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[3.10] bpo-28737: Document when tp_dealloc should call PyObject_GC_UnTrack() (GH-29246) #29249

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Merged
ambv merged 1 commit into from
Oct 28, 2021
Merged

[3.10] bpo-28737: Document when tp_dealloc should call PyObject_GC_UnTrack() (GH-29246) #29249

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16 changes: 8 additions & 8 deletions Doc/c-api/gcsupport.rst
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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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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
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14 changes: 13 additions & 1 deletion Doc/extending/newtypes.rst
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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::
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