bpo-31095: Fix potential crash during GC (GH-3197)

(cherry picked from commit a6296d3)

Author: methane

Doc/extending/newtypes.rst

@@ -748,8 +748,9 @@ 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)
@@ -767,13 +768,6 @@ to use it::
        return 0;
    }
 
-   static void
-   Noddy_dealloc(Noddy* self)
-   {
-       Noddy_clear(self);
-       self->ob_type->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
@@ -796,6 +790,23 @@ decrementing of reference counts.  With :c:func:`Py_CLEAR`, the
        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 */

Doc/includes/noddy4.c

@@ -46,6 +46,7 @@ Noddy_clear(Noddy *self)
 static void
 Noddy_dealloc(Noddy* self)
 {
+    PyObject_GC_UnTrack(self);
     Noddy_clear(self);
     Py_TYPE(self)->tp_free((PyObject*)self);
 }

Misc/NEWS.d/next/Core and Builtins/2017-08-01-18-48-30.bpo-31095.bXWZDb.rst

@@ -0,0 +1,2 @@
+Fix potential crash during GC caused by ``tp_dealloc`` which doesn't call
+``PyObject_GC_UnTrack()``.

Modules/_collectionsmodule.c

@@ -1271,6 +1271,8 @@ dequeiter_traverse(dequeiterobject *dio, visitproc visit, void *arg)
 static void
 dequeiter_dealloc(dequeiterobject *dio)
 {
+    /* bpo-31095: UnTrack is needed before calling any callbacks */
+    PyObject_GC_UnTrack(dio);
     Py_XDECREF(dio->deque);
     PyObject_GC_Del(dio);
 }
@@ -1556,6 +1558,8 @@ static PyMemberDef defdict_members[] = {
 static void
 defdict_dealloc(defdictobject *dd)
 {
+    /* bpo-31095: UnTrack is needed before calling any callbacks */
+    PyObject_GC_UnTrack(dd);
     Py_CLEAR(dd->default_factory);
     PyDict_Type.tp_dealloc((PyObject *)dd);
 }

Modules/_functoolsmodule.c

@@ -143,6 +143,7 @@ partial_new(PyTypeObject *type, PyObject *args, PyObject *kw)
 static void
 partial_dealloc(partialobject *pto)
 {
+    /* bpo-31095: UnTrack is needed before calling any callbacks */
     PyObject_GC_UnTrack(pto);
     if (pto->weakreflist != NULL)
         PyObject_ClearWeakRefs((PyObject *) pto);

Modules/_io/bytesio.c

@@ -745,6 +745,7 @@ bytesio_setstate(bytesio *self, PyObject *state)
 static void
 bytesio_dealloc(bytesio *self)
 {
+    /* bpo-31095: UnTrack is needed before calling any callbacks */
     _PyObject_GC_UNTRACK(self);
     if (self->buf != NULL) {
         PyMem_Free(self->buf);

Modules/_json.c

@@ -840,7 +840,8 @@ py_encode_basestring_ascii(PyObject* self UNUSED, PyObject *pystr)
 static void
 scanner_dealloc(PyObject *self)
 {
-    /* Deallocate scanner object */
+    /* bpo-31095: UnTrack is needed before calling any callbacks */
+    PyObject_GC_UnTrack(self);
     scanner_clear(self);
     Py_TYPE(self)->tp_free(self);
 }
@@ -2298,7 +2299,8 @@ encoder_listencode_list(PyEncoderObject *s, PyObject *rval, PyObject *seq, Py_ss
 static void
 encoder_dealloc(PyObject *self)
 {
-    /* Deallocate Encoder */
+    /* bpo-31095: UnTrack is needed before calling any callbacks */
+    PyObject_GC_UnTrack(self);
     encoder_clear(self);
     Py_TYPE(self)->tp_free(self);
 }

Modules/_ssl.c

@@ -2214,6 +2214,8 @@ context_clear(PySSLContext *self)
 static void
 context_dealloc(PySSLContext *self)
 {
+    /* bpo-31095: UnTrack is needed before calling any callbacks */
+    PyObject_GC_UnTrack(self);
     context_clear(self);
     SSL_CTX_free(self->ctx);
 #ifdef OPENSSL_NPN_NEGOTIATED

Objects/dictobject.c

@@ -1076,6 +1076,7 @@ dict_dealloc(register PyDictObject *mp)
 {
     register PyDictEntry *ep;
     Py_ssize_t fill = mp->ma_fill;
+    /* bpo-31095: UnTrack is needed before calling any callbacks */
     PyObject_GC_UnTrack(mp);
     Py_TRASHCAN_SAFE_BEGIN(mp)
     for (ep = mp->ma_table; fill > 0; ep++) {
@@ -2576,6 +2577,8 @@ dictiter_new(PyDictObject *dict, PyTypeObject *itertype)
 static void
 dictiter_dealloc(dictiterobject *di)
 {
+    /* bpo-31095: UnTrack is needed before calling any callbacks */
+    _PyObject_GC_UNTRACK(di);
     Py_XDECREF(di->di_dict);
     Py_XDECREF(di->di_result);
     PyObject_GC_Del(di);
@@ -2855,6 +2858,8 @@ typedef struct {
 static void
 dictview_dealloc(dictviewobject *dv)
 {
+    /* bpo-31095: UnTrack is needed before calling any callbacks */
+    _PyObject_GC_UNTRACK(dv);
     Py_XDECREF(dv->dv_dict);
     PyObject_GC_Del(dv);
 }

Objects/setobject.c

@@ -549,6 +549,7 @@ set_dealloc(PySetObject *so)
 {
     register setentry *entry;
     Py_ssize_t fill = so->fill;
+    /* bpo-31095: UnTrack is needed before calling any callbacks */
     PyObject_GC_UnTrack(so);
     Py_TRASHCAN_SAFE_BEGIN(so)
     if (so->weakreflist != NULL)
@@ -811,6 +812,8 @@ typedef struct {
 static void
 setiter_dealloc(setiterobject *si)
 {
+    /* bpo-31095: UnTrack is needed before calling any callbacks */
+    _PyObject_GC_UNTRACK(si);
     Py_XDECREF(si->si_set);
     PyObject_GC_Del(si);
 }