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gh-99574: Add Py_SETREF() to the limited C API
Add Py_SETREF() and Py_XSETREF() macros to the limited C API.
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Lines changed: 82 additions & 64 deletions

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‎Doc/data/stable_abi.dat‎

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‎Doc/whatsnew/3.12.rst‎

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@@ -798,6 +798,10 @@ New Features
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get a frame variable by its name.
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(Contributed by Victor Stinner in :gh:`91248`.)
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* Add :c:macro:`Py_SETREF` and :c:macro:`Py_XSETREF` macros to the limited C
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API.
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(Contributed by Victor Stinner in :gh:`99574`.)
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Porting to Python 3.12
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----------------------
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‎Include/cpython/object.h‎

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@@ -305,70 +305,6 @@ _PyObject_GenericSetAttrWithDict(PyObject *, PyObject *,
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PyAPI_FUNC(PyObject *) _PyObject_FunctionStr(PyObject *);
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/* Safely decref `dst` and set `dst` to `src`.
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*
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* As in case of Py_CLEAR "the obvious" code can be deadly:
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*
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* Py_DECREF(dst);
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* dst = src;
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*
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* The safe way is:
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*
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* Py_SETREF(dst, src);
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*
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* That arranges to set `dst` to `src` _before_ decref'ing, so that any code
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* triggered as a side-effect of `dst` getting torn down no longer believes
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* `dst` points to a valid object.
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*
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* Temporary variables are used to only evalutate macro arguments once and so
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* avoid the duplication of side effects. _Py_TYPEOF() or memcpy() is used to
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* avoid a miscompilation caused by type punning. See Py_CLEAR() comment for
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* implementation details about type punning.
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*
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* The memcpy() implementation does not emit a compiler warning if 'src' has
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* not the same type than 'src': any pointer type is accepted for 'src'.
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*/
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#ifdef _Py_TYPEOF
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#define Py_SETREF(dst, src) \
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do { \
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_Py_TYPEOF(dst)* _tmp_dst_ptr = &(dst); \
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_Py_TYPEOF(dst) _tmp_old_dst = (*_tmp_dst_ptr); \
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*_tmp_dst_ptr = (src); \
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Py_DECREF(_tmp_old_dst); \
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} while (0)
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#else
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#define Py_SETREF(dst, src) \
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do { \
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PyObject **_tmp_dst_ptr = _Py_CAST(PyObject**, &(dst)); \
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PyObject *_tmp_old_dst = (*_tmp_dst_ptr); \
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PyObject *_tmp_src = _PyObject_CAST(src); \
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memcpy(_tmp_dst_ptr, &_tmp_src, sizeof(PyObject*)); \
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Py_DECREF(_tmp_old_dst); \
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} while (0)
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#endif
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/* Py_XSETREF() is a variant of Py_SETREF() that uses Py_XDECREF() instead of
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* Py_DECREF().
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*/
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#ifdef _Py_TYPEOF
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#define Py_XSETREF(dst, src) \
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do { \
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_Py_TYPEOF(dst)* _tmp_dst_ptr = &(dst); \
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_Py_TYPEOF(dst) _tmp_old_dst = (*_tmp_dst_ptr); \
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*_tmp_dst_ptr = (src); \
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Py_XDECREF(_tmp_old_dst); \
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} while (0)
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#else
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#define Py_XSETREF(dst, src) \
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do { \
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PyObject **_tmp_dst_ptr = _Py_CAST(PyObject**, &(dst)); \
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PyObject *_tmp_old_dst = (*_tmp_dst_ptr); \
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PyObject *_tmp_src = _PyObject_CAST(src); \
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memcpy(_tmp_dst_ptr, &_tmp_src, sizeof(PyObject*)); \
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Py_XDECREF(_tmp_old_dst); \
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} while (0)
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#endif
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PyAPI_DATA(PyTypeObject) _PyNone_Type;
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PyAPI_DATA(PyTypeObject) _PyNotImplemented_Type;

‎Include/object.h‎

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@@ -637,6 +637,75 @@ static inline void Py_DECREF(PyObject *op)
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#endif
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#if !defined(Py_LIMITED_API) || Py_LIMITED_API+0 >= 0x030C0000
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/* Safely decref `dst` and set `dst` to `src`.
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*
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* As in case of Py_CLEAR "the obvious" code can be deadly:
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*
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* Py_DECREF(dst);
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* dst = src;
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*
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* The safe way is:
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*
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* Py_SETREF(dst, src);
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*
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* That arranges to set `dst` to `src` _before_ decref'ing, so that any code
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* triggered as a side-effect of `dst` getting torn down no longer believes
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* `dst` points to a valid object.
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*
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* Temporary variables are used to only evalutate macro arguments once and so
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* avoid the duplication of side effects. _Py_TYPEOF() or memcpy() is used to
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* avoid a miscompilation caused by type punning. See Py_CLEAR() comment for
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* implementation details about type punning.
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*
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* The memcpy() implementation does not emit a compiler warning if 'src' has
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* not the same type than 'src': any pointer type is accepted for 'src'.
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*/
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#ifdef _Py_TYPEOF
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#define Py_SETREF(dst, src) \
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do { \
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_Py_TYPEOF(dst)* _tmp_dst_ptr = &(dst); \
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_Py_TYPEOF(dst) _tmp_old_dst = (*_tmp_dst_ptr); \
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*_tmp_dst_ptr = (src); \
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Py_DECREF(_tmp_old_dst); \
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} while (0)
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#else
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#define Py_SETREF(dst, src) \
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do { \
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PyObject **_tmp_dst_ptr = _Py_CAST(PyObject**, &(dst)); \
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PyObject *_tmp_old_dst = (*_tmp_dst_ptr); \
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PyObject *_tmp_src = _PyObject_CAST(src); \
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memcpy(_tmp_dst_ptr, &_tmp_src, sizeof(PyObject*)); \
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Py_DECREF(_tmp_old_dst); \
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} while (0)
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#endif
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/* Py_XSETREF() is a variant of Py_SETREF() that uses Py_XDECREF() instead of
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* Py_DECREF().
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*/
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#ifdef _Py_TYPEOF
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#define Py_XSETREF(dst, src) \
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do { \
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_Py_TYPEOF(dst)* _tmp_dst_ptr = &(dst); \
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_Py_TYPEOF(dst) _tmp_old_dst = (*_tmp_dst_ptr); \
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*_tmp_dst_ptr = (src); \
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Py_XDECREF(_tmp_old_dst); \
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} while (0)
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#else
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#define Py_XSETREF(dst, src) \
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do { \
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PyObject **_tmp_dst_ptr = _Py_CAST(PyObject**, &(dst)); \
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PyObject *_tmp_old_dst = (*_tmp_dst_ptr); \
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PyObject *_tmp_src = _PyObject_CAST(src); \
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memcpy(_tmp_dst_ptr, &_tmp_src, sizeof(PyObject*)); \
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Py_XDECREF(_tmp_old_dst); \
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} while (0)
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#endif
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#endif // Py_LIMITED_API
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/* Function to use in case the object pointer can be NULL: */
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static inline void Py_XINCREF(PyObject *op)
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{

‎Misc/NEWS.d/next/C API/2022-11-18-13-22-31.gh-issue-99574.kmYSuk.rst‎

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Add :c:macro:`Py_SETREF()` and :c:macro:`Py_XSETREF()` macros to the limited
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C API. Patch by Victor Stinner.

‎Misc/stable_abi.toml‎

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added = '3.12' # Before 3.12, available in "structmember.h" w/o Py_ prefix
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[const.Py_AUDIT_READ]
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added = '3.12' # Before 3.12, available in "structmember.h"
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[macro.Py_SETREF]
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added = '3.12'
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[macro.Py_XSETREF]
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added = '3.12'

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