Issue 28737: Document that tp_dealloc handler must call PyObject_GC_UnTrack if Py_TPFLAGS_HAVE_GC is set (original) (raw)

Issue28737

Created on 2016-11-18 16:47 by colesbury, last changed 2022-04-11 14:58 by admin. This issue is now closed.

Messages (6)
msg281146 - (view)	Author: Sam Gross (colesbury) *	Date: 2016-11-18 16:47
In general, an a PyTypeObject that has Py_TPFLAGS_HAVE_GC set must call PyObject_GC_UnTrack() before it frees any PyObject* references it owns. The only reference to this requirement I found is in https://docs.python.org/3/c-api/gcsupport.html#c._PyObject_GC_TRACK. This requirement should be documented in: 1. https://docs.python.org/3/c-api/typeobj.html#c.PyTypeObject.tp_dealloc 2. https://docs.python.org/3/extending/newtypes.html A call to PyObject_GC_UnTrack() should also be added to he official "noddy4" example. Currently, the example is incorrect and can crash if a referred-to object triggers a GC from it's destructor. See the following example which segfaults: https://github.com/colesbury/noddy It may be worthwhile to have _Py_Dealloc call PyObject_GC_UnTrack() if the PyTypeObject has Py_TPFLAGS_HAVE_GC set. Considering that the official Python extension example is missing the call, it seems likely that extension writers often forget to include it.
msg405112 - (view)	Author: Sam Gross (colesbury) *	Date: 2021-10-27 16:46
Antoine Pitrou already fixed the "noddy4" example (now renamed to "custom4") and updated the newtypes_tutorial, but I think it's still worth mentioning PyObject_GC_Untrack in a few additional places.
msg405120 - (view)	Author: Łukasz Langa (lukasz.langa) *	Date: 2021-10-27 19:15
New changeset 35e1ff38ee67ee543d9fcb268c3552c5397f9b3f by Sam Gross in branch 'main': bpo-28737: Document when tp_dealloc should call PyObject_GC_UnTrack() (GH-29246) https://github.com/python/cpython/commit/35e1ff38ee67ee543d9fcb268c3552c5397f9b3f
msg405232 - (view)	Author: Łukasz Langa (lukasz.langa) *	Date: 2021-10-28 19:23
New changeset 9e0012116ac9e8d26bf19ef8741deeecf2b6f72b by Sam Gross in branch '3.10': [3.10] bpo-28737: Document when tp_dealloc should call PyObject_GC_UnTrack() (GH-29246) (GH-29249) https://github.com/python/cpython/commit/9e0012116ac9e8d26bf19ef8741deeecf2b6f72b
msg405233 - (view)	Author: Łukasz Langa (lukasz.langa) *	Date: 2021-10-28 19:23
New changeset 193504acf3bfb7cff1edf7f568c2405b857fa1f7 by Miss Islington (bot) in branch '3.9': bpo-28737: Document when tp_dealloc should call PyObject_GC_UnTrack() (GH-29246) (GH-29248) https://github.com/python/cpython/commit/193504acf3bfb7cff1edf7f568c2405b857fa1f7
msg405234 - (view)	Author: Łukasz Langa (lukasz.langa) *	Date: 2021-10-28 19:23
Thanks, Sam! ✨ 🍰 ✨