Objects/weakrefobject.c

#include "Python.h"
#include "structmember.h"


#define GET_WEAKREFS_LISTPTR(o) \
        ((PyWeakReference **) PyObject_GET_WEAKREFS_LISTPTR(o))


Py_ssize_t
_PyWeakref_GetWeakrefCount(PyWeakReference *head)
{
    Py_ssize_t count = 0;

    while (head != NULL) {
        ++count;
        head = head->wr_next;
    }
    return count;
}


static void
init_weakref(PyWeakReference *self, PyObject *ob, PyObject *callback)
{
    self->hash = -1;
    self->wr_object = ob;
    Py_XINCREF(callback);
    self->wr_callback = callback;
}

static PyWeakReference *
new_weakref(PyObject *ob, PyObject *callback)
{
    PyWeakReference *result;

    result = PyObject_GC_New(PyWeakReference, &_PyWeakref_RefType);
    if (result) {
        init_weakref(result, ob, callback);
        PyObject_GC_Track(result);
    }
    return result;
}


/* This function clears the passed-in reference and removes it from the
 * list of weak references for the referent.  This is the only code that
 * removes an item from the doubly-linked list of weak references for an
 * object; it is also responsible for clearing the callback slot.
 */
static void
clear_weakref(PyWeakReference *self)
{
    PyObject *callback = self->wr_callback;

    if (self->wr_object != Py_None) {
        PyWeakReference **list = GET_WEAKREFS_LISTPTR(self->wr_object);

        if (*list == self)
            /* If 'self' is the end of the list (and thus self->wr_next == NULL)
               then the weakref list itself (and thus the value of *list) will
               end up being set to NULL. */
            *list = self->wr_next;
        self->wr_object = Py_None;
        if (self->wr_prev != NULL)
            self->wr_prev->wr_next = self->wr_next;
        if (self->wr_next != NULL)
            self->wr_next->wr_prev = self->wr_prev;
        self->wr_prev = NULL;
        self->wr_next = NULL;
    }
    if (callback != NULL) {
        Py_DECREF(callback);
        self->wr_callback = NULL;
    }
}

/* Cyclic gc uses this to *just* clear the passed-in reference, leaving
 * the callback intact and uncalled.  It must be possible to call self's
 * tp_dealloc() after calling this, so self has to be left in a sane enough
 * state for that to work.  We expect tp_dealloc to decref the callback
 * then.  The reason for not letting clear_weakref() decref the callback
 * right now is that if the callback goes away, that may in turn trigger
 * another callback (if a weak reference to the callback exists) -- running
 * arbitrary Python code in the middle of gc is a disaster.  The convolution
 * here allows gc to delay triggering such callbacks until the world is in
 * a sane state again.
 */
void
_PyWeakref_ClearRef(PyWeakReference *self)
{
    PyObject *callback;

    assert(self != NULL);
    assert(PyWeakref_Check(self));
    /* Preserve and restore the callback around clear_weakref. */
    callback = self->wr_callback;
    self->wr_callback = NULL;
    clear_weakref(self);
    self->wr_callback = callback;
}

static void
weakref_dealloc(PyObject *self)
{
    PyObject_GC_UnTrack(self);
    clear_weakref((PyWeakReference *) self);
    Py_TYPE(self)->tp_free(self);
}


static int
gc_traverse(PyWeakReference *self, visitproc visit, void *arg)
{
    Py_VISIT(self->wr_callback);
    return 0;
}


static int
gc_clear(PyWeakReference *self)
{
    clear_weakref(self);
    return 0;
}


static PyObject *
weakref_call(PyWeakReference *self, PyObject *args, PyObject *kw)
{
    static char *kwlist[] = {NULL};

    if (PyArg_ParseTupleAndKeywords(args, kw, ":__call__", kwlist)) {
        PyObject *object = PyWeakref_GET_OBJECT(self);
        Py_INCREF(object);
        return (object);
    }
    return NULL;
}


static Py_hash_t
weakref_hash(PyWeakReference *self)
{
    if (self->hash != -1)
        return self->hash;
    if (PyWeakref_GET_OBJECT(self) == Py_None) {
        PyErr_SetString(PyExc_TypeError, "weak object has gone away");
        return -1;
    }
    self->hash = PyObject_Hash(PyWeakref_GET_OBJECT(self));
    return self->hash;
}


static PyObject *
weakref_repr(PyWeakReference *self)
{
    PyObject *name, *repr;
    _Py_IDENTIFIER(__name__);

    if (PyWeakref_GET_OBJECT(self) == Py_None)
        return PyUnicode_FromFormat("<weakref at %p; dead>", self);

    name = _PyObject_GetAttrId(PyWeakref_GET_OBJECT(self), &PyId___name__);
    if (name == NULL || !PyUnicode_Check(name)) {
        if (name == NULL)
            PyErr_Clear();
        repr = PyUnicode_FromFormat(
            "<weakref at %p; to '%s' at %p>",
            self,
            Py_TYPE(PyWeakref_GET_OBJECT(self))->tp_name,
            PyWeakref_GET_OBJECT(self));
    }
    else {
        repr = PyUnicode_FromFormat(
            "<weakref at %p; to '%s' at %p (%U)>",
            self,
            Py_TYPE(PyWeakref_GET_OBJECT(self))->tp_name,
            PyWeakref_GET_OBJECT(self),
            name);
    }
    Py_XDECREF(name);
    return repr;
}

/* Weak references only support equality, not ordering. Two weak references
   are equal if the underlying objects are equal. If the underlying object has
   gone away, they are equal if they are identical. */

static PyObject *
weakref_richcompare(PyWeakReference* self, PyWeakReference* other, int op)
{
    if ((op != Py_EQ && op != Py_NE) ||
        !PyWeakref_Check(self) ||
        !PyWeakref_Check(other)) {
        Py_RETURN_NOTIMPLEMENTED;
    }
    if (PyWeakref_GET_OBJECT(self) == Py_None
        || PyWeakref_GET_OBJECT(other) == Py_None) {
        int res = (self == other);
        if (op == Py_NE)
            res = !res;
        if (res)
            Py_RETURN_TRUE;
        else
            Py_RETURN_FALSE;
    }
    return PyObject_RichCompare(PyWeakref_GET_OBJECT(self),
                                PyWeakref_GET_OBJECT(other), op);
}

/* Given the head of an object's list of weak references, extract the
 * two callback-less refs (ref and proxy).  Used to determine if the
 * shared references exist and to determine the back link for newly
 * inserted references.
 */
static void
get_basic_refs(PyWeakReference *head,
               PyWeakReference **refp, PyWeakReference **proxyp)
{
    *refp = NULL;
    *proxyp = NULL;

    if (head != NULL && head->wr_callback == NULL) {
        /* We need to be careful that the "basic refs" aren't
           subclasses of the main types.  That complicates this a
           little. */
        if (PyWeakref_CheckRefExact(head)) {
            *refp = head;
            head = head->wr_next;
        }
        if (head != NULL
            && head->wr_callback == NULL
            && PyWeakref_CheckProxy(head)) {
            *proxyp = head;
            /* head = head->wr_next; */
        }
    }
}

/* Insert 'newref' in the list after 'prev'.  Both must be non-NULL. */
static void
insert_after(PyWeakReference *newref, PyWeakReference *prev)
{
    newref->wr_prev = prev;
    newref->wr_next = prev->wr_next;
    if (prev->wr_next != NULL)
        prev->wr_next->wr_prev = newref;
    prev->wr_next = newref;
}

/* Insert 'newref' at the head of the list; 'list' points to the variable
 * that stores the head.
 */
static void
insert_head(PyWeakReference *newref, PyWeakReference **list)
{
    PyWeakReference *next = *list;

    newref->wr_prev = NULL;
    newref->wr_next = next;
    if (next != NULL)
        next->wr_prev = newref;
    *list = newref;
}

static int
parse_weakref_init_args(char *funcname, PyObject *args, PyObject *kwargs,
                        PyObject **obp, PyObject **callbackp)
{
    /* XXX Should check that kwargs == NULL or is empty. */
    return PyArg_UnpackTuple(args, funcname, 1, 2, obp, callbackp);
}

static PyObject *
weakref___new__(PyTypeObject *type, PyObject *args, PyObject *kwargs)
{
    PyWeakReference *self = NULL;
    PyObject *ob, *callback = NULL;

    if (parse_weakref_init_args("__new__", args, kwargs, &ob, &callback)) {
        PyWeakReference *ref, *proxy;
        PyWeakReference **list;

        if (!PyType_SUPPORTS_WEAKREFS(Py_TYPE(ob))) {
            PyErr_Format(PyExc_TypeError,
                         "cannot create weak reference to '%s' object",
                         Py_TYPE(ob)->tp_name);
            return NULL;
        }
        if (callback == Py_None)
            callback = NULL;
        list = GET_WEAKREFS_LISTPTR(ob);
        get_basic_refs(*list, &ref, &proxy);
        if (callback == NULL && type == &_PyWeakref_RefType) {
            if (ref != NULL) {
                /* We can re-use an existing reference. */
                Py_INCREF(ref);
                return (PyObject *)ref;
            }
        }
        /* We have to create a new reference. */
        /* Note: the tp_alloc() can trigger cyclic GC, so the weakref
           list on ob can be mutated.  This means that the ref and
           proxy pointers we got back earlier may have been collected,
           so we need to compute these values again before we use
           them. */
        self = (PyWeakReference *) (type->tp_alloc(type, 0));
        if (self != NULL) {
            init_weakref(self, ob, callback);
            if (callback == NULL && type == &_PyWeakref_RefType) {
                insert_head(self, list);
            }
            else {
                PyWeakReference *prev;

                get_basic_refs(*list, &ref, &proxy);
                prev = (proxy == NULL) ? ref : proxy;
                if (prev == NULL)
                    insert_head(self, list);
                else
                    insert_after(self, prev);
            }
        }
    }
    return (PyObject *)self;
}

static int
weakref___init__(PyObject *self, PyObject *args, PyObject *kwargs)
{
    PyObject *tmp;

    if (parse_weakref_init_args("__init__", args, kwargs, &tmp, &tmp))
        return 0;
    else
        return -1;
}


PyTypeObject
_PyWeakref_RefType = {
    PyVarObject_HEAD_INIT(&PyType_Type, 0)
    "weakref",
    sizeof(PyWeakReference),
    0,
    weakref_dealloc,            /*tp_dealloc*/
    0,                          /*tp_print*/
    0,                          /*tp_getattr*/
    0,                          /*tp_setattr*/
    0,                          /*tp_reserved*/
    (reprfunc)weakref_repr,     /*tp_repr*/
    0,                          /*tp_as_number*/
    0,                          /*tp_as_sequence*/
    0,                          /*tp_as_mapping*/
    (hashfunc)weakref_hash,     /*tp_hash*/
    (ternaryfunc)weakref_call,  /*tp_call*/
    0,                          /*tp_str*/
    0,                          /*tp_getattro*/
    0,                          /*tp_setattro*/
    0,                          /*tp_as_buffer*/
    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC
        | Py_TPFLAGS_BASETYPE,  /*tp_flags*/
    0,                          /*tp_doc*/
    (traverseproc)gc_traverse,  /*tp_traverse*/
    (inquiry)gc_clear,          /*tp_clear*/
    (richcmpfunc)weakref_richcompare,   /*tp_richcompare*/
    0,                          /*tp_weaklistoffset*/
    0,                          /*tp_iter*/
    0,                          /*tp_iternext*/
    0,                          /*tp_methods*/
    0,                          /*tp_members*/
    0,                          /*tp_getset*/
    0,                          /*tp_base*/
    0,                          /*tp_dict*/
    0,                          /*tp_descr_get*/
    0,                          /*tp_descr_set*/
    0,                          /*tp_dictoffset*/
    weakref___init__,           /*tp_init*/
    PyType_GenericAlloc,        /*tp_alloc*/
    weakref___new__,            /*tp_new*/
    PyObject_GC_Del,            /*tp_free*/
};


static int
proxy_checkref(PyWeakReference *proxy)
{
    if (PyWeakref_GET_OBJECT(proxy) == Py_None) {
        PyErr_SetString(PyExc_ReferenceError,
                        "weakly-referenced object no longer exists");
        return 0;
    }
    return 1;
}


/* If a parameter is a proxy, check that it is still "live" and wrap it,
 * replacing the original value with the raw object.  Raises ReferenceError
 * if the param is a dead proxy.
 */
#define UNWRAP(o) \
        if (PyWeakref_CheckProxy(o)) { \
            if (!proxy_checkref((PyWeakReference *)o)) \
                return NULL; \
            o = PyWeakref_GET_OBJECT(o); \
        }

#define UNWRAP_I(o) \
        if (PyWeakref_CheckProxy(o)) { \
            if (!proxy_checkref((PyWeakReference *)o)) \
                return -1; \
            o = PyWeakref_GET_OBJECT(o); \
        }

#define WRAP_UNARY(method, generic) \
    static PyObject * \
    method(PyObject *proxy) { \
        UNWRAP(proxy); \
        return generic(proxy); \
    }

#define WRAP_BINARY(method, generic) \
    static PyObject * \
    method(PyObject *x, PyObject *y) { \
        UNWRAP(x); \
        UNWRAP(y); \
        return generic(x, y); \
    }

/* Note that the third arg needs to be checked for NULL since the tp_call
 * slot can receive NULL for this arg.
 */
#define WRAP_TERNARY(method, generic) \
    static PyObject * \
    method(PyObject *proxy, PyObject *v, PyObject *w) { \
        UNWRAP(proxy); \
        UNWRAP(v); \
        if (w != NULL) \
            UNWRAP(w); \
        return generic(proxy, v, w); \
    }

#define WRAP_METHOD(method, special) \
    static PyObject * \
    method(PyObject *proxy) { \
            _Py_IDENTIFIER(special); \
            UNWRAP(proxy); \
                return _PyObject_CallMethodId(proxy, &PyId_##special, ""); \
        }


/* direct slots */

WRAP_BINARY(proxy_getattr, PyObject_GetAttr)
WRAP_UNARY(proxy_str, PyObject_Str)
WRAP_TERNARY(proxy_call, PyEval_CallObjectWithKeywords)

static PyObject *
proxy_repr(PyWeakReference *proxy)
{
    return PyUnicode_FromFormat(
        "<weakproxy at %p to %s at %p>",
        proxy,
        Py_TYPE(PyWeakref_GET_OBJECT(proxy))->tp_name,
        PyWeakref_GET_OBJECT(proxy));
}


static int
proxy_setattr(PyWeakReference *proxy, PyObject *name, PyObject *value)
{
    if (!proxy_checkref(proxy))
        return -1;
    return PyObject_SetAttr(PyWeakref_GET_OBJECT(proxy), name, value);
}

static PyObject *
proxy_richcompare(PyObject *proxy, PyObject *v, int op)
{
    UNWRAP(proxy);
    UNWRAP(v);
    return PyObject_RichCompare(proxy, v, op);
}

/* number slots */
WRAP_BINARY(proxy_add, PyNumber_Add)
WRAP_BINARY(proxy_sub, PyNumber_Subtract)
WRAP_BINARY(proxy_mul, PyNumber_Multiply)
WRAP_BINARY(proxy_floor_div, PyNumber_FloorDivide)
WRAP_BINARY(proxy_true_div, PyNumber_TrueDivide)
WRAP_BINARY(proxy_mod, PyNumber_Remainder)
WRAP_BINARY(proxy_divmod, PyNumber_Divmod)
WRAP_TERNARY(proxy_pow, PyNumber_Power)
WRAP_UNARY(proxy_neg, PyNumber_Negative)
WRAP_UNARY(proxy_pos, PyNumber_Positive)
WRAP_UNARY(proxy_abs, PyNumber_Absolute)
WRAP_UNARY(proxy_invert, PyNumber_Invert)
WRAP_BINARY(proxy_lshift, PyNumber_Lshift)
WRAP_BINARY(proxy_rshift, PyNumber_Rshift)
WRAP_BINARY(proxy_and, PyNumber_And)
WRAP_BINARY(proxy_xor, PyNumber_Xor)
WRAP_BINARY(proxy_or, PyNumber_Or)
WRAP_UNARY(proxy_int, PyNumber_Long)
WRAP_UNARY(proxy_float, PyNumber_Float)
WRAP_BINARY(proxy_iadd, PyNumber_InPlaceAdd)
WRAP_BINARY(proxy_isub, PyNumber_InPlaceSubtract)
WRAP_BINARY(proxy_imul, PyNumber_InPlaceMultiply)
WRAP_BINARY(proxy_ifloor_div, PyNumber_InPlaceFloorDivide)
WRAP_BINARY(proxy_itrue_div, PyNumber_InPlaceTrueDivide)
WRAP_BINARY(proxy_imod, PyNumber_InPlaceRemainder)
WRAP_TERNARY(proxy_ipow, PyNumber_InPlacePower)
WRAP_BINARY(proxy_ilshift, PyNumber_InPlaceLshift)
WRAP_BINARY(proxy_irshift, PyNumber_InPlaceRshift)
WRAP_BINARY(proxy_iand, PyNumber_InPlaceAnd)
WRAP_BINARY(proxy_ixor, PyNumber_InPlaceXor)
WRAP_BINARY(proxy_ior, PyNumber_InPlaceOr)
WRAP_UNARY(proxy_index, PyNumber_Index)

static int
proxy_bool(PyWeakReference *proxy)
{
    PyObject *o = PyWeakref_GET_OBJECT(proxy);
    if (!proxy_checkref(proxy))
        return -1;
    return PyObject_IsTrue(o);
}

static void
proxy_dealloc(PyWeakReference *self)
{
    if (self->wr_callback != NULL)
        PyObject_GC_UnTrack((PyObject *)self);
    clear_weakref(self);
    PyObject_GC_Del(self);
}

/* sequence slots */

static int
proxy_contains(PyWeakReference *proxy, PyObject *value)
{
    if (!proxy_checkref(proxy))
        return -1;
    return PySequence_Contains(PyWeakref_GET_OBJECT(proxy), value);
}


/* mapping slots */

static Py_ssize_t
proxy_length(PyWeakReference *proxy)
{
    if (!proxy_checkref(proxy))
        return -1;
    return PyObject_Length(PyWeakref_GET_OBJECT(proxy));
}

WRAP_BINARY(proxy_getitem, PyObject_GetItem)

static int
proxy_setitem(PyWeakReference *proxy, PyObject *key, PyObject *value)
{
    if (!proxy_checkref(proxy))
        return -1;

    if (value == NULL)
        return PyObject_DelItem(PyWeakref_GET_OBJECT(proxy), key);
    else
        return PyObject_SetItem(PyWeakref_GET_OBJECT(proxy), key, value);
}

/* iterator slots */

static PyObject *
proxy_iter(PyWeakReference *proxy)
{
    if (!proxy_checkref(proxy))
        return NULL;
    return PyObject_GetIter(PyWeakref_GET_OBJECT(proxy));
}

static PyObject *
proxy_iternext(PyWeakReference *proxy)
{
    if (!proxy_checkref(proxy))
        return NULL;
    return PyIter_Next(PyWeakref_GET_OBJECT(proxy));
}


WRAP_METHOD(proxy_bytes, __bytes__)


static PyMethodDef proxy_methods[] = {
        {"__bytes__", (PyCFunction)proxy_bytes, METH_NOARGS},
        {NULL, NULL}
};


static PyNumberMethods proxy_as_number = {
    proxy_add,              /*nb_add*/
    proxy_sub,              /*nb_subtract*/
    proxy_mul,              /*nb_multiply*/
    proxy_mod,              /*nb_remainder*/
    proxy_divmod,           /*nb_divmod*/
    proxy_pow,              /*nb_power*/
    proxy_neg,              /*nb_negative*/
    proxy_pos,              /*nb_positive*/
    proxy_abs,              /*nb_absolute*/
    (inquiry)proxy_bool,    /*nb_bool*/
    proxy_invert,           /*nb_invert*/
    proxy_lshift,           /*nb_lshift*/
    proxy_rshift,           /*nb_rshift*/
    proxy_and,              /*nb_and*/
    proxy_xor,              /*nb_xor*/
    proxy_or,               /*nb_or*/
    proxy_int,              /*nb_int*/
    0,                      /*nb_reserved*/
    proxy_float,            /*nb_float*/
    proxy_iadd,             /*nb_inplace_add*/
    proxy_isub,             /*nb_inplace_subtract*/
    proxy_imul,             /*nb_inplace_multiply*/
    proxy_imod,             /*nb_inplace_remainder*/
    proxy_ipow,             /*nb_inplace_power*/
    proxy_ilshift,          /*nb_inplace_lshift*/
    proxy_irshift,          /*nb_inplace_rshift*/
    proxy_iand,             /*nb_inplace_and*/
    proxy_ixor,             /*nb_inplace_xor*/
    proxy_ior,              /*nb_inplace_or*/
    proxy_floor_div,        /*nb_floor_divide*/
    proxy_true_div,         /*nb_true_divide*/
    proxy_ifloor_div,       /*nb_inplace_floor_divide*/
    proxy_itrue_div,        /*nb_inplace_true_divide*/
    proxy_index,            /*nb_index*/
};

static PySequenceMethods proxy_as_sequence = {
    (lenfunc)proxy_length,      /*sq_length*/
    0,                          /*sq_concat*/
    0,                          /*sq_repeat*/
    0,                          /*sq_item*/
    0,                          /*sq_slice*/
    0,                          /*sq_ass_item*/
    0,                           /*sq_ass_slice*/
    (objobjproc)proxy_contains, /* sq_contains */
};

static PyMappingMethods proxy_as_mapping = {
    (lenfunc)proxy_length,        /*mp_length*/
    proxy_getitem,                /*mp_subscript*/
    (objobjargproc)proxy_setitem, /*mp_ass_subscript*/
};


PyTypeObject
_PyWeakref_ProxyType = {
    PyVarObject_HEAD_INIT(&PyType_Type, 0)
    "weakproxy",
    sizeof(PyWeakReference),
    0,
    /* methods */
    (destructor)proxy_dealloc,          /* tp_dealloc */
    0,                                  /* tp_print */
    0,                                  /* tp_getattr */
    0,                                  /* tp_setattr */
    0,                                  /* tp_reserved */
    (reprfunc)proxy_repr,               /* tp_repr */
    &proxy_as_number,                   /* tp_as_number */
    &proxy_as_sequence,                 /* tp_as_sequence */
    &proxy_as_mapping,                  /* tp_as_mapping */
    0,                                  /* tp_hash */
    0,                                  /* tp_call */
    proxy_str,                          /* tp_str */
    proxy_getattr,                      /* tp_getattro */
    (setattrofunc)proxy_setattr,        /* tp_setattro */
    0,                                  /* tp_as_buffer */
    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC, /* tp_flags */
    0,                                  /* tp_doc */
    (traverseproc)gc_traverse,          /* tp_traverse */
    (inquiry)gc_clear,                  /* tp_clear */
    proxy_richcompare,                  /* tp_richcompare */
    0,                                  /* tp_weaklistoffset */
    (getiterfunc)proxy_iter,            /* tp_iter */
    (iternextfunc)proxy_iternext,       /* tp_iternext */
        proxy_methods,                      /* tp_methods */
};


PyTypeObject
_PyWeakref_CallableProxyType = {
    PyVarObject_HEAD_INIT(&PyType_Type, 0)
    "weakcallableproxy",
    sizeof(PyWeakReference),
    0,
    /* methods */
    (destructor)proxy_dealloc,          /* tp_dealloc */
    0,                                  /* tp_print */
    0,                                  /* tp_getattr */
    0,                                  /* tp_setattr */
    0,                                  /* tp_reserved */
    (unaryfunc)proxy_repr,              /* tp_repr */
    &proxy_as_number,                   /* tp_as_number */
    &proxy_as_sequence,                 /* tp_as_sequence */
    &proxy_as_mapping,                  /* tp_as_mapping */
    0,                                  /* tp_hash */
    proxy_call,                         /* tp_call */
    proxy_str,                          /* tp_str */
    proxy_getattr,                      /* tp_getattro */
    (setattrofunc)proxy_setattr,        /* tp_setattro */
    0,                                  /* tp_as_buffer */
    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC, /* tp_flags */
    0,                                  /* tp_doc */
    (traverseproc)gc_traverse,          /* tp_traverse */
    (inquiry)gc_clear,                  /* tp_clear */
    proxy_richcompare,                  /* tp_richcompare */
    0,                                  /* tp_weaklistoffset */
    (getiterfunc)proxy_iter,            /* tp_iter */
    (iternextfunc)proxy_iternext,       /* tp_iternext */
};



PyObject *
PyWeakref_NewRef(PyObject *ob, PyObject *callback)
{
    PyWeakReference *result = NULL;
    PyWeakReference **list;
    PyWeakReference *ref, *proxy;

    if (!PyType_SUPPORTS_WEAKREFS(Py_TYPE(ob))) {
        PyErr_Format(PyExc_TypeError,
                     "cannot create weak reference to '%s' object",
                     Py_TYPE(ob)->tp_name);
        return NULL;
    }
    list = GET_WEAKREFS_LISTPTR(ob);
    get_basic_refs(*list, &ref, &proxy);
    if (callback == Py_None)
        callback = NULL;
    if (callback == NULL)
        /* return existing weak reference if it exists */
        result = ref;
    if (result != NULL)
        Py_INCREF(result);
    else {
        /* Note: new_weakref() can trigger cyclic GC, so the weakref
           list on ob can be mutated.  This means that the ref and
           proxy pointers we got back earlier may have been collected,
           so we need to compute these values again before we use
           them. */
        result = new_weakref(ob, callback);
        if (result != NULL) {
            get_basic_refs(*list, &ref, &proxy);
            if (callback == NULL) {
                if (ref == NULL)
                    insert_head(result, list);
                else {
                    /* Someone else added a ref without a callback
                       during GC.  Return that one instead of this one
                       to avoid violating the invariants of the list
                       of weakrefs for ob. */
                    Py_DECREF(result);
                    Py_INCREF(ref);
                    result = ref;
                }
            }
            else {
                PyWeakReference *prev;

                prev = (proxy == NULL) ? ref : proxy;
                if (prev == NULL)
                    insert_head(result, list);
                else
                    insert_after(result, prev);
            }
        }
    }
    return (PyObject *) result;
}


PyObject *
PyWeakref_NewProxy(PyObject *ob, PyObject *callback)
{
    PyWeakReference *result = NULL;
    PyWeakReference **list;
    PyWeakReference *ref, *proxy;

    if (!PyType_SUPPORTS_WEAKREFS(Py_TYPE(ob))) {
        PyErr_Format(PyExc_TypeError,
                     "cannot create weak reference to '%s' object",
                     Py_TYPE(ob)->tp_name);
        return NULL;
    }
    list = GET_WEAKREFS_LISTPTR(ob);
    get_basic_refs(*list, &ref, &proxy);
    if (callback == Py_None)
        callback = NULL;
    if (callback == NULL)
        /* attempt to return an existing weak reference if it exists */
        result = proxy;
    if (result != NULL)
        Py_INCREF(result);
    else {
        /* Note: new_weakref() can trigger cyclic GC, so the weakref
           list on ob can be mutated.  This means that the ref and
           proxy pointers we got back earlier may have been collected,
           so we need to compute these values again before we use
           them. */
        result = new_weakref(ob, callback);
        if (result != NULL) {
            PyWeakReference *prev;

            if (PyCallable_Check(ob))
                Py_TYPE(result) = &_PyWeakref_CallableProxyType;
            else
                Py_TYPE(result) = &_PyWeakref_ProxyType;
            get_basic_refs(*list, &ref, &proxy);
            if (callback == NULL) {
                if (proxy != NULL) {
                    /* Someone else added a proxy without a callback
                       during GC.  Return that one instead of this one
                       to avoid violating the invariants of the list
                       of weakrefs for ob. */
                    Py_DECREF(result);
                    Py_INCREF(result = proxy);
                    goto skip_insert;
                }
                prev = ref;
            }
            else
                prev = (proxy == NULL) ? ref : proxy;

            if (prev == NULL)
                insert_head(result, list);
            else
                insert_after(result, prev);
        skip_insert:
            ;
        }
    }
    return (PyObject *) result;
}


PyObject *
PyWeakref_GetObject(PyObject *ref)
{
    if (ref == NULL || !PyWeakref_Check(ref)) {
        PyErr_BadInternalCall();
        return NULL;
    }
    return PyWeakref_GET_OBJECT(ref);
}

/* Note that there's an inlined copy-paste of handle_callback() in gcmodule.c's
 * handle_weakrefs().
 */
static void
handle_callback(PyWeakReference *ref, PyObject *callback)
{
    PyObject *cbresult = PyObject_CallFunctionObjArgs(callback, ref, NULL);

    if (cbresult == NULL)
        PyErr_WriteUnraisable(callback);
    else
        Py_DECREF(cbresult);
}

/* This function is called by the tp_dealloc handler to clear weak references.
 *
 * This iterates through the weak references for 'object' and calls callbacks
 * for those references which have one.  It returns when all callbacks have
 * been attempted.
 */
void
PyObject_ClearWeakRefs(PyObject *object)
{
    PyWeakReference **list;

    if (object == NULL
        || !PyType_SUPPORTS_WEAKREFS(Py_TYPE(object))
        || object->ob_refcnt != 0) {
        PyErr_BadInternalCall();
        return;
    }
    list = GET_WEAKREFS_LISTPTR(object);
    /* Remove the callback-less basic and proxy references */
    if (*list != NULL && (*list)->wr_callback == NULL) {
        clear_weakref(*list);
        if (*list != NULL && (*list)->wr_callback == NULL)
            clear_weakref(*list);
    }
    if (*list != NULL) {
        PyWeakReference *current = *list;
        Py_ssize_t count = _PyWeakref_GetWeakrefCount(current);
        int restore_error = PyErr_Occurred() ? 1 : 0;
        PyObject *err_type, *err_value, *err_tb;

        if (restore_error)
            PyErr_Fetch(&err_type, &err_value, &err_tb);
        if (count == 1) {
            PyObject *callback = current->wr_callback;

            current->wr_callback = NULL;
            clear_weakref(current);
            if (callback != NULL) {
                if (((PyObject *)current)->ob_refcnt > 0)
                    handle_callback(current, callback);
                Py_DECREF(callback);
            }
        }
        else {
            PyObject *tuple;
            Py_ssize_t i = 0;

            tuple = PyTuple_New(count * 2);
            if (tuple == NULL) {
                if (restore_error)
                    PyErr_Fetch(&err_type, &err_value, &err_tb);
                return;
            }

            for (i = 0; i < count; ++i) {
                PyWeakReference *next = current->wr_next;

                if (((PyObject *)current)->ob_refcnt > 0)
                {
                    Py_INCREF(current);
                    PyTuple_SET_ITEM(tuple, i * 2, (PyObject *) current);
                    PyTuple_SET_ITEM(tuple, i * 2 + 1, current->wr_callback);
                }
                else {
                    Py_DECREF(current->wr_callback);
                }
                current->wr_callback = NULL;
                clear_weakref(current);
                current = next;
            }
            for (i = 0; i < count; ++i) {
                PyObject *callback = PyTuple_GET_ITEM(tuple, i * 2 + 1);

                /* The tuple may have slots left to NULL */
                if (callback != NULL) {
                    PyObject *item = PyTuple_GET_ITEM(tuple, i * 2);
                    handle_callback((PyWeakReference *)item, callback);
                }
            }
            Py_DECREF(tuple);
        }
        if (restore_error)
            PyErr_Restore(err_type, err_value, err_tb);
    }
}