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GH-99293: Fix stale method caches and assertion errors in SWIG-generated modules#99294
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modules.Extension modules generated by SWIG up to version 4.1.0 clear thePy_TPFLAGS_VALID_VERSION_TAG bit from tp_flags when modifying the type, asthey should, but do not update tp_version_tag as typeobject.c expects. (Forexample, merely one of many instances I've found:https://github.com/OSGeo/gdal/blob/6bd07b20b3e55c2fc94da611244a615a4fd2991f/swig/python/extensions/osr_wrap.cpp#L2296)In release builds this means a potentially stale cached entry is used, andin debug builds (or release builds that happen to enable assertions, whichas it turns out is actually a good idea) it triggers an assertion error.SWIG 4.1.0 avoids this problem by using PyType_Modified(), but there are alot of older versions of SWIG and of checked-in generated extension modulesaround.
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LGTM. However, let's wait a bit for others to review.
| #if MCACHE_STATS | ||
| cache->hits++; | ||
| #endif | ||
| assert(_PyType_HasFeature(type, Py_TPFLAGS_VALID_VERSION_TAG)); | ||
| return entry->value; | ||
| } | ||
Fidget-SpinnerNov 9, 2022 • edited
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For other reviewers wondering what happens down this code path ifPy_TPFLAGS_VALID_VERSION_TAG is not set:
- A normal lookup happens where we walk the MRO.
- We assign a new version tag to the type. This sets
Py_TPFLAGS_VALID_VERSION_TAG.
So this should be safe.
| @@ -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 noticedPy_TPFLAGS_VALID_VERSION_TAG isn't actually documented.
| 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.) |
Fidget-Spinner commentedNov 9, 2022 • edited
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Copying a comment I made elsewhere: The code will One fix is to add a guard to all specialised |
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Extension modules generated by SWIG up to version 4.1.0 clear the Py_TPFLAGS_VALID_VERSION_TAG bit from tp_flags when modifying the type, as they should, but do not update tp_version_tag as typeobject.c expects. (For example, merely one of many instances I've found:https://github.com/OSGeo/gdal/blob/6bd07b20b3e55c2fc94da611244a615a4fd2991f/swig/python/extensions/osr_wrap.cpp#L2296)
In release builds this means a potentially stale cached entry is used, and in debug builds (or release builds that happen to enable assertions, which as it turns out is actually a good idea) it triggers an assertion error.
SWIG 4.1.0 avoids this problem by using PyType_Modified(), but there are a lot of older versions of SWIG and of checked-in generated extension modules around.