Issue #18112: PEP 442 implementation (safe object finalization).

6 files changed, 350 insertions, 113 deletions

diff --git a/Modules/_io/bufferedio.c b/Modules/_io/bufferedio.c
index 6a885cf4bfb..6fe5d586a9c 100644
--- a/Modules/_io/bufferedio.c
+++ b/Modules/_io/bufferedio.c
@@ -190,7 +190,8 @@ PyTypeObject PyBufferedIOBase_Type = {
     0,                          /*tp_getattro*/
     0,                          /*tp_setattro*/
     0,                          /*tp_as_buffer*/
-    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,  /*tp_flags*/
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE
+        | Py_TPFLAGS_HAVE_FINALIZE,  /*tp_flags*/
     bufferediobase_doc,         /* tp_doc */
     0,                          /* tp_traverse */
     0,                          /* tp_clear */
@@ -209,6 +210,16 @@ PyTypeObject PyBufferedIOBase_Type = {
     0,                          /* tp_init */
     0,                          /* tp_alloc */
     0,                          /* tp_new */
+    0,                          /* tp_free */
+    0,                          /* tp_is_gc */
+    0,                          /* tp_bases */
+    0,                          /* tp_mro */
+    0,                          /* tp_cache */
+    0,                          /* tp_subclasses */
+    0,                          /* tp_weaklist */
+    0,                          /* tp_del */
+    0,                          /* tp_version_tag */
+    0,                          /* tp_finalize */
 };
 
 
@@ -220,7 +231,7 @@ typedef struct {
     int detached;
     int readable;
     int writable;
-    int deallocating;
+    char finalizing;
 
     /* True if this is a vanilla Buffered object (rather than a user derived
        class) *and* the raw stream is a vanilla FileIO object. */
@@ -384,8 +395,8 @@ _enter_buffered_busy(buffered *self)
 static void
 buffered_dealloc(buffered *self)
 {
-    self->deallocating = 1;
-    if (self->ok && _PyIOBase_finalize((PyObject *) self) < 0)
+    self->finalizing = 1;
+    if (_PyIOBase_finalize((PyObject *) self) < 0)
         return;
     _PyObject_GC_UNTRACK(self);
     self->ok = 0;
@@ -428,8 +439,6 @@ buffered_traverse(buffered *self, visitproc visit, void *arg)
 static int
 buffered_clear(buffered *self)
 {
-    if (self->ok && _PyIOBase_finalize((PyObject *) self) < 0)
-        return -1;
     self->ok = 0;
     Py_CLEAR(self->raw);
     Py_CLEAR(self->dict);
@@ -508,7 +517,7 @@ buffered_close(buffered *self, PyObject *args)
         goto end;
     }
 
-    if (self->deallocating) {
+    if (self->finalizing) {
         PyObject *r = buffered_dealloc_warn(self, (PyObject *) self);
         if (r)
             Py_DECREF(r);
@@ -1749,6 +1758,7 @@ static PyMethodDef bufferedreader_methods[] = {
 
 static PyMemberDef bufferedreader_members[] = {
     {"raw", T_OBJECT, offsetof(buffered, raw), READONLY},
+    {"_finalizing", T_BOOL, offsetof(buffered, finalizing), 0},
     {NULL}
 };
 
@@ -1781,7 +1791,7 @@ PyTypeObject PyBufferedReader_Type = {
     0,                          /*tp_setattro*/
     0,                          /*tp_as_buffer*/
     Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE
-            | Py_TPFLAGS_HAVE_GC, /*tp_flags*/
+        | Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_HAVE_FINALIZE, /*tp_flags*/
     bufferedreader_doc,         /* tp_doc */
     (traverseproc)buffered_traverse, /* tp_traverse */
     (inquiry)buffered_clear,    /* tp_clear */
@@ -1800,6 +1810,16 @@ PyTypeObject PyBufferedReader_Type = {
     (initproc)bufferedreader_init, /* tp_init */
     0,                          /* tp_alloc */
     PyType_GenericNew,          /* tp_new */
+    0,                          /* tp_free */
+    0,                          /* tp_is_gc */
+    0,                          /* tp_bases */
+    0,                          /* tp_mro */
+    0,                          /* tp_cache */
+    0,                          /* tp_subclasses */
+    0,                          /* tp_weaklist */
+    0,                          /* tp_del */
+    0,                          /* tp_version_tag */
+    0,                          /* tp_finalize */
 };
 
 
@@ -2130,6 +2150,7 @@ static PyMethodDef bufferedwriter_methods[] = {
 
 static PyMemberDef bufferedwriter_members[] = {
     {"raw", T_OBJECT, offsetof(buffered, raw), READONLY},
+    {"_finalizing", T_BOOL, offsetof(buffered, finalizing), 0},
     {NULL}
 };
 
@@ -2162,7 +2183,7 @@ PyTypeObject PyBufferedWriter_Type = {
     0,                          /*tp_setattro*/
     0,                          /*tp_as_buffer*/
     Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE
-        | Py_TPFLAGS_HAVE_GC,   /*tp_flags*/
+        | Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_HAVE_FINALIZE,   /*tp_flags*/
     bufferedwriter_doc,         /* tp_doc */
     (traverseproc)buffered_traverse, /* tp_traverse */
     (inquiry)buffered_clear,    /* tp_clear */
@@ -2181,6 +2202,16 @@ PyTypeObject PyBufferedWriter_Type = {
     (initproc)bufferedwriter_init, /* tp_init */
     0,                          /* tp_alloc */
     PyType_GenericNew,          /* tp_new */
+    0,                          /* tp_free */
+    0,                          /* tp_is_gc */
+    0,                          /* tp_bases */
+    0,                          /* tp_mro */
+    0,                          /* tp_cache */
+    0,                          /* tp_subclasses */
+    0,                          /* tp_weaklist */
+    0,                          /* tp_del */
+    0,                          /* tp_version_tag */
+    0,                          /* tp_finalize */
 };
 
 
@@ -2416,7 +2447,7 @@ PyTypeObject PyBufferedRWPair_Type = {
     0,                          /*tp_setattro*/
     0,                          /*tp_as_buffer*/
     Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE
-        | Py_TPFLAGS_HAVE_GC,   /* tp_flags */
+        | Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_HAVE_FINALIZE,   /* tp_flags */
     bufferedrwpair_doc,         /* tp_doc */
     (traverseproc)bufferedrwpair_traverse, /* tp_traverse */
     (inquiry)bufferedrwpair_clear, /* tp_clear */
@@ -2435,6 +2466,16 @@ PyTypeObject PyBufferedRWPair_Type = {
     (initproc)bufferedrwpair_init, /* tp_init */
     0,                          /* tp_alloc */
     PyType_GenericNew,          /* tp_new */
+    0,                          /* tp_free */
+    0,                          /* tp_is_gc */
+    0,                          /* tp_bases */
+    0,                          /* tp_mro */
+    0,                          /* tp_cache */
+    0,                          /* tp_subclasses */
+    0,                          /* tp_weaklist */
+    0,                          /* tp_del */
+    0,                          /* tp_version_tag */
+    0,                          /* tp_finalize */
 };
 
 
@@ -2522,6 +2563,7 @@ static PyMethodDef bufferedrandom_methods[] = {
 
 static PyMemberDef bufferedrandom_members[] = {
     {"raw", T_OBJECT, offsetof(buffered, raw), READONLY},
+    {"_finalizing", T_BOOL, offsetof(buffered, finalizing), 0},
     {NULL}
 };
 
@@ -2554,7 +2596,7 @@ PyTypeObject PyBufferedRandom_Type = {
     0,                          /*tp_setattro*/
     0,                          /*tp_as_buffer*/
     Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE
-        | Py_TPFLAGS_HAVE_GC,   /*tp_flags*/
+        | Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_HAVE_FINALIZE,   /*tp_flags*/
     bufferedrandom_doc,         /* tp_doc */
     (traverseproc)buffered_traverse, /* tp_traverse */
     (inquiry)buffered_clear,    /* tp_clear */
@@ -2573,4 +2615,14 @@ PyTypeObject PyBufferedRandom_Type = {
     (initproc)bufferedrandom_init, /* tp_init */
     0,                          /* tp_alloc */
     PyType_GenericNew,          /* tp_new */
+    0,                          /* tp_free */
+    0,                          /* tp_is_gc */
+    0,                          /* tp_bases */
+    0,                          /* tp_mro */
+    0,                          /* tp_cache */
+    0,                          /* tp_subclasses */
+    0,                          /* tp_weaklist */
+    0,                          /* tp_del */
+    0,                          /* tp_version_tag */
+    0,                          /* tp_finalize */
 };
diff --git a/Modules/_io/fileio.c b/Modules/_io/fileio.c
index 5280991727d..e88ae877f2f 100644
--- a/Modules/_io/fileio.c
+++ b/Modules/_io/fileio.c
@@ -51,7 +51,7 @@ typedef struct {
     unsigned int writable : 1;
     signed int seekable : 2; /* -1 means unknown */
     unsigned int closefd : 1;
-    unsigned int deallocating: 1;
+    char finalizing;
     PyObject *weakreflist;
     PyObject *dict;
 } fileio;
@@ -128,7 +128,7 @@ fileio_close(fileio *self)
         self->fd = -1;
         Py_RETURN_NONE;
     }
-    if (self->deallocating) {
+    if (self->finalizing) {
         PyObject *r = fileio_dealloc_warn(self, (PyObject *) self);
         if (r)
             Py_DECREF(r);
@@ -447,7 +447,7 @@ fileio_clear(fileio *self)
 static void
 fileio_dealloc(fileio *self)
 {
-    self->deallocating = 1;
+    self->finalizing = 1;
     if (_PyIOBase_finalize((PyObject *) self) < 0)
         return;
     _PyObject_GC_UNTRACK(self);
@@ -1182,6 +1182,11 @@ static PyGetSetDef fileio_getsetlist[] = {
     {NULL},
 };
 
+static PyMemberDef fileio_members[] = {
+    {"_finalizing", T_BOOL, offsetof(fileio, finalizing), 0},
+    {NULL}
+};
+
 PyTypeObject PyFileIO_Type = {
     PyVarObject_HEAD_INIT(NULL, 0)
     "_io.FileIO",
@@ -1203,7 +1208,7 @@ PyTypeObject PyFileIO_Type = {
     0,                                          /* tp_setattro */
     0,                                          /* tp_as_buffer */
     Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE
-                    | Py_TPFLAGS_HAVE_GC,       /* tp_flags */
+        | Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_HAVE_FINALIZE,       /* tp_flags */
     fileio_doc,                                 /* tp_doc */
     (traverseproc)fileio_traverse,              /* tp_traverse */
     (inquiry)fileio_clear,                      /* tp_clear */
@@ -1212,7 +1217,7 @@ PyTypeObject PyFileIO_Type = {
     0,                                          /* tp_iter */
     0,                                          /* tp_iternext */
     fileio_methods,                             /* tp_methods */
-    0,                                          /* tp_members */
+    fileio_members,                             /* tp_members */
     fileio_getsetlist,                          /* tp_getset */
     0,                                          /* tp_base */
     0,                                          /* tp_dict */
@@ -1223,4 +1228,13 @@ PyTypeObject PyFileIO_Type = {
     PyType_GenericAlloc,                        /* tp_alloc */
     fileio_new,                                 /* tp_new */
     PyObject_GC_Del,                            /* tp_free */
+    0,                                          /* tp_is_gc */
+    0,                                          /* tp_bases */
+    0,                                          /* tp_mro */
+    0,                                          /* tp_cache */
+    0,                                          /* tp_subclasses */
+    0,                                          /* tp_weaklist */
+    0,                                          /* tp_del */
+    0,                                          /* tp_version_tag */
+    0,                                          /* tp_finalize */
 };
diff --git a/Modules/_io/iobase.c b/Modules/_io/iobase.c
index e38473a7452..a4f2c0e917d 100644
--- a/Modules/_io/iobase.c
+++ b/Modules/_io/iobase.c
@@ -196,21 +196,17 @@ iobase_close(PyObject *self, PyObject *args)
 
 /* Finalization and garbage collection support */
 
-int
-_PyIOBase_finalize(PyObject *self)
+static void
+iobase_finalize(PyObject *self)
 {
     PyObject *res;
-    PyObject *tp, *v, *tb;
-    int closed = 1;
-    int is_zombie;
+    PyObject *error_type, *error_value, *error_traceback;
+    int closed;
+    _Py_IDENTIFIER(_finalizing);
+
+    /* Save the current exception, if any. */
+    PyErr_Fetch(&error_type, &error_value, &error_traceback);
 
-    /* If _PyIOBase_finalize() is called from a destructor, we need to
-       resurrect the object as calling close() can invoke arbitrary code. */
-    is_zombie = (Py_REFCNT(self) == 0);
-    if (is_zombie) {
-        ++Py_REFCNT(self);
-    }
-    PyErr_Fetch(&tp, &v, &tb);
     /* If `closed` doesn't exist or can't be evaluated as bool, then the
        object is probably in an unusable state, so ignore. */
     res = PyObject_GetAttr(self, _PyIO_str_closed);
@@ -223,6 +219,10 @@ _PyIOBase_finalize(PyObject *self)
             PyErr_Clear();
     }
     if (closed == 0) {
+        /* Signal close() that it was called as part of the object
+           finalization process. */
+        if (_PyObject_SetAttrId(self, &PyId__finalizing, Py_True))
+            PyErr_Clear();
         res = PyObject_CallMethodObjArgs((PyObject *) self, _PyIO_str_close,
                                           NULL);
         /* Silencing I/O errors is bad, but printing spurious tracebacks is
@@ -233,31 +233,25 @@ _PyIOBase_finalize(PyObject *self)
         else
             Py_DECREF(res);
     }
-    PyErr_Restore(tp, v, tb);
-    if (is_zombie) {
-        if (--Py_REFCNT(self) != 0) {
-            /* The object lives again. The following code is taken from
-               slot_tp_del in typeobject.c. */
-            Py_ssize_t refcnt = Py_REFCNT(self);
-            _Py_NewReference(self);
-            Py_REFCNT(self) = refcnt;
-            /* If Py_REF_DEBUG, _Py_NewReference bumped _Py_RefTotal, so
-             * we need to undo that. */
-            _Py_DEC_REFTOTAL;
-            /* If Py_TRACE_REFS, _Py_NewReference re-added self to the object
-             * chain, so no more to do there.
-             * If COUNT_ALLOCS, the original decref bumped tp_frees, and
-             * _Py_NewReference bumped tp_allocs:  both of those need to be
-             * undone.
-             */
-#ifdef COUNT_ALLOCS
-            --Py_TYPE(self)->tp_frees;
-            --Py_TYPE(self)->tp_allocs;
-#endif
-            return -1;
-        }
+
+    /* Restore the saved exception. */
+    PyErr_Restore(error_type, error_value, error_traceback);
+}
+
+int
+_PyIOBase_finalize(PyObject *self)
+{
+    int is_zombie;
+
+    /* If _PyIOBase_finalize() is called from a destructor, we need to
+       resurrect the object as calling close() can invoke arbitrary code. */
+    is_zombie = (Py_REFCNT(self) == 0);
+    if (is_zombie)
+        return PyObject_CallFinalizerFromDealloc(self);
+    else {
+        PyObject_CallFinalizer(self);
+        return 0;
     }
-    return 0;
 }
 
 static int
@@ -270,8 +264,6 @@ iobase_traverse(iobase *self, visitproc visit, void *arg)
 static int
 iobase_clear(iobase *self)
 {
-    if (_PyIOBase_finalize((PyObject *) self) < 0)
-        return -1;
     Py_CLEAR(self->dict);
     return 0;
 }
@@ -741,7 +733,7 @@ PyTypeObject PyIOBase_Type = {
     0,                          /*tp_setattro*/
     0,                          /*tp_as_buffer*/
     Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE
-        | Py_TPFLAGS_HAVE_GC,   /*tp_flags*/
+        | Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_HAVE_FINALIZE,   /*tp_flags*/
     iobase_doc,                 /* tp_doc */
     (traverseproc)iobase_traverse, /* tp_traverse */
     (inquiry)iobase_clear,      /* tp_clear */
@@ -760,6 +752,16 @@ PyTypeObject PyIOBase_Type = {
     0,                          /* tp_init */
     0,                          /* tp_alloc */
     PyType_GenericNew,          /* tp_new */
+    0,                          /* tp_free */
+    0,                          /* tp_is_gc */
+    0,                          /* tp_bases */
+    0,                          /* tp_mro */
+    0,                          /* tp_cache */
+    0,                          /* tp_subclasses */
+    0,                          /* tp_weaklist */
+    0,                          /* tp_del */
+    0,                          /* tp_version_tag */
+    (destructor)iobase_finalize, /* tp_finalize */
 };
 
 
@@ -905,7 +907,7 @@ PyTypeObject PyRawIOBase_Type = {
     0,                          /*tp_getattro*/
     0,                          /*tp_setattro*/
     0,                          /*tp_as_buffer*/
-    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,  /*tp_flags*/
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE | Py_TPFLAGS_HAVE_FINALIZE,  /*tp_flags*/
     rawiobase_doc,              /* tp_doc */
     0,                          /* tp_traverse */
     0,                          /* tp_clear */
@@ -924,4 +926,14 @@ PyTypeObject PyRawIOBase_Type = {
     0,                          /* tp_init */
     0,                          /* tp_alloc */
     0,                          /* tp_new */
+    0,                          /* tp_free */
+    0,                          /* tp_is_gc */
+    0,                          /* tp_bases */
+    0,                          /* tp_mro */
+    0,                          /* tp_cache */
+    0,                          /* tp_subclasses */
+    0,                          /* tp_weaklist */
+    0,                          /* tp_del */
+    0,                          /* tp_version_tag */
+    0,                          /* tp_finalize */
 };
diff --git a/Modules/_io/textio.c b/Modules/_io/textio.c
index 283411b10ca..5d2438d8125 100644
--- a/Modules/_io/textio.c
+++ b/Modules/_io/textio.c
@@ -173,7 +173,8 @@ PyTypeObject PyTextIOBase_Type = {
     0,                          /*tp_getattro*/
     0,                          /*tp_setattro*/
     0,                          /*tp_as_buffer*/
-    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,  /*tp_flags*/
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE
+        | Py_TPFLAGS_HAVE_FINALIZE,  /*tp_flags*/
     textiobase_doc,             /* tp_doc */
     0,                          /* tp_traverse */
     0,                          /* tp_clear */
@@ -192,6 +193,16 @@ PyTypeObject PyTextIOBase_Type = {
     0,                          /* tp_init */
     0,                          /* tp_alloc */
     0,                          /* tp_new */
+    0,                          /* tp_free */
+    0,                          /* tp_is_gc */
+    0,                          /* tp_bases */
+    0,                          /* tp_mro */
+    0,                          /* tp_cache */
+    0,                          /* tp_subclasses */
+    0,                          /* tp_weaklist */
+    0,                          /* tp_del */
+    0,                          /* tp_version_tag */
+    0,                          /* tp_finalize */
 };
 
 
@@ -691,7 +702,7 @@ typedef struct
     char seekable;
     char has_read1;
     char telling;
-    char deallocating;
+    char finalizing;
     /* Specialized encoding func (see below) */
     encodefunc_t encodefunc;
     /* Whether or not it's the start of the stream */
@@ -1112,8 +1123,6 @@ textiowrapper_init(textio *self, PyObject *args, PyObject *kwds)
 static int
 _textiowrapper_clear(textio *self)
 {
-    if (self->ok && _PyIOBase_finalize((PyObject *) self) < 0)
-        return -1;
     self->ok = 0;
     Py_CLEAR(self->buffer);
     Py_CLEAR(self->encoding);
@@ -1131,9 +1140,10 @@ _textiowrapper_clear(textio *self)
 static void
 textiowrapper_dealloc(textio *self)
 {
-    self->deallocating = 1;
-    if (_textiowrapper_clear(self) < 0)
+    self->finalizing = 1;
+    if (_PyIOBase_finalize((PyObject *) self) < 0)
         return;
+    _textiowrapper_clear(self);
     _PyObject_GC_UNTRACK(self);
     if (self->weakreflist != NULL)
         PyObject_ClearWeakRefs((PyObject *)self);
@@ -2573,7 +2583,7 @@ textiowrapper_close(textio *self, PyObject *args)
     }
     else {
         PyObject *exc = NULL, *val, *tb;
-        if (self->deallocating) {
+        if (self->finalizing) {
             res = _PyObject_CallMethodId(self->buffer, &PyId__dealloc_warn, "O", self);
             if (res)
                 Py_DECREF(res);
@@ -2734,6 +2744,7 @@ static PyMemberDef textiowrapper_members[] = {
     {"encoding", T_OBJECT, offsetof(textio, encoding), READONLY},
     {"buffer", T_OBJECT, offsetof(textio, buffer), READONLY},
     {"line_buffering", T_BOOL, offsetof(textio, line_buffering), READONLY},
+    {"_finalizing", T_BOOL, offsetof(textio, finalizing), 0},
     {NULL}
 };
 
@@ -2770,7 +2781,7 @@ PyTypeObject PyTextIOWrapper_Type = {
     0,                          /*tp_setattro*/
     0,                          /*tp_as_buffer*/
     Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE
-            | Py_TPFLAGS_HAVE_GC, /*tp_flags*/
+        | Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_HAVE_FINALIZE, /*tp_flags*/
     textiowrapper_doc,          /* tp_doc */
     (traverseproc)textiowrapper_traverse, /* tp_traverse */
     (inquiry)textiowrapper_clear, /* tp_clear */
@@ -2789,4 +2800,14 @@ PyTypeObject PyTextIOWrapper_Type = {
     (initproc)textiowrapper_init, /* tp_init */
     0,                          /* tp_alloc */
     PyType_GenericNew,          /* tp_new */
+    0,                          /* tp_free */
+    0,                          /* tp_is_gc */
+    0,                          /* tp_bases */
+    0,                          /* tp_mro */
+    0,                          /* tp_cache */
+    0,                          /* tp_subclasses */
+    0,                          /* tp_weaklist */
+    0,                          /* tp_del */
+    0,                          /* tp_version_tag */
+    0,                          /* tp_finalize */
 };
diff --git a/Modules/_testcapimodule.c b/Modules/_testcapimodule.c
index 9463227abf1..6527a533625 100644
--- a/Modules/_testcapimodule.c
+++ b/Modules/_testcapimodule.c
@@ -2491,6 +2491,85 @@ test_pytime_object_to_timespec(PyObject *self, PyObject *args)
     return Py_BuildValue("Nl", _PyLong_FromTime_t(sec), nsec);
 }
 
+static void
+slot_tp_del(PyObject *self)
+{
+    _Py_IDENTIFIER(__tp_del__);
+    PyObject *del, *res;
+    PyObject *error_type, *error_value, *error_traceback;
+
+    /* Temporarily resurrect the object. */
+    assert(self->ob_refcnt == 0);
+    self->ob_refcnt = 1;
+
+    /* Save the current exception, if any. */
+    PyErr_Fetch(&error_type, &error_value, &error_traceback);
+
+    /* Execute __del__ method, if any. */
+    del = _PyObject_LookupSpecial(self, &PyId___tp_del__);
+    if (del != NULL) {
+        res = PyEval_CallObject(del, NULL);
+        if (res == NULL)
+            PyErr_WriteUnraisable(del);
+        else
+            Py_DECREF(res);
+        Py_DECREF(del);
+    }
+
+    /* Restore the saved exception. */
+    PyErr_Restore(error_type, error_value, error_traceback);
+
+    /* Undo the temporary resurrection; can't use DECREF here, it would
+     * cause a recursive call.
+     */
+    assert(self->ob_refcnt > 0);
+    if (--self->ob_refcnt == 0)
+        return;         /* this is the normal path out */
+
+    /* __del__ resurrected it!  Make it look like the original Py_DECREF
+     * never happened.
+     */
+    {
+        Py_ssize_t refcnt = self->ob_refcnt;
+        _Py_NewReference(self);
+        self->ob_refcnt = refcnt;
+    }
+    assert(!PyType_IS_GC(Py_TYPE(self)) ||
+           _Py_AS_GC(self)->gc.gc_refs != _PyGC_REFS_UNTRACKED);
+    /* If Py_REF_DEBUG, _Py_NewReference bumped _Py_RefTotal, so
+     * we need to undo that. */
+    _Py_DEC_REFTOTAL;
+    /* If Py_TRACE_REFS, _Py_NewReference re-added self to the object
+     * chain, so no more to do there.
+     * If COUNT_ALLOCS, the original decref bumped tp_frees, and
+     * _Py_NewReference bumped tp_allocs:  both of those need to be
+     * undone.
+     */
+#ifdef COUNT_ALLOCS
+    --Py_TYPE(self)->tp_frees;
+    --Py_TYPE(self)->tp_allocs;
+#endif
+}
+
+static PyObject *
+with_tp_del(PyObject *self, PyObject *args)
+{
+    PyObject *obj;
+    PyTypeObject *tp;
+
+    if (!PyArg_ParseTuple(args, "O:with_tp_del", &obj))
+        return NULL;
+    tp = (PyTypeObject *) obj;
+    if (!PyType_Check(obj) || !PyType_HasFeature(tp, Py_TPFLAGS_HEAPTYPE)) {
+        PyErr_Format(PyExc_TypeError,
+                     "heap type expected, got %R", obj);
+        return NULL;
+    }
+    tp->tp_del = slot_tp_del;
+    Py_INCREF(obj);
+    return obj;
+}
+
 static PyObject *
 _test_incref(PyObject *ob)
 {
@@ -2789,6 +2868,7 @@ static PyMethodDef TestMethods[] = {
     {"pytime_object_to_time_t", test_pytime_object_to_time_t,  METH_VARARGS},
     {"pytime_object_to_timeval", test_pytime_object_to_timeval,  METH_VARARGS},
     {"pytime_object_to_timespec", test_pytime_object_to_timespec,  METH_VARARGS},
+    {"with_tp_del",             with_tp_del,                     METH_VARARGS},
     {"test_pymem",
      (PyCFunction)test_pymem_alloc0, METH_NOARGS},
     {"test_pymem_alloc0",
diff --git a/Modules/gcmodule.c b/Modules/gcmodule.c
index d96d2c7b68a..dac7feb44c9 100644
--- a/Modules/gcmodule.c
+++ b/Modules/gcmodule.c
@@ -223,10 +223,10 @@ GC_TENTATIVELY_UNREACHABLE
 #define GC_REACHABLE                    _PyGC_REFS_REACHABLE
 #define GC_TENTATIVELY_UNREACHABLE      _PyGC_REFS_TENTATIVELY_UNREACHABLE
 
-#define IS_TRACKED(o) ((AS_GC(o))->gc.gc_refs != GC_UNTRACKED)
-#define IS_REACHABLE(o) ((AS_GC(o))->gc.gc_refs == GC_REACHABLE)
+#define IS_TRACKED(o) (_PyGC_REFS(o) != GC_UNTRACKED)
+#define IS_REACHABLE(o) (_PyGC_REFS(o) == GC_REACHABLE)
 #define IS_TENTATIVELY_UNREACHABLE(o) ( \
-    (AS_GC(o))->gc.gc_refs == GC_TENTATIVELY_UNREACHABLE)
+    _PyGC_REFS(o) == GC_TENTATIVELY_UNREACHABLE)
 
 /*** list functions ***/
 
@@ -341,8 +341,8 @@ update_refs(PyGC_Head *containers)
 {
     PyGC_Head *gc = containers->gc.gc_next;
     for (; gc != containers; gc = gc->gc.gc_next) {
-        assert(gc->gc.gc_refs == GC_REACHABLE);
-        gc->gc.gc_refs = Py_REFCNT(FROM_GC(gc));
+        assert(_PyGCHead_REFS(gc) == GC_REACHABLE);
+        _PyGCHead_SET_REFS(gc, Py_REFCNT(FROM_GC(gc)));
         /* Python's cyclic gc should never see an incoming refcount
          * of 0:  if something decref'ed to 0, it should have been
          * deallocated immediately at that time.
@@ -361,7 +361,7 @@ update_refs(PyGC_Head *containers)
          * so serious that maybe this should be a release-build
          * check instead of an assert?
          */
-        assert(gc->gc.gc_refs != 0);
+        assert(_PyGCHead_REFS(gc) != 0);
     }
 }
 
@@ -376,9 +376,9 @@ visit_decref(PyObject *op, void *data)
          * generation being collected, which can be recognized
          * because only they have positive gc_refs.
          */
-        assert(gc->gc.gc_refs != 0); /* else refcount was too small */
-        if (gc->gc.gc_refs > 0)
-            gc->gc.gc_refs--;
+        assert(_PyGCHead_REFS(gc) != 0); /* else refcount was too small */
+        if (_PyGCHead_REFS(gc) > 0)
+            _PyGCHead_DECREF(gc);
     }
     return 0;
 }
@@ -407,7 +407,7 @@ visit_reachable(PyObject *op, PyGC_Head *reachable)
 {
     if (PyObject_IS_GC(op)) {
         PyGC_Head *gc = AS_GC(op);
-        const Py_ssize_t gc_refs = gc->gc.gc_refs;
+        const Py_ssize_t gc_refs = _PyGCHead_REFS(gc);
 
         if (gc_refs == 0) {
             /* This is in move_unreachable's 'young' list, but
@@ -415,7 +415,7 @@ visit_reachable(PyObject *op, PyGC_Head *reachable)
              * we need to do is tell move_unreachable that it's
              * reachable.
              */
-            gc->gc.gc_refs = 1;
+            _PyGCHead_SET_REFS(gc, 1);
         }
         else if (gc_refs == GC_TENTATIVELY_UNREACHABLE) {
             /* This had gc_refs = 0 when move_unreachable got
@@ -425,7 +425,7 @@ visit_reachable(PyObject *op, PyGC_Head *reachable)
              * again.
              */
             gc_list_move(gc, reachable);
-            gc->gc.gc_refs = 1;
+            _PyGCHead_SET_REFS(gc, 1);
         }
         /* Else there's nothing to do.
          * If gc_refs > 0, it must be in move_unreachable's 'young'
@@ -469,7 +469,7 @@ move_unreachable(PyGC_Head *young, PyGC_Head *unreachable)
     while (gc != young) {
         PyGC_Head *next;
 
-        if (gc->gc.gc_refs) {
+        if (_PyGCHead_REFS(gc)) {
             /* gc is definitely reachable from outside the
              * original 'young'.  Mark it as such, and traverse
              * its pointers to find any other objects that may
@@ -480,8 +480,8 @@ move_unreachable(PyGC_Head *young, PyGC_Head *unreachable)
              */
             PyObject *op = FROM_GC(gc);
             traverseproc traverse = Py_TYPE(op)->tp_traverse;
-            assert(gc->gc.gc_refs > 0);
-            gc->gc.gc_refs = GC_REACHABLE;
+            assert(_PyGCHead_REFS(gc) > 0);
+            _PyGCHead_SET_REFS(gc, GC_REACHABLE);
             (void) traverse(op,
                             (visitproc)visit_reachable,
                             (void *)young);
@@ -500,7 +500,7 @@ move_unreachable(PyGC_Head *young, PyGC_Head *unreachable)
              */
             next = gc->gc.gc_next;
             gc_list_move(gc, unreachable);
-            gc->gc.gc_refs = GC_TENTATIVELY_UNREACHABLE;
+            _PyGCHead_SET_REFS(gc, GC_TENTATIVELY_UNREACHABLE);
         }
         gc = next;
     }
@@ -520,22 +520,19 @@ untrack_dicts(PyGC_Head *head)
     }
 }
 
-/* Return true if object has a finalization method. */
+/* Return true if object has a pre-PEP 442 finalization method. */
 static int
-has_finalizer(PyObject *op)
+has_legacy_finalizer(PyObject *op)
 {
-    if (PyGen_CheckExact(op))
-        return PyGen_NeedsFinalizing((PyGenObject *)op);
-    else
-        return op->ob_type->tp_del != NULL;
+    return op->ob_type->tp_del != NULL;
 }
 
-/* Move the objects in unreachable with __del__ methods into `finalizers`.
+/* Move the objects in unreachable with tp_del slots into `finalizers`.
  * Objects moved into `finalizers` have gc_refs set to GC_REACHABLE; the
  * objects remaining in unreachable are left at GC_TENTATIVELY_UNREACHABLE.
  */
 static void
-move_finalizers(PyGC_Head *unreachable, PyGC_Head *finalizers)
+move_legacy_finalizers(PyGC_Head *unreachable, PyGC_Head *finalizers)
 {
     PyGC_Head *gc;
     PyGC_Head *next;
@@ -549,14 +546,14 @@ move_finalizers(PyGC_Head *unreachable, PyGC_Head *finalizers)
         assert(IS_TENTATIVELY_UNREACHABLE(op));
         next = gc->gc.gc_next;
 
-        if (has_finalizer(op)) {
+        if (has_legacy_finalizer(op)) {
             gc_list_move(gc, finalizers);
-            gc->gc.gc_refs = GC_REACHABLE;
+            _PyGCHead_SET_REFS(gc, GC_REACHABLE);
         }
     }
 }
 
-/* A traversal callback for move_finalizer_reachable. */
+/* A traversal callback for move_legacy_finalizer_reachable. */
 static int
 visit_move(PyObject *op, PyGC_Head *tolist)
 {
@@ -564,7 +561,7 @@ visit_move(PyObject *op, PyGC_Head *tolist)
         if (IS_TENTATIVELY_UNREACHABLE(op)) {
             PyGC_Head *gc = AS_GC(op);
             gc_list_move(gc, tolist);
-            gc->gc.gc_refs = GC_REACHABLE;
+            _PyGCHead_SET_REFS(gc, GC_REACHABLE);
         }
     }
     return 0;
@@ -574,7 +571,7 @@ visit_move(PyObject *op, PyGC_Head *tolist)
  * into finalizers set.
  */
 static void
-move_finalizer_reachable(PyGC_Head *finalizers)
+move_legacy_finalizer_reachable(PyGC_Head *finalizers)
 {
     traverseproc traverse;
     PyGC_Head *gc = finalizers->gc.gc_next;
@@ -747,7 +744,7 @@ debug_cycle(char *msg, PyObject *op)
                        msg, Py_TYPE(op)->tp_name, op);
 }
 
-/* Handle uncollectable garbage (cycles with finalizers, and stuff reachable
+/* Handle uncollectable garbage (cycles with tp_del slots, and stuff reachable
  * only from such cycles).
  * If DEBUG_SAVEALL, all objects in finalizers are appended to the module
  * garbage list (a Python list), else only the objects in finalizers with
@@ -757,7 +754,7 @@ debug_cycle(char *msg, PyObject *op)
  * The finalizers list is made empty on a successful return.
  */
 static int
-handle_finalizers(PyGC_Head *finalizers, PyGC_Head *old)
+handle_legacy_finalizers(PyGC_Head *finalizers, PyGC_Head *old)
 {
     PyGC_Head *gc = finalizers->gc.gc_next;
 
@@ -769,7 +766,7 @@ handle_finalizers(PyGC_Head *finalizers, PyGC_Head *old)
     for (; gc != finalizers; gc = gc->gc.gc_next) {
         PyObject *op = FROM_GC(gc);
 
-        if ((debug & DEBUG_SAVEALL) || has_finalizer(op)) {
+        if ((debug & DEBUG_SAVEALL) || has_legacy_finalizer(op)) {
             if (PyList_Append(garbage, op) < 0)
                 return -1;
         }
@@ -779,6 +776,62 @@ handle_finalizers(PyGC_Head *finalizers, PyGC_Head *old)
     return 0;
 }
 
+static void
+finalize_garbage(PyGC_Head *collectable, PyGC_Head *old)
+{
+    destructor finalize;
+    PyGC_Head *gc = collectable->gc.gc_next;
+
+    for (; gc != collectable; gc = gc->gc.gc_next) {
+        PyObject *op = FROM_GC(gc);
+
+        if (!_PyGCHead_FINALIZED(gc) &&
+            PyType_HasFeature(Py_TYPE(op), Py_TPFLAGS_HAVE_FINALIZE) &&
+            (finalize = Py_TYPE(op)->tp_finalize) != NULL) {
+            _PyGCHead_SET_FINALIZED(gc, 1);
+            Py_INCREF(op);
+            finalize(op);
+            if (Py_REFCNT(op) == 1) {
+                /* op will be destroyed */
+                gc = gc->gc.gc_prev;
+            }
+            Py_DECREF(op);
+        }
+    }
+}
+
+/* Walk the collectable list and check that they are really unreachable
+   from the outside (some objects could have been resurrected by a
+   finalizer). */
+static int
+check_garbage(PyGC_Head *collectable)
+{
+    PyGC_Head *gc;
+    for (gc = collectable->gc.gc_next; gc != collectable;
+         gc = gc->gc.gc_next) {
+        _PyGCHead_SET_REFS(gc, Py_REFCNT(FROM_GC(gc)));
+        assert(_PyGCHead_REFS(gc) != 0);
+    }
+    subtract_refs(collectable);
+    for (gc = collectable->gc.gc_next; gc != collectable;
+         gc = gc->gc.gc_next) {
+        assert(_PyGCHead_REFS(gc) >= 0);
+        if (_PyGCHead_REFS(gc) != 0)
+            return -1;
+    }
+    return 0;
+}
+
+static void
+revive_garbage(PyGC_Head *collectable)
+{
+    PyGC_Head *gc;
+    for (gc = collectable->gc.gc_next; gc != collectable;
+         gc = gc->gc.gc_next) {
+        _PyGCHead_SET_REFS(gc, GC_REACHABLE);
+    }
+}
+
 /* Break reference cycles by clearing the containers involved.  This is
  * tricky business as the lists can be changing and we don't know which
  * objects may be freed.  It is possible I screwed something up here.
@@ -792,7 +845,6 @@ delete_garbage(PyGC_Head *collectable, PyGC_Head *old)
         PyGC_Head *gc = collectable->gc.gc_next;
         PyObject *op = FROM_GC(gc);
 
-        assert(IS_TENTATIVELY_UNREACHABLE(op));
         if (debug & DEBUG_SAVEALL) {
             PyList_Append(garbage, op);
         }
@@ -806,7 +858,7 @@ delete_garbage(PyGC_Head *collectable, PyGC_Head *old)
         if (collectable->gc.gc_next == gc) {
             /* object is still alive, move it, it may die later */
             gc_list_move(gc, old);
-            gc->gc.gc_refs = GC_REACHABLE;
+            _PyGCHead_SET_REFS(gc, GC_REACHABLE);
         }
     }
 }
@@ -929,19 +981,15 @@ collect(int generation, Py_ssize_t *n_collected, Py_ssize_t *n_uncollectable,
     }
 
     /* All objects in unreachable are trash, but objects reachable from
-     * finalizers can't safely be deleted.  Python programmers should take
-     * care not to create such things.  For Python, finalizers means
-     * instance objects with __del__ methods.  Weakrefs with callbacks
-     * can also call arbitrary Python code but they will be dealt with by
-     * handle_weakrefs().
+     * legacy finalizers (e.g. tp_del) can't safely be deleted.
      */
     gc_list_init(&finalizers);
-    move_finalizers(&unreachable, &finalizers);
-    /* finalizers contains the unreachable objects with a finalizer;
+    move_legacy_finalizers(&unreachable, &finalizers);
+    /* finalizers contains the unreachable objects with a legacy finalizer;
      * unreachable objects reachable *from* those are also uncollectable,
      * and we move those into the finalizers list too.
      */
-    move_finalizer_reachable(&finalizers);
+    move_legacy_finalizer_reachable(&finalizers);
 
     /* Collect statistics on collectable objects found and print
      * debugging information.
@@ -957,11 +1005,20 @@ collect(int generation, Py_ssize_t *n_collected, Py_ssize_t *n_uncollectable,
     /* Clear weakrefs and invoke callbacks as necessary. */
     m += handle_weakrefs(&unreachable, old);
 
-    /* Call tp_clear on objects in the unreachable set.  This will cause
-     * the reference cycles to be broken.  It may also cause some objects
-     * in finalizers to be freed.
-     */
-    delete_garbage(&unreachable, old);
+    /* Call tp_finalize on objects which have one. */
+    finalize_garbage(&unreachable, old);
+
+    if (check_garbage(&unreachable)) {
+        revive_garbage(&unreachable);
+        gc_list_merge(&unreachable, old);
+    }
+    else {
+        /* Call tp_clear on objects in the unreachable set.  This will cause
+         * the reference cycles to be broken.  It may also cause some objects
+         * in finalizers to be freed.
+         */
+        delete_garbage(&unreachable, old);
+    }
 
     /* Collect statistics on uncollectable objects found and print
      * debugging information. */
@@ -992,7 +1049,7 @@ collect(int generation, Py_ssize_t *n_collected, Py_ssize_t *n_uncollectable,
      * reachable list of garbage.  The programmer has to deal with
      * this if they insist on creating this type of structure.
      */
-    (void)handle_finalizers(&finalizers, old);
+    (void)handle_legacy_finalizers(&finalizers, old);
 
     /* Clear free list only during the collection of the highest
      * generation */
@@ -1662,7 +1719,8 @@ _PyObject_GC_Malloc(size_t basicsize)
         sizeof(PyGC_Head) + basicsize);
     if (g == NULL)
         return PyErr_NoMemory();
-    g->gc.gc_refs = GC_UNTRACKED;
+    g->gc.gc_refs = 0;
+    _PyGCHead_SET_REFS(g, GC_UNTRACKED);
     generations[0].count++; /* number of allocated GC objects */
     if (generations[0].count > generations[0].threshold &&
         enabled &&