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gh-133931: Introduce _PyObject_XSetRefDelayed to replace Py_XSETREF#134377

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corona10 merged 21 commits intopython:mainfromcorona10:gh-133931
Jun 17, 2025

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@corona10corona10 commentedMay 20, 2025•
edited by bedevere-appbot
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_PyObject_XSetRefDelayed(PyObject **ptr, PyObject *value)
{
PyObject *old = *ptr;
FT_ATOMIC_STORE_PTR_RELEASE(*ptr, Py_NewRef(value));
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The default build implementation inpycore_pymem.h doesn't include thePy_NewRef().

My preference is to keep thePy_NewRef(value) in the callers, i.e.:

_PyObject_XSetRefDelayed(dictptr,Py_NewRef(value));

to match the semantics ofPy_XSETREF.

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Interesting... CI passed in my local but not in the CI.. :(

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Ah okay..

test_aclose (test.test_asyncgen.TestUnawaitedWarnings.test_aclose) ... Assertion failed: (!_Py_IsImmortal(op)), function _Py_ExplicitMergeRefcount, file object.c, line 467.

@corona10corona10 changed the titlegh-133931: Introduce _PyObject_XSetRefDelayed to replace Py_XSETREF[WIP] gh-133931: Introduce _PyObject_XSetRefDelayed to replace Py_XSETREFMay 21, 2025
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Why do we need_PyObject_XSetRefDelayed? It looks like all uses of it are wrapped in critical sections where the oldPy_XSETREF would work.

Also, what is "delayed"? Either choose a clearer name, or add an explanatory comment in the header file.

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@corona10corona10 changed the title[WIP] gh-133931: Introduce _PyObject_XSetRefDelayed to replace Py_XSETREFgh-133931: Introduce _PyObject_XSetRefDelayed to replace Py_XSETREFJun 7, 2025
@corona10corona10 marked this pull request as ready for reviewJune 7, 2025 13:34
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corona10 commentedJun 7, 2025•
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Why do we need _PyObject_XSetRefDelayed? It looks like all uses of it are wrapped in critical sections where the old Py_XSETREF would work.

IIUC, the critical section is for the root object to prevent other threads from mutating the child object, and a delayed reference to prevent use-after-free from other threads that reference the child object.

@corona10corona10 requested a review fromcolesburyJune 7, 2025 13:42
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Gentle ping@kumaraditya303@vstinner

@corona10corona10 requested a review fromvstinnerJune 15, 2025 06:32
@@ -718,15 +720,19 @@ gen_set_name(PyObject *self, PyObject *value, void *Py_UNUSED(ignored))
"__name__ must be set to a string object");
return -1;
}
Py_XSETREF(op->gi_name, Py_NewRef(value));
Py_BEGIN_CRITICAL_SECTION(self);
// To prevent use-after-free from other threads that reference the gi_name.
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Maybe addgh-133931: prefix to these comments.

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#ifdef Py_GIL_DISABLED
// Same as `Py_XSETREF` but in free-threading, it stores the object atomically
// and queues the old object to be decrefed at a safe point using QSBR.
PyAPI_FUNC(void) _PyObject_XSetRefDelayed(PyObject **p_obj, PyObject *obj);
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Why not adding this function topycore_object.h header instead?

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Because _PyObject_XDecRefDelayed is also declared here?

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I think it's fine to place_PyObject_XDecRefDelayed and_PyObject_XSetRefDelayed inpycore_object.h instead if you prefer. (I agree that they should probably be in the same header as each other).

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@@ -3967,7 +3967,8 @@ _PyObject_SetDict(PyObject *obj, PyObject *value)
return -1;
}
Py_BEGIN_CRITICAL_SECTION(obj);
// To prevent use-after-free from other threads that reference the __dict__
// gh-133931: To prevent use-after-free from other threads that reference
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@efimov-mikhailefimov-mikhailJun 16, 2025•
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Is it correct issue reference for__dict__ too?

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Looks good - a few comments below

#ifdef Py_GIL_DISABLED
// Same as `Py_XSETREF` but in free-threading, it stores the object atomically
// and queues the old object to be decrefed at a safe point using QSBR.
PyAPI_FUNC(void) _PyObject_XSetRefDelayed(PyObject **p_obj, PyObject *obj);
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I think it's fine to place_PyObject_XDecRefDelayed and_PyObject_XSetRefDelayed inpycore_object.h instead if you prefer. (I agree that they should probably be in the same header as each other).

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@corona10corona10 requested a review fromcolesburyJune 17, 2025 02:48
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IMO, it's better to use "gh-133980" comment for__dict__ and "gh-133931" comment forgi_name andgi_qualname.
Moreover, we've moved_PyObject_XSetRefDelayed and so we have to make little changes in includes.

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LGTM

@corona10corona10 requested a review fromvstinnerJune 17, 2025 14:25
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LGTM

@corona10corona10 merged commit52be7f4 intopython:mainJun 17, 2025
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lkollar pushed a commit to lkollar/cpython that referenced this pull requestJun 19, 2025
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@kumaraditya303kumaraditya303kumaraditya303 approved these changes

@ericsnowcurrentlyericsnowcurrentlyAwaiting requested review from ericsnowcurrentlyericsnowcurrently is a code owner

@markshannonmarkshannonAwaiting requested review from markshannonmarkshannon is a code owner

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