Free threaded: slicing a shared memoryview raises `ValueError: operation forbidden on released memoryview object`

Bug report

Bug description:

Hello (and thanks for your amazing work),

I found that, on free threading Python, slicing a shared memoryview from several threads raises ValueError: operation forbidden on released memoryview object, even though nothing calls .release() and the underlying object is an immutable bytes. Stock (GIL) builds are unaffected.

data = bytes(1 << 20)
mv = memoryview(data)                   # ONE view, shared
# in each of 8 threads:  bytes(mv[0:64])

See the attached reproduction script. Results are not deterministic: not all threads raise an exception. Here are the results on my machine after running it:

$ PYTHON_GIL=0 python3.15t repro_shared_memoryview_threads.py
python 3.15.0rc1 gil_enabled=False: 3/8 threads FAILED -> ValueError('operation forbidden on released memoryview object')
$ PYTHON_GIL=1 python3.15t repro_shared_memoryview_threads.py
python 3.15.0rc1 gil_enabled=True: all 8 threads OK

CPython versions tested on:

3.15

Operating systems tested on:

Linux

Linked PRs

• gh-155882

Beginne mit dem angehängten Skript repro_shared_memoryview_threads.py und vergleiche dessen Ergebnisse unter PYTHON_GIL=0 und PYTHON_GIL=1 in Python 3.15. Untersuche das Verhalten von shared memoryview in der free-threaded Runtime; die Aufgabe ist erledigt, wenn gleichzeitiges Slicing nicht mehr den gemeldeten ValueError auslöst. Der verknüpfte PR gh-155882 zeigt, dass die Arbeit daran bereits begonnen hat.