Max recursion during unpickling with a proxy object

Bug report

Bug description:

Description of Problem

A wrapper class was needed to wrap an underlying object while exposing the underlying object's attributes. https://stackoverflow.com/questions/68926132/creation-of-a-class-wrapper-in-python

When creating a wrapper object for an object to be placed on a multiprocessing queue, if the wrapper object overrides the getattr method, and the object is retrieved from the queue, a max recursion depth error is observed. The expected behavior was for the wrapper object to be returned and have the underlying object exposed in the process that was handling the object.

Example Snippet

from multiprocessing import Queue

class Foo:
    """
    Any old object
    """
    def __init__(self, var: int):
        self._var = var

    @property
    def var(self) -> int:
        return self._var

class FooWrapper:
    """
    Wrapper class to expose underlying object attrs
    """
    def __init__(self, foo: Foo):
        self.foo = foo

    def __getattr__(self, name):
        return getattr(self.foo, name)

if __name__=="__main__":
    queue = Queue()
    foo = Foo(2)
    foo_wrapper = FooWrapper(foo)
    queue.put(foo_wrapper)
    message_wrapper = queue.get()  # This fails due to max recursion depth reached

>>
    return getattr(self.foo, name)
                   ^^^^^^^^
  [Previous line repeated 994 more times]
RecursionError: maximum recursion depth exceeded

CPython versions tested on:

3.11

Operating systems tested on:

Linux, Windows

まず、Python 3.11 で提供された FooWrapper と multiprocessing.Queue の再現コードを実行し、続いて queue.get() のアンピックリング経路と wrapper の getattr を追跡します。ラップされたオブジェクトが復元される前に、なぜ属性アクセスが再帰するのかを特定します。Foo.var を引き続き公開しつつ、wrapper を RecursionError なしでキューに入れて取得でき、再現コードに対するリグレッションカバレッジがあることを完了条件とします。