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GH-99293: Fix stale method caches and assertion errors in SWIG-generated modules #99294

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GH-99293: Fix stale method caches and assertion errors in SWIG-generated modules #99294

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aacb0f6
Fix stale method caches and assertion errors in SWIG-generated extension
Yhg1s Nov 9, 2022
0e8b65d
📜🤖 Added by blurb_it.
blurb-it[bot] Nov 9, 2022
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1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Core and Builtins/2022-11-09-16-02-27.gh-issue-99293.oPstNV.rst
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@@ -0,0 +1 @@
Fix an issue where extension modules could inadvertently trigger an assertion error in typeobject.c by clearing the Py_TPFLAGS_VALID_VERSION_TAG tp_flag without clearing the tp_version_tag field. (Extension modules should use PyType_Modified() instead, however.)
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I just noticed Py_TPFLAGS_VALID_VERSION_TAG isn't actually documented.

Suggested change
Fix an issue where extension modules could inadvertently trigger an assertion error in typeobject.c by clearing the Py_TPFLAGS_VALID_VERSION_TAG tp_flag without clearing the tp_version_tag field. (Extension modules should use PyType_Modified() instead, however.)
Fix an issue where extension modules could inadvertently trigger an assertion error in ``typeobject.c`` by clearing the ``Py_TPFLAGS_VALID_VERSION_TAG`` :c:member:`~PyTypeObject.tp_flags` without clearing the :c:member:`~PyTypeObject.tp_version_tag` field. (Extension modules should use :c:func:`PyType_Modified` instead, however.)

9 changes: 7 additions & 2 deletions Objects/typeobject.c
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Expand Up @@ -4176,12 +4176,17 @@ _PyType_Lookup(PyTypeObject *type, PyObject *name)
unsigned int h = MCACHE_HASH_METHOD(type, name);
struct type_cache *cache = get_type_cache();
struct type_cache_entry *entry = &cache->hashtable[h];
/* Some extension modules (e.g. those generated by SWIG) don't always
* change/clear tp_version_tag when they clear the
* Py_TPFLAGS_VALID_VERSION_TAG tp_flag, so check both tp_version_tag
* and the tp_flag. (Invalidated cache entries are replaced below.)
*/
if (entry->version == type->tp_version_tag &&
entry->name == name) {
entry->name == name &&
_PyType_HasFeature(type, Py_TPFLAGS_VALID_VERSION_TAG)) {
#if MCACHE_STATS
cache->hits++;
#endif
assert(_PyType_HasFeature(type, Py_TPFLAGS_VALID_VERSION_TAG));
return entry->value;
}

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@Fidget-Spinner Fidget-Spinner Nov 9, 2022 •
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For other reviewers wondering what happens down this code path if Py_TPFLAGS_VALID_VERSION_TAG is not set:

  1. A normal lookup happens where we walk the MRO.
  2. We assign a new version tag to the type. This sets Py_TPFLAGS_VALID_VERSION_TAG.

So this should be safe.

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