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bpo-35810: Incref heap-allocated types in PyObject_Init (GH-11661)
* Incref heap-allocated types in PyObject_Init * Add documentation and porting notes to What's New
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‎Doc/whatsnew/3.8.rst

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@@ -509,6 +509,12 @@ Build and C API Changes
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``1`` for objects implementing ``__index__()``.
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(Contributed by Serhiy Storchaka in :issue:`36048`.)
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* Heap-allocated type objects will now increase their reference count
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in :c:func:`PyObject_Init` (and its parallel macro ``PyObject_INIT``)
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instead of in :c:func:`PyType_GenericAlloc`. Types that modify instance
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allocation or deallocation may need to be adjusted.
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(Contributed by Eddie Elizondo in :issue:`35810`.)
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Deprecated
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==========
@@ -732,6 +738,67 @@ Changes in the C API
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(Contributed by Inada Naoki in :issue:`36381`.)
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Changes in the C API
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--------------------------
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* Instances of heap-allocated types (such as those created with
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:c:func:`PyType_FromSpec`) hold a reference to their type object.
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Increasing the reference count of these type objects has been moved from
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:c:func:`PyType_GenericAlloc` to the more low-level functions,
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:c:func:`PyObject_Init` and :c:func:`PyObject_INIT`.
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This makes types created through :c:func:`PyType_FromSpec` behave like
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other classes in managed code.
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Statically allocated types are not affected.
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For the vast majority of cases, there should be no side effect.
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However, types that manually increase the reference count after allocating
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an instance (perhaps to work around the bug) may now become immortal.
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To avoid this, these classes need to call Py_DECREF on the type object
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during instance deallocation.
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To correctly port these types into 3.8, please apply the following
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changes:
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* Remove :c:macro:`Py_INCREF` on the type object after allocating an
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instance - if any.
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This may happen after calling :c:func:`PyObject_New`,
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:c:func:`PyObject_NewVar`, :c:func:`PyObject_GC_New`,
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:c:func:`PyObject_GC_NewVar`, or any other custom allocator that uses
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:c:func:`PyObject_Init` or :c:func:`PyObject_INIT`.
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Example::
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static foo_struct *
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foo_new(PyObject *type) {
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foo_struct *foo = PyObject_GC_New(foo_struct, (PyTypeObject *) type);
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if (foo == NULL)
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return NULL;
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#if PY_VERSION_HEX < 0x03080000
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// Workaround for Python issue 35810; no longer necessary in Python 3.8
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PY_INCREF(type)
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#endif
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return foo;
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}
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* Ensure that all custom ``tp_dealloc`` functions of heap-allocated types
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decrease the type's reference count.
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Example::
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static void
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foo_dealloc(foo_struct *instance) {
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PyObject *type = Py_TYPE(instance);
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PyObject_GC_Del(instance);
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#if PY_VERSION_HEX >= 0x03080000
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// This was not needed before Python 3.8 (Python issue 35810)
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Py_DECREF(type);
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#endif
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}
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(Contributed by Eddie Elizondo in :issue:`35810`.)
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CPython bytecode changes
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------------------------
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‎Include/objimpl.h

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@@ -138,6 +138,9 @@ _PyObject_INIT(PyObject *op, PyTypeObject *typeobj)
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{
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assert(op != NULL);
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Py_TYPE(op) = typeobj;
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if (PyType_GetFlags(typeobj) & Py_TPFLAGS_HEAPTYPE) {
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Py_INCREF(typeobj);
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}
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_Py_NewReference(op);
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return op;
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}

‎Misc/NEWS.d/next/C API/2019-01-23-12-38-11.bpo-35810.wpbWeb.rst

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@@ -0,0 +1,4 @@
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Modify ``PyObject_Init`` to correctly increase the refcount of heap-
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allocated Type objects. Also fix the refcounts of the heap-allocated types
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that were either doing this manually or not decreasing the type's refcount
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in tp_dealloc

‎Modules/_curses_panel.c

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@@ -250,7 +250,10 @@ PyCursesPanel_New(PANEL *pan, PyCursesWindowObject *wo)
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static void
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PyCursesPanel_Dealloc(PyCursesPanelObject *po)
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{
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PyObject *obj = (PyObject *) panel_userptr(po->pan);
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PyObject *tp, *obj;
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tp = (PyObject *) Py_TYPE(po);
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obj = (PyObject *) panel_userptr(po->pan);
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if (obj) {
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(void)set_panel_userptr(po->pan, NULL);
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Py_DECREF(obj);
@@ -261,6 +264,7 @@ PyCursesPanel_Dealloc(PyCursesPanelObject *po)
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remove_lop(po);
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}
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PyObject_DEL(po);
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Py_DECREF(tp);
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}
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/* panel_above(NULL) returns the bottom panel in the stack. To get

‎Modules/_tkinter.c

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@@ -617,7 +617,6 @@ Tkapp_New(const char *screenName, const char *className,
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v = PyObject_New(TkappObject, (PyTypeObject *) Tkapp_Type);
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if (v == NULL)
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return NULL;
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Py_INCREF(Tkapp_Type);
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v->interp = Tcl_CreateInterp();
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v->wantobjects = wantobjects;
@@ -802,7 +801,6 @@ newPyTclObject(Tcl_Obj *arg)
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self = PyObject_New(PyTclObject, (PyTypeObject *) PyTclObject_Type);
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if (self == NULL)
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return NULL;
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Py_INCREF(PyTclObject_Type);
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Tcl_IncrRefCount(arg);
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self->value = arg;
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self->string = NULL;
@@ -2722,7 +2720,6 @@ Tktt_New(PyObject *func)
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v = PyObject_New(TkttObject, (PyTypeObject *) Tktt_Type);
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if (v == NULL)
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return NULL;
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Py_INCREF(Tktt_Type);
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Py_INCREF(func);
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v->token = NULL;

‎Objects/object.c

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@@ -230,6 +230,9 @@ PyObject_Init(PyObject *op, PyTypeObject *tp)
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return PyErr_NoMemory();
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/* Any changes should be reflected in PyObject_INIT (objimpl.h) */
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Py_TYPE(op) = tp;
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if (PyType_GetFlags(tp) & Py_TPFLAGS_HEAPTYPE) {
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Py_INCREF(tp);
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}
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_Py_NewReference(op);
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return op;
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}
@@ -240,9 +243,8 @@ PyObject_InitVar(PyVarObject *op, PyTypeObject *tp, Py_ssize_t size)
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if (op == NULL)
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return (PyVarObject *) PyErr_NoMemory();
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/* Any changes should be reflected in PyObject_INIT_VAR */
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op->ob_size = size;
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Py_TYPE(op) = tp;
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_Py_NewReference((PyObject *)op);
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Py_SIZE(op) = size;
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PyObject_Init((PyObject *)op, tp);
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return op;
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}
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‎Objects/structseq.c

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@@ -63,12 +63,17 @@ static void
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structseq_dealloc(PyStructSequence *obj)
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{
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Py_ssize_t i, size;
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PyTypeObject *tp;
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tp = (PyTypeObject *) Py_TYPE(obj);
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size = REAL_SIZE(obj);
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for (i = 0; i < size; ++i) {
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Py_XDECREF(obj->ob_item[i]);
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}
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PyObject_GC_Del(obj);
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if (PyType_GetFlags(tp) & Py_TPFLAGS_HEAPTYPE) {
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Py_DECREF(tp);
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}
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}
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/*[clinic input]

‎Objects/typeobject.c

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@@ -987,9 +987,6 @@ PyType_GenericAlloc(PyTypeObject *type, Py_ssize_t nitems)
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memset(obj, '\0', size);
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if (type->tp_flags & Py_TPFLAGS_HEAPTYPE)
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Py_INCREF(type);
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if (type->tp_itemsize == 0)
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(void)PyObject_INIT(obj, type);
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else

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