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

See also:

• https://pythondev.readthedocs.io/garbage_collector.html#reference-cycles
• https://vstinner.github.io/debug-python-refleak.html
• https://vstinner.github.io/subinterpreter-leaks.html

Linked PRs

• gh-108240: Fix a reference cycle in _socket.CAPI capsule #108241
• gh-108240: Add _PyCapsule_SetTraverse() internal function #108339
• gh-108240: _PyCapsule_SetTraverse() rejects NULL callbacks #108417
• gh-108240: Add pycore_capsule.h internal header file #108596