Set Cache-Control: no-cache if cache buster check fails

If an asset URL fetch fails due to version skew across multiple servers in a
deployment, prevent the 4xx response from being cached by the CDN or browser.

The proper solution is for the downstream app to either deploy assets ahead of
the app deployment, or use sticky sessions. This change reduces the downside if
eg. stickiness fails for any reason.

Fixes #27

Author: robertknight

src/h_assets/view.py

@@ -16,7 +16,14 @@ def wrapper(context, request):
         if request.query_string and not environment.check_cache_buster(
             request.path, request.query_string
         ):
-            return HTTPNotFound()
+            response = HTTPNotFound()
+
+            # Disable downstream caching of failed responses, in case this
+            # happened due to version skew during a deployment. See
+            # https://github.com/hypothesis/h-assets/issues/27.
+            response.cache_control.no_cache = True
+
+            return response
 
         return static(context, request)
 

tests/unit/h_assets/view_test.py

@@ -50,6 +50,9 @@ def test_it_returns_404_if_cache_buster_invalid(self, environment, static_view):
         static_view.return_value.assert_not_called()
         assert isinstance(response, HTTPNotFound)
 
+        # Returns "*" though set to `True`
+        assert response.cache_control.no_cache  # pylint: disable=no-member
+
     @pytest.fixture
     def environment(self):
         return create_autospec(Environment, instance=True, spec_set=True)