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gh-157176: Fix GC tracking in PyStructSequence_New #157179

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gh-157176: Fix GC tracking in PyStructSequence_New #157179

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ad2aa7b
fix(structseq): enable GC tracking in PyStructSequence_New
ashm-dev Sep 8, 2026
60b9a41
Merge branch 'main' into gh-157176
ashm-dev Sep 8, 2026
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5 changes: 5 additions & 0 deletions Lib/test/test_structseq.py
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Original file line number Diff line number Diff line change
Expand Up @@ -365,5 +365,10 @@ def test_replace_gc_tracked(self):

self.assertTrue(gc.is_tracked(replaced_struct))

def test_gc_tracked(self):
# PyStructSequence objects created via C API or Python should be GC-tracked
self.assertTrue(gc.is_tracked(time.gmtime()))
self.assertTrue(gc.is_tracked(os.stat(__file__)))

if __name__ == "__main__":
unittest.main()
1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2026-09-08-15-55-00.gh-issue-157176.yZ8a1B.rst
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@@ -0,0 +1 @@
Fix GC tracking for :class:`!PyStructSequence` instances created via :c:func:`PyStructSequence_New`. Patch by Shamil Abdulaev.
3 changes: 1 addition & 2 deletions Objects/structseq.c
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Expand Up @@ -84,6 +84,7 @@ PyStructSequence_New(PyTypeObject *type)
for (i = 0; i < size; i++)
obj->ob_item[i] = NULL;

_PyObject_GC_TRACK(obj);

@sergey-miryanov sergey-miryanov Sep 8, 2026

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This looks safe, but we usually try to avoid exposing partially constructed objects to the GC. I'm not sure I get why this change fixes your repro. Could you explain a bit?

@ashm-dev ashm-dev Sep 8, 2026

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  • Why it fixes the repro: time.gmtime() is not tracked by the GC. Because of that, the GC cannot see the cycle type(t) -> t -> type(t), so the entire heap type leaks at shutdown. Tracking t lets GC traverse t -> Py_TYPE(t) and collect the cycle.

  • Partially constructed object: All items are pre-initialized to NULL. Both structseq_traverse (Py_VISIT) and structseq_dealloc (Py_XDECREF) safely ignore NULL. This follows PyTuple_New(), which also tracks the object with NULL items since C API callers have no separate "finish" step.

@sergey-miryanov sergey-miryanov Sep 8, 2026

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So, this change affects all call sites where PyStructSequence_New is called without a subsequent call to _PyObject_GC_TRACK. But at first glance, many of them don't seem to need tracking for PyStructSequence_New instances.

@vstinner Do we need frozen version of PyStructSequence?

return (PyObject*)obj;
}

Expand Down Expand Up @@ -265,7 +266,6 @@ structseq_new_impl(PyTypeObject *type, PyObject *arg, PyObject *dict)
}
}

_PyObject_GC_TRACK(res);
return (PyObject*) res;
}

Expand Down Expand Up @@ -449,7 +449,6 @@ structseq_replace(PyObject *op, PyObject *args, PyObject *kwargs)
}
}

_PyObject_GC_TRACK(result);
return (PyObject *)result;

error:
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