Fix metadata dependency cache leaking across the MRO

libcst/_metadata_dependent.py

@@ -82,17 +82,19 @@ def get_inherited_dependencies(cls) -> Collection["ProviderT"]:
 
         Recursively searches the MRO of the subclass for metadata dependencies.
         """
-        try:
-            # pyre-fixme[16]: use a hidden attribute to cache the property
-            return cls._INHERITED_METADATA_DEPENDENCIES_CACHE
-        except AttributeError:
+        # Only consult this class's own cache entry: a plain attribute lookup
+        # would walk the MRO and return a base class's cached value, so a bare
+        # base class resolved first would hide the subclass's dependencies.
+        cache = cls.__dict__.get("_INHERITED_METADATA_DEPENDENCIES_CACHE")
+        if cache is None:
             dependencies = set()
             for c in inspect.getmro(cls):
                 if issubclass(c, MetadataDependent):
                     dependencies.update(c.METADATA_DEPENDENCIES)
+            cache = frozenset(dependencies)
             # pyre-fixme[16]: use a hidden attribute to cache the property
-            cls._INHERITED_METADATA_DEPENDENCIES_CACHE = frozenset(dependencies)
-            return cls._INHERITED_METADATA_DEPENDENCIES_CACHE
+            cls._INHERITED_METADATA_DEPENDENCIES_CACHE = cache
+        return cache
 
     @contextmanager
     def resolve(self, wrapper: "MetadataWrapper") -> Iterator[None]:

libcst/metadata/tests/test_metadata_provider.py

@@ -200,6 +200,28 @@ def visit_Pass(self, node: cst.Pass) -> None:
         # Check each visitor is called once
         mock.visited_simple.assert_called_once()
 
+    def test_bare_base_class_does_not_poison_inherited_metadata(self) -> None:
+        """
+        Tests that resolving a bare base class first does not hide the
+        dependencies declared by a subclass.
+        """
+        test_runner = self
+
+        class SimpleProvider(VisitorMetadataProvider[int]):
+            def visit_Pass(self, node: cst.Pass) -> None:
+                self.set_metadata(node, 1)
+
+        class DependentVisitor(CSTTransformer):
+            METADATA_DEPENDENCIES = (SimpleProvider,)
+
+            def visit_Pass(self, node: cst.Pass) -> None:
+                test_runner.assertEqual(self.get_metadata(SimpleProvider, node), 1)
+
+        wrapper = MetadataWrapper(parse_module("pass"))
+        # Resolving the bare base class caches an empty dependency set on it.
+        wrapper.visit(CSTTransformer())
+        wrapper.visit(DependentVisitor())
+
     def test_provider_inherited_metadata(self) -> None:
         """
         Tests that providers inherit access to metadata declared by their base