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_socket extension C capsule cannot be visited by the GC and so _socket.socket type may stay alive longer than expected #108240

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@vstinner

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@vstinner

Bug report

Checklist

  • I am confident this is a bug in CPython, not a bug in a third-party project
  • I have searched the CPython issue tracker,
    and am confident this bug has not been reported before

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Output from running 'python -VV' on the command line:

Python 3.13.0a0 (heads/main:d63972e289, Aug 21 2023, 22:29:24) [GCC 13.2.1 20230728 (Red Hat 13.2.1-1)]

A clear and concise description of the bug:

The following script will never delete _socket.socket type:

import _socket

# uncomment to workaround the bug
#_socket.CAPI = None

_socket = None

import sys
del sys.modules['_socket']

import gc
gc.collect()

print("exit")

Example on a Python debug build:

$ ./python -X showrefcount script.py 
exit
[408 refs, 260 blocks]

The GC cannot visit the strong reference to the type stored in _socket.CAPI capsule (C API). In Python, creating a heap type creates a reference cycle. For example, a type MRO contains the type. Methods also contain a strong reference to their type.

$ ./python
Python 3.13.0a0 (heads/main:d63972e289, Aug 21 2023, 22:29:24) [GCC 13.2.1 20230728 (Red Hat 13.2.1-1)] on linux
>>> import _socket
>>> t=_socket.socket
>>> t.__mro__[0] is t
True

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