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

[Yhg1s]

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.

• Issue: Assertion errors/stale cache in typeobject.c when tp_version_tag is not cleared. #99293

[Fidget-Spinner]

LGTM. However, let's wait a bit for others to review.

[Fidget-Spinner]

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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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.)

[Fidget-Spinner]

Copying a comment I made elsewhere:

The code will crash return incorrect cached methods and values on 3.11 when running specialised instructions. Those only use tp_version_tag as a guard and don't check Py_TPFLAGS_VALID_VERSION_TAG for speed reasons.

One fix is to add a guard to all specialised LOAD_METHOD/ATTR instructions to check for the flag (this will slow down all those instructions a little). Another fix is to ask everyone to regenerate their code with a newer SWIG for Python 3.11.0.