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Crash because of ownership assert in PyObject_ClearManagedDict on Free Threading #152254

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Crash because of ownership assert in PyObject_ClearManagedDict on Free Threading#152254
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sobolevn
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interpreter-core(Objects, Python, Grammar, and Parser dirs)type-crashA hard crash of the interpreter, possibly with a core dump

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@sobolevn
sobolevn
opened on Jun 26, 2026

Crash report

Refs #151763
Original description: https://gist.github.com/devdanzin/99b84915b197ae0ade7face262b8af66

Repro:

from unittest.mock import MagicMock
from _testcapi import set_nomemory

set_nomemory(200)              # fail allocations from the 200th onward (lands in shutdown GC)
(MagicMock(), undefined_name)  # build a MagicMock (managed __dict__), then NameError abandons
                               # it into a traceback cycle -> survives to shutdown GC

I have a fix ready.

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  1. sobolevn
    self-assigned this
    on Jun 26, 2026
  2. sobolevn
    added
    interpreter-core(Objects, Python, Grammar, and Parser dirs)
    on Jun 26, 2026
  3. sobolevn
    added a parent issue on Jun 26, 2026
  4. sobolevn
    added
    type-crashA hard crash of the interpreter, possibly with a core dump
    on Jun 26, 2026
  5. github-project-automation
    added this to Project_VKon Oct 18, 2026
  6. github-project-automation
    moved this to Todo in Project_VKon Oct 18, 2026
  7. vstinner
    changed the title [-]Crash because of ownership assert in `PyObject_ClearManagedDict`[/-] [+]Crash because of ownership assert in `PyObject_ClearManagedDict` on Free Threading[/+] on Nov 14, 2026

  8. vstinner commented on Nov 14, 2026

    @vstinner
    vstinner
    on Nov 14, 2026
    Member

    Ah, the reproducer requires Free Threading. Using Free Threading, I get:

    $ ./python repro.py 
object address  : 0x2000181b890
object refcount : 3
object type     : 0x9ff7e0
object type name: NameError
object repr     : 
lost sys.stderr
python: Objects/dictobject.c:207: set_keys: Assertion `_Py_IsOwnedByCurrentThread((PyObject *)mp) || IS_DICT_SHARED(mp)' failed.
Aborted                    (core dumped) ./python repro.py
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interpreter-core(Objects, Python, Grammar, and Parser dirs)type-crashA hard crash of the interpreter, possibly with a core dump

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