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

首先在 Python 3.11 上執行提供的 FooWrapper 與 multiprocessing.Queue 重現程式，接著追蹤 queue.get() 的反序列化路徑以及 wrapper 的 getattr。找出為什麼在包裝的物件還原之前，屬性存取會發生遞迴。完成條件是：wrapper 可以放入佇列並取出而不會發生 RecursionError，同時仍然能夠公開 Foo.var，並為該重現程式提供回歸測試涵蓋。