Constrain lru_cache proxy detection to BaseObjectProxy subclasses.

The previous check for an __self_setattr__ attribute on the bound
instance was too loose, since an unrelated class could expose that name
and be misrouted into the proxy cache storage path. Detection now gates
on issubclass(type(instance), BaseObjectProxy), using type() so the real
proxy type is seen rather than the wrapped object's type reported by the
proxy __class__ override, then stores via __self_setattr__.

Author: GrahamDumpleton

src/wrapt/caching.py

@@ -9,7 +9,7 @@
 from functools import lru_cache as _functools_lru_cache
 from functools import partial
 
-from .__wrapt__ import BoundFunctionWrapper, FunctionWrapper
+from .__wrapt__ import BaseObjectProxy, BoundFunctionWrapper, FunctionWrapper
 from .decorators import decorator
 from .synchronization import synchronized
 
@@ -74,14 +74,14 @@ def __call__(self, *args, **kwargs):
                     # If the instance the method is bound to is a wrapt
                     # object proxy, a plain setattr() would fall through and
                     # store the cache on the wrapped object rather than the
-                    # proxy. Proxies expose __self_setattr__() which stores
-                    # the attribute on the proxy itself, so use it when it is
-                    # available and fall back to setattr() otherwise.
-
-                    set_attr = getattr(instance, "__self_setattr__", None)
-
-                    if set_attr is not None:
-                        set_attr(cache_attr, cache)
+                    # proxy. Use type() rather than isinstance() so the check
+                    # sees the real proxy type and is not fooled by the proxy
+                    # overriding __class__ to report the wrapped object's
+                    # type. Proxies expose __self_setattr__() which stores the
+                    # attribute on the proxy itself.
+
+                    if issubclass(type(instance), BaseObjectProxy):
+                        instance.__self_setattr__(cache_attr, cache)
                     else:
                         setattr(instance, cache_attr, cache)