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bpo-40217: Ensure Py_VISIT(Py_TYPE(self)) is always called for PyType_FromSpec types #19414

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Merged
merged 1 commit into from
Apr 27, 2020
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bpo-40217: Ensure Py_VISIT(Py_TYPE(self)) is always called for PyType_FromSpec types #19414

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84 changes: 83 additions & 1 deletion Objects/typeobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1021,6 +1021,38 @@ type_call(PyTypeObject *type, PyObject *args, PyObject *kwds)
return obj;
}

PyObject *
PyType_FromSpec_Alloc(PyTypeObject *type, Py_ssize_t nitems)
{
PyObject *obj;
const size_t size = _Py_SIZE_ROUND_UP(
_PyObject_VAR_SIZE(type, nitems+1) + sizeof(traverseproc),
SIZEOF_VOID_P);
/* note that we need to add one, for the sentinel and space for the
provided tp-traverse: See bpo-40217 for more details */

if (PyType_IS_GC(type))
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PEP 7: please add { ... }. Same remark for other if.

obj = _PyObject_GC_Malloc(size);
else
obj = (PyObject *)PyObject_MALLOC(size);

if (obj == NULL)
return PyErr_NoMemory();

obj = obj;
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what is the purpose of that?


memset(obj, '\0', size);

if (type->tp_itemsize == 0)
(void)PyObject_INIT(obj, type);
else
(void) PyObject_INIT_VAR((PyVarObject *)obj, type, nitems);

if (PyType_IS_GC(type))
_PyObject_GC_TRACK(obj);
return obj;
}

PyObject *
PyType_GenericAlloc(PyTypeObject *type, Py_ssize_t nitems)
{
Expand Down Expand Up @@ -2846,6 +2878,36 @@ static const short slotoffsets[] = {
#include "typeslots.inc"
};

static int
PyType_FromSpec_tp_traverse(PyObject *self, visitproc visit, void *arg)
{
PyTypeObject *parent = Py_TYPE(self);

// Only a instance of a type that is directly created by
// PyType_FromSpec (not subclasses) must visit its parent.
if (parent->tp_traverse == PyType_FromSpec_tp_traverse) {
Py_VISIT(parent);
}

// Search for the original type that was created using PyType_FromSpec
PyTypeObject *base;
base = parent;
while (base->tp_traverse != PyType_FromSpec_tp_traverse) {
base = base->tp_base;
assert(base);
}

// Extract the user defined traverse function that we placed at the end
// of the type and call it.
size_t size = Py_SIZE(base);
size_t _offset = _PyObject_VAR_SIZE(&PyType_Type, size+1);
traverseproc fun = *(traverseproc*)((char*)base + _offset);
if (fun == NULL) {
return 0;
}
return fun(self, visit, arg);
}

PyObject *
PyType_FromSpecWithBases(PyType_Spec *spec, PyObject *bases)
{
Expand Down Expand Up @@ -2880,7 +2942,7 @@ PyType_FromSpecWithBases(PyType_Spec *spec, PyObject *bases)
}
}

res = (PyHeapTypeObject*)PyType_GenericAlloc(&PyType_Type, nmembers);
res = (PyHeapTypeObject*)PyType_FromSpec_Alloc(&PyType_Type, nmembers);
if (res == NULL)
return NULL;
res_start = (char*)res;
Expand Down Expand Up @@ -2985,6 +3047,26 @@ PyType_FromSpecWithBases(PyType_Spec *spec, PyObject *bases)
memcpy(PyHeapType_GET_MEMBERS(res), slot->pfunc, len);
type->tp_members = PyHeapType_GET_MEMBERS(res);
}
else if (slot->slot == Py_tp_traverse) {

/* Types created by PyType_FromSpec own a strong reference to their
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Please put bpo-40217 somewhere in the comment.

* type, but this was added in Python 3.8. The tp_traverse function
* needs to call Py_VISIT on the type but all existing traverse
* functions cannot be updated (especially the ones from existing user
* functions) so we need to provide a tp_traverse that manually calls
* Py_VISIT(Py_TYPE(self)) and then call the provided tp_traverse. In
* this way, user functions do not need to be updated, preserve
* backwards compatibility.
*
* We store the user-provided traverse function at the end of the type
* (we have allocated space for it) so we can call it from our
* PyType_FromSpec_tp_traverse wrapper. */

type->tp_traverse = PyType_FromSpec_tp_traverse;
size_t _offset = _PyObject_VAR_SIZE(&PyType_Type, nmembers+1);
traverseproc *user_traverse = (traverseproc*)((char*)type + _offset);
*user_traverse = slot->pfunc;
}
else {
/* Copy other slots directly */
*(void**)(res_start + slotoffsets[slot->slot]) = slot->pfunc;
Expand Down