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Fix segmentation fault on Python 3.13 #960

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Fix segmentation fault on Python 3.13 #960

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452fddb
Fix segmentation fault on Python 3.13
shadchin Nov 27, 2025
d14b921
Re-enable the regular tests for 3.13
shadchin Nov 27, 2025
639f7ee
Add include
shadchin Nov 27, 2025
6ffe4e3
Disable `Py_TPFLAGS_INLINE_VALUES`
shadchin Dec 13, 2025
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3 changes: 1 addition & 2 deletions .github/workflows/ci.yml
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Original file line number Diff line number Diff line change
Expand Up @@ -60,8 +60,7 @@ jobs:
- "3.10"
- "3.11"
- "3.12"
# TODO: Enable this once fixed https://github.com/jcrist/msgspec/issues/910
# - "3.13"
- "3.13"
- "3.14"

steps:
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4 changes: 4 additions & 0 deletions src/msgspec/_core.c
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Expand Up @@ -5328,6 +5328,10 @@ Struct_alloc(PyTypeObject *type) {
PyObject *obj;
bool is_gc = MS_TYPE_IS_GC(type);

#if PY313_PLUS && !PY314_PLUS
type->tp_flags &= ~Py_TPFLAGS_INLINE_VALUES;
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@ZeroIntensity ZeroIntensity Dec 16, 2025

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I'm almost certain you'll trigger assertion failures (or worse, get spurious crashes) with this. Supertypes don't expect their flags to be different in subtypes.

#endif

if (is_gc) {
obj = PyObject_GC_New(PyObject, type);
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@ZeroIntensity ZeroIntensity Dec 16, 2025

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Manually calling PyObject_GC_New/PyObject_New doesn't really work for subtypes. It might work in practice sometimes, but it's not something that's supposed to be supported.

I believe the root of this bug comes from the fact that on 3.13, inline values are initialized in the tp_alloc slot of type objects, which is skipped here. In 3.14, we moved that logic to PyObject_GC_New because we decided to support more types.

So, to fix this, just call type->tp_alloc instead; that will invoke the supertype (PyType_GenericAlloc) and initialize the inline values.

}
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