Calling ShareableList.count in threads aborts: Assertion 'self->exports == 0' failed

[devdanzin]

Crash report

What happened?

It's possible to abort the interpreter by calling multiprocessing.shared_memory.ShareableList.count in threads with PYTHON_GIL=0 in a debug build:

import gc
import multiprocessing.shared_memory
from threading import Thread

obj = multiprocessing.shared_memory.ShareableList("Uq..SeDAmB+EBrkLl.SG.Z+Z.ZdsV..wT+zLxKwdN\b")

for x in range(10):
    Thread(target=obj.count, args=(1,)).start()

del obj
gc.collect()

Result:

Exception ignored in: <function SharedMemory.__del__ at 0x200006bbfb0>
Traceback (most recent call last):
  File "/home/danzin/projects/mycpython/Lib/multiprocessing/shared_memory.py", line 189, in __del__
    self.close()
  File "/home/danzin/projects/mycpython/Lib/multiprocessing/shared_memory.py", line 229, in close
    self._buf.release()
BufferError: memoryview has 2 exported buffers
python: Objects/memoryobject.c:1143: memory_dealloc: Assertion `self->exports == 0' failed.
Aborted

Found using fusil by @vstinner.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

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

Python 3.14.0a2+ experimental free-threading build (heads/main-dirty:c9b399fbdb0, Nov 19 2024, 20:12:48) [GCC 11.4.0]