bpo-31095: fix potential crash during GC (GH-2974) (#3196)

(cherry picked from commit a6296d34a478b4f697ea9db798146195075d496c)

14 files changed, 60 insertions, 14 deletions

diff --git a/Doc/extending/newtypes.rst b/Doc/extending/newtypes.rst
index 003b4e5..abd5da9 100644
--- a/Doc/extending/newtypes.rst
+++ b/Doc/extending/newtypes.rst
@@ -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 */
diff --git a/Doc/includes/noddy4.c b/Doc/includes/noddy4.c
index eb9622a..08ba4c3 100644
--- a/Doc/includes/noddy4.c
+++ b/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);
 }
diff --git a/Misc/NEWS.d/next/Core and Builtins/2017-08-01-18-48-30.bpo-31095.bXWZDb.rst b/Misc/NEWS.d/next/Core and Builtins/2017-08-01-18-48-30.bpo-31095.bXWZDb.rst
new file mode 100644
index 0000000..ca1f8ba
--- /dev/null
+++ b/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()``.
diff --git a/Modules/_collectionsmodule.c b/Modules/_collectionsmodule.c
index 08fceca..25d2d4c 100644
--- a/Modules/_collectionsmodule.c
+++ b/Modules/_collectionsmodule.c
@@ -1641,6 +1641,8 @@ 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);
 }
@@ -2021,6 +2023,8 @@ 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);
 }
diff --git a/Modules/_elementtree.c b/Modules/_elementtree.c
index fedef10..5dba9f7 100644
--- a/Modules/_elementtree.c
+++ b/Modules/_elementtree.c
@@ -653,6 +653,7 @@ element_gc_clear(ElementObject *self)
 static void
 element_dealloc(ElementObject* self)
 {
+    /* bpo-31095: UnTrack is needed before calling any callbacks */
     PyObject_GC_UnTrack(self);
     Py_TRASHCAN_SAFE_BEGIN(self)
 
@@ -2025,6 +2026,9 @@ static void
 elementiter_dealloc(ElementIterObject *it)
 {
     ParentLocator *p = it->parent_stack;
+    /* bpo-31095: UnTrack is needed before calling any callbacks */
+    PyObject_GC_UnTrack(it);
+
     while (p) {
         ParentLocator *temp = p;
         Py_XDECREF(p->parent);
@@ -2034,8 +2038,6 @@ elementiter_dealloc(ElementIterObject *it)
 
     Py_XDECREF(it->sought_tag);
     Py_XDECREF(it->root_element);
-
-    PyObject_GC_UnTrack(it);
     PyObject_GC_Del(it);
 }
 
diff --git a/Modules/_functoolsmodule.c b/Modules/_functoolsmodule.c
index 985bcb6..d27e1c4 100644
--- a/Modules/_functoolsmodule.c
+++ b/Modules/_functoolsmodule.c
@@ -116,6 +116,7 @@ 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);
@@ -1039,7 +1040,11 @@ lru_cache_clear_list(lru_list_elem *link)
 static void
 lru_cache_dealloc(lru_cache_object *obj)
 {
-    lru_list_elem *list = lru_cache_unlink_list(obj);
+    lru_list_elem *list;
+    /* bpo-31095: UnTrack is needed before calling any callbacks */
+    PyObject_GC_UnTrack(obj);
+
+    list = lru_cache_unlink_list(obj);
     Py_XDECREF(obj->maxsize_O);
     Py_XDECREF(obj->func);
     Py_XDECREF(obj->cache);
diff --git a/Modules/_io/bytesio.c b/Modules/_io/bytesio.c
index 9e5d78b..d1c9ccb 100644
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -1133,6 +1133,8 @@ bytesiobuf_traverse(bytesiobuf *self, visitproc visit, void *arg)
 static void
 bytesiobuf_dealloc(bytesiobuf *self)
 {
+    /* bpo-31095: UnTrack is needed before calling any callbacks */
+    PyObject_GC_UnTrack(self);
     Py_CLEAR(self->source);
     Py_TYPE(self)->tp_free(self);
 }
diff --git a/Modules/_json.c b/Modules/_json.c
index 346bd55..135e2e7 100644
--- a/Modules/_json.c
+++ b/Modules/_json.c
@@ -655,7 +655,8 @@ py_encode_basestring(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);
 }
@@ -1793,7 +1794,8 @@ bail:
 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);
 }
diff --git a/Modules/_ssl.c b/Modules/_ssl.c
index 3b2d3ad..8648267 100644
--- a/Modules/_ssl.c
+++ b/Modules/_ssl.c
@@ -2465,6 +2465,8 @@ 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
diff --git a/Modules/_struct.c b/Modules/_struct.c
index 2273129..5817b5e 100644
--- a/Modules/_struct.c
+++ b/Modules/_struct.c
@@ -1581,6 +1581,8 @@ typedef struct {
 static void
 unpackiter_dealloc(unpackiterobject *self)
 {
+    /* bpo-31095: UnTrack is needed before calling any callbacks */
+    PyObject_GC_UnTrack(self);
     Py_XDECREF(self->so);
     PyBuffer_Release(&self->buf);
     PyObject_GC_Del(self);
diff --git a/Objects/dictobject.c b/Objects/dictobject.c
index ff77bee..a8b26d2 100644
--- a/Objects/dictobject.c
+++ b/Objects/dictobject.c
@@ -1636,6 +1636,8 @@ dict_dealloc(PyDictObject *mp)
     PyObject **values = mp->ma_values;
     PyDictKeysObject *keys = mp->ma_keys;
     Py_ssize_t i, n;
+
+    /* bpo-31095: UnTrack is needed before calling any callbacks */
     PyObject_GC_UnTrack(mp);
     Py_TRASHCAN_SAFE_BEGIN(mp)
     if (values != NULL) {
@@ -2961,6 +2963,8 @@ 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);
@@ -3319,6 +3323,8 @@ dictiter_reduce(dictiterobject *di)
 static void
 dictview_dealloc(_PyDictViewObject *dv)
 {
+    /* bpo-31095: UnTrack is needed before calling any callbacks */
+    _PyObject_GC_UNTRACK(dv);
     Py_XDECREF(dv->dv_dict);
     PyObject_GC_Del(dv);
 }
diff --git a/Objects/setobject.c b/Objects/setobject.c
index 3dbdb4e..1f532bc 100644
--- a/Objects/setobject.c
+++ b/Objects/setobject.c
@@ -472,6 +472,7 @@ set_dealloc(PySetObject *so)
     setentry *entry;
     Py_ssize_t used = so->used;
 
+    /* bpo-31095: UnTrack is needed before calling any callbacks */
     PyObject_GC_UnTrack(so);
     Py_TRASHCAN_SAFE_BEGIN(so)
     if (so->weakreflist != NULL)
@@ -736,6 +737,8 @@ 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);
 }
diff --git a/Parser/asdl_c.py b/Parser/asdl_c.py
index ac9d2b6..60c5a75 100755
--- a/Parser/asdl_c.py
+++ b/Parser/asdl_c.py
@@ -630,6 +630,8 @@ typedef struct {
 static void
 ast_dealloc(AST_object *self)
 {
+    /* bpo-31095: UnTrack is needed before calling any callbacks */
+    PyObject_GC_UnTrack(self);
     Py_CLEAR(self->dict);
     Py_TYPE(self)->tp_free(self);
 }
diff --git a/Python/Python-ast.c b/Python/Python-ast.c
index ad53ba35..d7586f2 100644
--- a/Python/Python-ast.c
+++ b/Python/Python-ast.c
@@ -486,6 +486,8 @@ typedef struct {
 static void
 ast_dealloc(AST_object *self)
 {
+    /* bpo-31095: UnTrack is needed before calling any callbacks */
+    PyObject_GC_UnTrack(self);
     Py_CLEAR(self->dict);
     Py_TYPE(self)->tp_free(self);
 }