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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"v3.13.0rc2 Segmentation Fault in gc.get_referents. PyCapsule-\u003etraverse_func is NULL","articleBody":"# Crash report\r\n\r\n### What happened?\r\n\r\nHello! Thank you for all your work on CPython. Hopefully this crash report is helpful.\r\n\r\nWhile working on https://github.com/matplotlib/matplotlib/pull/28861, we noticed that our tests were segfaulting, and only on python3.13. Here is a minimal reproduction of that crash. I've also run it with python3.13-dbg and `-X dev` to hopefully provide more details.\r\n\r\n**Error Message**\r\n```\r\npython: ../Objects/capsule.c:321: capsule_traverse: Assertion `capsule-\u003etraverse_func != NULL' failed.\r\n```\r\n\r\n**Minimal Reproducible Example:**\r\n```console\r\n$ python3.13 -m venv venv_313\r\n$ source venv_313/bin/activate\r\n$ pip install matplotlib==3.9.2\r\n$ python -X dev test.py\r\n```\r\n\r\n**test.py**\r\n```python\r\nimport gc\r\nfrom collections import deque\r\nfrom matplotlib.figure import Figure\r\nfrom matplotlib.lines import Line2D\r\nfrom matplotlib.collections import PathCollection\r\n\r\ndef get_children(parent):\r\n    if parent.__class__.__qualname__ == \"PyCapsule\":\r\n        print(f\"About to call gc.get_referents on {repr(parent)}\")\r\n    children = gc.get_referents(parent) # gc.get_referrers does not segfault, but gc.get_referents does\r\n    if parent.__class__.__qualname__ == \"PyCapsule\":\r\n        print(f\"There are {len(children)} children\")\r\n    return children\r\n\r\ndef breadth_first_search(start):\r\n    to_visit = deque([start])\r\n    explored = set()\r\n    try:\r\n        gc.disable()  # Can repro without this, but it should make behavior more consistent\r\n        while len(to_visit) \u003e 0:\r\n            parent = to_visit.popleft()\r\n            for child in get_children(parent):\r\n                if id(child) in explored:\r\n                    continue\r\n                explored.add(id(child))\r\n                to_visit.append(child)\r\n    finally:\r\n        gc.enable()\r\n\r\ndef find_reference_cycles():\r\n    fig = Figure()\r\n    ax = fig.add_subplot()\r\n    ax.plot([1])\r\n    fig.clear()\r\n    breadth_first_search(fig)\r\n\r\nif __name__ == \"__main__\":\r\n    find_reference_cycles()\r\n```\r\n\r\nwith python3.13 on Ubuntu 22.04.4\r\n```console\r\n$ python -VV\r\nPython 3.13.0rc2 (main, Sep  9 2024, 22:55:42) [GCC 11.4.0]\r\n$ python -X dev test.py\r\nAbout to call gc.get_referents on \u003ccapsule object \"pybind11_function_record_capsule\" at 0x7f79954f34d0\u003e\r\nFatal Python error: Segmentation fault\r\n\r\nCurrent thread 0x00007f79e687f280 (most recent call first):\r\n  File \"/home/justin/tmp/matplotlib_py313_segfault/test.py\", line 10 in get_children\r\n  File \"/home/justin/tmp/matplotlib_py313_segfault/test.py\", line 22 in breadth_first_search\r\n  File \"/home/justin/tmp/matplotlib_py313_segfault/test.py\", line 35 in find_reference_cycles\r\n  File \"/home/justin/tmp/matplotlib_py313_segfault/test.py\", line 38 in \u003cmodule\u003e\r\n\r\nExtension modules: numpy._core._multiarray_umath, numpy.linalg._umath_linalg, PIL._imaging, kiwisolver._cext (total: 4)\r\nSegmentation fault (core dumped)\r\n```\r\n\r\nwith python3.13-dbg on Ubuntu 22.04.4:\r\n```console\r\n$ python -VV\r\nPython 3.13.0rc2 (main, Sep  9 2024, 22:55:42) [GCC 11.4.0]\r\n$ python -X dev test.py\r\nAbout to call gc.get_referents on \u003ccapsule object \"pybind11_function_record_capsule\" at 0x7f0c31adba80\u003e\r\npython: ../Objects/capsule.c:321: capsule_traverse: Assertion `capsule-\u003etraverse_func != NULL' failed.\r\nFatal Python error: Aborted\r\n\r\nCurrent thread 0x00007f0c82d1a280 (most recent call first):\r\n  File \"/home/justin/tmp/matplotlib_py313_segfault/test.py\", line 10 in get_children\r\n  File \"/home/justin/tmp/matplotlib_py313_segfault/test.py\", line 22 in breadth_first_search\r\n  File \"/home/justin/tmp/matplotlib_py313_segfault/test.py\", line 35 in find_reference_cycles\r\n  File \"/home/justin/tmp/matplotlib_py313_segfault/test.py\", line 38 in \u003cmodule\u003e\r\n\r\nExtension modules: numpy._core._multiarray_umath, numpy.linalg._umath_linalg, PIL._imaging, kiwisolver._cext (total: 4)\r\nAborted (core dumped)\r\n```\r\n\r\nIt does not crash with python3.12 on Ubuntu 22.04.4\r\n```console\r\n$ python -VV\r\nPython 3.12.6 (main, Sep 10 2024, 00:05:17) [GCC 11.4.0]\r\n\r\n$ python -X dev test.py\r\nAbout to call gc.get_referents on \u003ccapsule object \"pybind11_function_record_capsule\" at 0x7f85a852a570\u003e\r\nThere are 0 children\r\n[ ... many similar log lines removed ... ]\r\n```\r\n\r\nSide-note:\r\nI tested this on both python3.12 and 3.13, but not the main branch. The Github Issue dropdown doesn't have a 3.13rc2 option for me to check.\r\n\r\n### CPython versions tested on:\r\n\r\n3.12\r\n\r\n### Operating systems tested on:\r\n\r\nLinux\r\n\r\n### Output from running 'python -VV' on the command line:\r\n\r\nPython 3.13.0rc2 (main, Sep  9 2024, 22:55:42) [GCC 11.4.0]\r\n\r\n\u003c!-- gh-linked-prs --\u003e\r\n### Linked PRs\r\n* gh-124559\r\n* gh-124588\n\u003c!-- /gh-linked-prs --\u003e\r\n","author":{"url":"https://github.com/justinjhendrick","@type":"Person","name":"justinjhendrick"},"datePublished":"2024-09-25T19:29:57.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":4},"url":"https://github.com/124538/cpython/issues/124538"}