Disambiguate missing cache values from None

We have observed an issue with spurious cache hits in development,
that we've been unable to reproduce so far. Looking into this did
however prompt an audit of the caching decorator, which reveals
we potentially have an issue where a cached "None" value is mis-
interpreted as "not cached" rather than "cached as None". This
commit clarifies the expected behaviour with a test and a more
robust cache check.

src/open_inwoner/utils/decorators.py

@@ -120,11 +120,12 @@ def wrapped(*args, **kwargs) -> RT:
 
             cache_key = cache_key_with_attr_placeholders.format(**key_kwargs)
             logger.debug("Resolved cache_key `%s` to `%s`", key, cache_key)
+
             _cache: BaseCache = caches[alias]
-            result = _cache.get(cache_key)
 
-            # The key exists in cache so we return the already cached data
-            if result is not None:
+            CACHE_MISS = object()
+            result = _cache.get(cache_key, default=CACHE_MISS)
+            if result is not CACHE_MISS:
                 logger.debug("Cache hit: '%s'", cache_key)
                 return result
 

src/open_inwoner/utils/tests/test_utils.py

@@ -55,11 +55,11 @@ def with_static_key(self):
 
         self.cache.get.assert_has_calls(
             [
-                mock.call("alpha:charlie:bravo:42"),
-                mock.call("alpha:bar:bravo:baz"),
-                mock.call("alpha:bar:bravo:5"),
-                mock.call("alpha:bar:bravo:baz:charlie:charlie:42"),
-                mock.call("static"),
+                mock.call("alpha:charlie:bravo:42", default=mock.ANY),
+                mock.call("alpha:bar:bravo:baz", default=mock.ANY),
+                mock.call("alpha:bar:bravo:5", default=mock.ANY),
+                mock.call("alpha:bar:bravo:baz:charlie:charlie:42", default=mock.ANY),
+                mock.call("static", default=mock.ANY),
             ]
         )
 
@@ -201,3 +201,19 @@ def foo():
 
         with self.assertRaises(ValueError):
             foo()
+
+    def test_returning_None_is_not_treated_as_a_cache_miss(self):
+        m = mock.Mock()
+
+        @cache("foo")
+        def returns_none():
+            m()
+            return None
+
+        # The second call should return the cached "None" from the first call,
+        # which the cache decorator should interpret as a valid cached value,
+        # not as a cache miss.
+        returns_none()
+        returns_none()
+
+        m.assert_called_once()