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bpo-31095: fix potential crash during GC (GH-2974)

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INADA Naoki 2017-08-24 14:55:17 +09:00 • committed by GitHub
parent bf9075a0c5
commit a6296d34a4
14 changed files with 60 additions and 13 deletions
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29
Doc/extending/newtypes.rst
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@ -728,8 +728,9 @@ functions. With :c:func:`Py_VISIT`, :c:func:`Noddy_traverse` can be simplified:
uniformity across these boring implementations.
We also need to provide a method for clearing any subobjects that can
participate in cycles. We implement the method and reimplement the deallocator
to use it::
participate in cycles.
::
static int
Noddy_clear(Noddy *self)
@ -747,13 +748,6 @@ to use it::
return 0;
}
static void
Noddy_dealloc(Noddy* self)
{
Noddy_clear(self);
Py_TYPE(self)->tp_free((PyObject*)self);
}
Notice the use of a temporary variable in :c:func:`Noddy_clear`. We use the
temporary variable so that we can set each member to *NULL* before decrementing
its reference count. We do this because, as was discussed earlier, if the
@ -776,6 +770,23 @@ be simplified::
return 0;
}
Note that :c:func:`Noddy_dealloc` may call arbitrary functions through
``__del__`` method or weakref callback. It means circular GC can be
triggered inside the function. Since GC assumes reference count is not zero,
we need to untrack the object from GC by calling :c:func:`PyObject_GC_UnTrack`
before clearing members. Here is reimplemented deallocator which uses
:c:func:`PyObject_GC_UnTrack` and :c:func:`Noddy_clear`.
::
static void
Noddy_dealloc(Noddy* self)
{
PyObject_GC_UnTrack(self);
Noddy_clear(self);
Py_TYPE(self)->tp_free((PyObject*)self);
}
Finally, we add the :const:`Py_TPFLAGS_HAVE_GC` flag to the class flags::
Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE | Py_TPFLAGS_HAVE_GC, /* tp_flags */