Added check for "*" value in Vary header

Loading a cached response containing a Vary: * header leads to a
deserialization error warning, even though it matches the specification.
Since these responses can never be served from the cache as per RFC 7234
section 4.1, storing them can be avoided to save disk space and get rid
of the warning.

Author: TheJokr

cachecontrol/controller.py

@@ -293,6 +293,15 @@ def cache_response(self, request, response, body=None, status_codes=None):
         if no_store:
             return
 
+        # https://tools.ietf.org/html/rfc7234#section-4.1:
+        # A Vary header field-value of "*" always fails to match.
+        # Storing such a response leads to a deserialization warning
+        # during cache lookup and is not allowed to ever be served,
+        # so storing it can be avoided.
+        if "*" in response_headers.get("vary", ""):
+            logger.debug('Response header has "Vary: *"')
+            return
+
         # If we've been given an etag, then keep the response
         if self.cache_etags and "etag" in response_headers:
             logger.debug("Caching due to etag")

cachecontrol/serialize.py

@@ -107,6 +107,8 @@ def prepare_response(self, request, cached):
         """
         # Special case the '*' Vary value as it means we cannot actually
         # determine if the cached response is suitable for this request.
+        # This case is also handled in the controller code when creating
+        # a cache entry, but is left here for backwards compatibility.
         if "*" in cached.get("vary", {}):
             return
 

tests/test_cache_control.py

@@ -112,6 +112,14 @@ def test_cache_response_no_store_with_etag(self, cc):
 
         assert not cc.cache.set.called
 
+    def test_no_cache_with_vary_star(self, cc):
+        # Vary: * indicates that the response can never be served
+        # from the cache, so storing it can be avoided.
+        resp = self.resp({"vary": "*"})
+        cc.cache_response(self.req(), resp)
+
+        assert not cc.cache.set.called
+
     def test_update_cached_response_with_valid_headers(self):
         cached_resp = Mock(headers={"ETag": "jfd9094r808", "Content-Length": 100})