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[3.10] bpo-28737: Document when tp_dealloc should call PyObject_GC_UnTrack() (GH-29246) by colesbury · Pull Request #29249 · python/cpython · GitHub

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16 changes: 8 additions & 8 deletions Doc/c-api/gcsupport.rst
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters. Learn more about bidirectional Unicode characters
Original file line number Diff line number Diff line change
Expand Up @@ -33,6 +33,14 @@ Constructors for container types must conform to two rules:
#. Once all the fields which may contain references to other containers are
initialized, it must call :c:func:`PyObject_GC_Track`.

Similarly, the deallocator for the object must conform to a similar pair of
rules:

#. Before fields which refer to other containers are invalidated,
:c:func:`PyObject_GC_UnTrack` must be called.

#. The object's memory must be deallocated using :c:func:`PyObject_GC_Del`.

.. warning::
If a type adds the Py_TPFLAGS_HAVE_GC, then it *must* implement at least
a :c:member:`~PyTypeObject.tp_traverse` handler or explicitly use one
Expand Down Expand Up @@ -100,14 +108,6 @@ Constructors for container types must conform to two rules:

.. versionadded:: 3.9

Similarly, the deallocator for the object must conform to a similar pair of
rules:

#. Before fields which refer to other containers are invalidated,
:c:func:`PyObject_GC_UnTrack` must be called.

#. The object's memory must be deallocated using :c:func:`PyObject_GC_Del`.


.. c:function:: void PyObject_GC_Del(void *op)

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12 changes: 12 additions & 0 deletions Doc/c-api/typeobj.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -668,6 +668,18 @@ and :c:type:`PyType_Type` effectively act as defaults.)
:c:func:`PyObject_GC_Del` if the instance was allocated using
:c:func:`PyObject_GC_New` or :c:func:`PyObject_GC_NewVar`.

If the type supports garbage collection (has the :const:`Py_TPFLAGS_HAVE_GC`
flag bit set), the destructor should call :c:func:`PyObject_GC_UnTrack`
before clearing any member fields.

.. code-block:: c

static void foo_dealloc(foo_object *self) {
PyObject_GC_UnTrack(self);
Py_CLEAR(self->ref);
Py_TYPE(self)->tp_free((PyObject *)self);
}

Finally, if the type is heap allocated (:const:`Py_TPFLAGS_HEAPTYPE`), the
deallocator should decrement the reference count for its type object after
calling the type deallocator. In order to avoid dangling pointers, the
Expand Down
14 changes: 13 additions & 1 deletion Doc/extending/newtypes.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -73,7 +73,19 @@ function::
newdatatype_dealloc(newdatatypeobject *obj)
{
free(obj->obj_UnderlyingDatatypePtr);
Py_TYPE(obj)->tp_free(obj);
Py_TYPE(obj)->tp_free((PyObject *)obj);
}

If your type supports garbage collection, the destructor should call
:c:func:`PyObject_GC_UnTrack` before clearing any member fields::

static void
newdatatype_dealloc(newdatatypeobject *obj)
{
PyObject_GC_UnTrack(obj);
Py_CLEAR(obj->other_obj);
...
Py_TYPE(obj)->tp_free((PyObject *)obj);
}

.. index::
Expand Down

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