e14cc8c3ca675eed56d72bc Mon Sep 17 00:00:00 2001
From: Kevin Christopher Henry <k@severian.com>
Date: Tue, 11 Oct 2016 03:42:00 -0400
Subject: [PATCH] Refs #19705 -- Made GZipMiddleware make ETags weak.

Django's conditional request processing can now produce 304 Not Modified
responses for content that is subject to compression.
---
 django/middleware/gzip.py |  8 ++++++--
 docs/ref/middleware.txt   |  3 +++
 tests/middleware/tests.py | 43 +++++++++++++++++++++++++++------------
 3 files changed, 39 insertions(+), 15 deletions(-)

diff --git a/django/middleware/gzip.py b/django/middleware/gzip.py
index 642e3188d3..d6e2134f17 100644
--- a/django/middleware/gzip.py
+++ b/django/middleware/gzip.py
@@ -41,8 +41,12 @@ class GZipMiddleware(MiddlewareMixin):
             response.content = compressed_content
             response['Content-Length'] = str(len(response.content))
 
-        if response.has_header('ETag'):
-            response['ETag'] = re.sub('"$', ';gzip"', response['ETag'])
+        # If there is a strong ETag, make it weak to fulfill the requirements
+        # of RFC 7232 section-2.1 while also allowing conditional request
+        # matches on ETags.
+        etag = response.get('ETag')
+        if etag and etag.startswith('"'):
+            response['ETag'] = 'W/' + etag
         response['Content-Encoding'] = 'gzip'
 
         return response
diff --git a/docs/ref/middleware.txt b/docs/ref/middleware.txt
index 432f5437ff..d007d252f5 100644
--- a/docs/ref/middleware.txt
+++ b/docs/ref/middleware.txt
@@ -155,6 +155,9 @@ It will NOT compress content if any of the following are true:
 * The request (the browser) hasn't sent an ``Accept-Encoding`` header
   containing ``gzip``.
 
+If the response has an ``ETag`` header, the ETag is made weak to comply with
+:rfc:`7232#section-2.1`.
+
 You can apply GZip compression to individual views using the
 :func:`~django.views.decorators.gzip.gzip_page()` decorator.
 
diff --git a/tests/middleware/tests.py b/tests/middleware/tests.py
index a9cb1f31ea..db8ed6ea90 100644
--- a/tests/middleware/tests.py
+++ b/tests/middleware/tests.py
@@ -855,27 +855,44 @@ class GZipMiddlewareTest(SimpleTestCase):
 @override_settings(USE_ETAGS=True)
 class ETagGZipMiddlewareTest(SimpleTestCase):
     """
-    Tests if the ETagMiddleware behaves correctly with GZipMiddleware.
+    ETags are handled properly by GZipMiddleware.
     """
     rf = RequestFactory()
     compressible_string = b'a' * 500
 
-    def test_compress_response(self):
+    def test_strong_etag_modified(self):
         """
-        ETag is changed after gzip compression is performed.
+        GZipMiddleware makes a strong ETag weak.
+        """
+        request = self.rf.get('/', HTTP_ACCEPT_ENCODING='gzip, deflate')
+        response = HttpResponse(self.compressible_string)
+        response['ETag'] = '"eggs"'
+        gzip_response = GZipMiddleware().process_response(request, response)
+        self.assertEqual(gzip_response['ETag'], 'W/"eggs"')
+
+    def test_weak_etag_not_modified(self):
+        """
+        GZipMiddleware doesn't modify a weak ETag.
+        """
+        request = self.rf.get('/', HTTP_ACCEPT_ENCODING='gzip, deflate')
+        response = HttpResponse(self.compressible_string)
+        response['ETag'] = 'W/"eggs"'
+        gzip_response = GZipMiddleware().process_response(request, response)
+        self.assertEqual(gzip_response['ETag'], 'W/"eggs"')
+
+    def test_etag_match(self):
+        """
+        GZipMiddleware allows 304 Not Modified responses.
         """
         request = self.rf.get('/', HTTP_ACCEPT_ENCODING='gzip, deflate')
         response = GZipMiddleware().process_response(
             request,
-            CommonMiddleware().process_response(request, HttpResponse(self.compressible_string))
+            ConditionalGetMiddleware().process_response(request, HttpResponse(self.compressible_string))
         )
-        gzip_etag = response.get('ETag')
-
-        request = self.rf.get('/', HTTP_ACCEPT_ENCODING='')
-        response = GZipMiddleware().process_response(
-            request,
-            CommonMiddleware().process_response(request, HttpResponse(self.compressible_string))
+        gzip_etag = response['ETag']
+        next_request = self.rf.get('/', HTTP_ACCEPT_ENCODING='gzip, deflate', HTTP_IF_NONE_MATCH=gzip_etag)
+        next_response = ConditionalGetMiddleware().process_response(
+            next_request,
+            HttpResponse(self.compressible_string)
         )
-        nogzip_etag = response.get('ETag')
-
-        self.assertNotEqual(gzip_etag, nogzip_etag)
+        self.assertEqual(next_response.status_code, 304)