bpo-44897: WIP: Integrate trashcan into _Py_Dealloc

Include/cpython/object.h

@@ -456,83 +456,15 @@ PyAPI_FUNC(int) _PyObject_CheckConsistency(
     int check_content);
 
 
-/* Trashcan mechanism, thanks to Christian Tismer.
-
-When deallocating a container object, it's possible to trigger an unbounded
-chain of deallocations, as each Py_DECREF in turn drops the refcount on "the
-next" object in the chain to 0.  This can easily lead to stack overflows,
-especially in threads (which typically have less stack space to work with).
-
-A container object can avoid this by bracketing the body of its tp_dealloc
-function with a pair of macros:
-
-static void
-mytype_dealloc(mytype *p)
-{
-    ... declarations go here ...
-
-    PyObject_GC_UnTrack(p);        // must untrack first
-    Py_TRASHCAN_BEGIN(p, mytype_dealloc)
-    ... The body of the deallocator goes here, including all calls ...
-    ... to Py_DECREF on contained objects.                         ...
-    Py_TRASHCAN_END                // there should be no code after this
-}
-
-CAUTION:  Never return from the middle of the body!  If the body needs to
-"get out early", put a label immediately before the Py_TRASHCAN_END
-call, and goto it.  Else the call-depth counter (see below) will stay
-above 0 forever, and the trashcan will never get emptied.
-
-How it works:  The BEGIN macro increments a call-depth counter.  So long
-as this counter is small, the body of the deallocator is run directly without
-further ado.  But if the counter gets large, it instead adds p to a list of
-objects to be deallocated later, skips the body of the deallocator, and
-resumes execution after the END macro.  The tp_dealloc routine then returns
-without deallocating anything (and so unbounded call-stack depth is avoided).
-
-When the call stack finishes unwinding again, code generated by the END macro
-notices this, and calls another routine to deallocate all the objects that
-may have been added to the list of deferred deallocations.  In effect, a
-chain of N deallocations is broken into (N-1)/(_PyTrash_UNWIND_LEVEL-1) pieces,
-with the call stack never exceeding a depth of _PyTrash_UNWIND_LEVEL.
-
-Since the tp_dealloc of a subclass typically calls the tp_dealloc of the base
-class, we need to ensure that the trashcan is only triggered on the tp_dealloc
-of the actual class being deallocated. Otherwise we might end up with a
-partially-deallocated object. To check this, the tp_dealloc function must be
-passed as second argument to Py_TRASHCAN_BEGIN().
-*/
-
-/* Forward declarations for PyThreadState */
-struct _ts;
-
-/* Python 3.9 private API, invoked by the macros below. */
-PyAPI_FUNC(int) _PyTrash_begin(struct _ts *tstate, PyObject *op);
-PyAPI_FUNC(void) _PyTrash_end(struct _ts *tstate);
-/* Python 3.10 private API, invoked by the Py_TRASHCAN_BEGIN(). */
-PyAPI_FUNC(int) _PyTrash_cond(PyObject *op, destructor dealloc);
-
-#define Py_TRASHCAN_BEGIN_CONDITION(op, cond) \
-    do { \
-        PyThreadState *_tstate = NULL; \
-        /* If "cond" is false, then _tstate remains NULL and the deallocator \
-         * is run normally without involving the trashcan */ \
-        if (cond) { \
-            _tstate = PyThreadState_Get(); \
-            if (_PyTrash_begin(_tstate, _PyObject_CAST(op))) { \
-                break; \
-            } \
-        }
-        /* The body of the deallocator is here. */
+/* Backwards compatibility macros.  The trashcan mechanism is now integrated
+ * with _Py_Dealloc and so these do nothing.  The assert() is present to handle
+ * the case that a 'goto' statement precedes Py_TRASHCAN_END.  */
+#define Py_TRASHCAN_BEGIN(op, dealloc) \
+    do {
 #define Py_TRASHCAN_END \
-        if (_tstate) { \
-            _PyTrash_end(_tstate); \
-        } \
+    assert(1); \
     } while (0);
 
-#define Py_TRASHCAN_BEGIN(op, dealloc) \
-    Py_TRASHCAN_BEGIN_CONDITION(op, \
-        _PyTrash_cond(_PyObject_CAST(op), (destructor)dealloc))
 
 /* For backwards compatibility, these macros enable the trashcan
  * unconditionally */

Modules/_elementtree.c

@@ -664,10 +664,6 @@ 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_BEGIN(self, element_dealloc)
-
     if (self->weakreflist != NULL)
         PyObject_ClearWeakRefs((PyObject *) self);
 
@@ -677,7 +673,6 @@ element_dealloc(ElementObject* self)
 
     RELEASE(sizeof(ElementObject), "destroy element");
     Py_TYPE(self)->tp_free((PyObject *)self);
-    Py_TRASHCAN_END
 }
 
 /* -------------------------------------------------------------------- */

Modules/_io/bufferedio.c

@@ -383,7 +383,6 @@ buffered_dealloc(buffered *self)
     self->finalizing = 1;
     if (_PyIOBase_finalize((PyObject *) self) < 0)
         return;
-    _PyObject_GC_UNTRACK(self);
     self->ok = 0;
     if (self->weakreflist != NULL)
         PyObject_ClearWeakRefs((PyObject *)self);
@@ -2144,7 +2143,6 @@ bufferedrwpair_clear(rwpair *self)
 static void
 bufferedrwpair_dealloc(rwpair *self)
 {
-    _PyObject_GC_UNTRACK(self);
     if (self->weakreflist != NULL)
         PyObject_ClearWeakRefs((PyObject *)self);
     Py_CLEAR(self->reader);

Modules/_io/bytesio.c

@@ -886,7 +886,6 @@ bytesio_setstate(bytesio *self, PyObject *state)
 static void
 bytesio_dealloc(bytesio *self)
 {
-    _PyObject_GC_UNTRACK(self);
     if (self->exports > 0) {
         PyErr_SetString(PyExc_SystemError,
                         "deallocated BytesIO object has exported buffers");

Modules/_io/fileio.c

@@ -527,7 +527,6 @@ fileio_dealloc(fileio *self)
     self->finalizing = 1;
     if (_PyIOBase_finalize((PyObject *) self) < 0)
         return;
-    _PyObject_GC_UNTRACK(self);
     if (self->weakreflist != NULL)
         PyObject_ClearWeakRefs((PyObject *) self);
     Py_CLEAR(self->dict);

Modules/_io/iobase.c

@@ -355,7 +355,6 @@ iobase_dealloc(iobase *self)
         }
         return;
     }
-    _PyObject_GC_UNTRACK(self);
     if (self->weakreflist != NULL)
         PyObject_ClearWeakRefs((PyObject *) self);
     Py_CLEAR(self->dict);

Modules/_io/stringio.c

@@ -596,7 +596,6 @@ stringio_clear(stringio *self)
 static void
 stringio_dealloc(stringio *self)
 {
-    _PyObject_GC_UNTRACK(self);
     self->ok = 0;
     if (self->buf) {
         PyMem_Free(self->buf);

Modules/_io/textio.c

@@ -1431,7 +1431,6 @@ textiowrapper_dealloc(textio *self)
     if (_PyIOBase_finalize((PyObject *) self) < 0)
         return;
     self->ok = 0;
-    _PyObject_GC_UNTRACK(self);
     if (self->weakreflist != NULL)
         PyObject_ClearWeakRefs((PyObject *)self);
     textiowrapper_clear(self);

Modules/_io/winconsoleio.c

@@ -431,7 +431,6 @@ winconsoleio_dealloc(winconsoleio *self)
     self->finalizing = 1;
     if (_PyIOBase_finalize((PyObject *) self) < 0)
         return;
-    _PyObject_GC_UNTRACK(self);
     if (self->weakreflist != NULL)
         PyObject_ClearWeakRefs((PyObject *) self);
     Py_CLEAR(self->dict);

Objects/bytearrayobject.c

@@ -2371,7 +2371,6 @@ typedef struct {
 static void
 bytearrayiter_dealloc(bytesiterobject *it)
 {
-    _PyObject_GC_UNTRACK(it);
     Py_XDECREF(it->it_seq);
     PyObject_GC_Del(it);
 }

Objects/bytesobject.c

@@ -3095,7 +3095,6 @@ typedef struct {
 static void
 striter_dealloc(striterobject *it)
 {
-    _PyObject_GC_UNTRACK(it);
     Py_XDECREF(it->it_seq);
     PyObject_GC_Del(it);
 }

Objects/cellobject.c

@@ -78,7 +78,6 @@ PyCell_Set(PyObject *op, PyObject *obj)
 static void
 cell_dealloc(PyCellObject *op)
 {
-    _PyObject_GC_UNTRACK(op);
     Py_XDECREF(op->ob_ref);
     PyObject_GC_Del(op);
 }

Objects/classobject.c

@@ -234,7 +234,6 @@ method_new(PyTypeObject* type, PyObject* args, PyObject *kw)
 static void
 method_dealloc(PyMethodObject *im)
 {
-    _PyObject_GC_UNTRACK(im);
     if (im->im_weakreflist != NULL)
         PyObject_ClearWeakRefs((PyObject *)im);
     Py_DECREF(im->im_func);
@@ -448,7 +447,6 @@ instancemethod_getattro(PyObject *self, PyObject *name)
 
 static void
 instancemethod_dealloc(PyObject *self) {
-    _PyObject_GC_UNTRACK(self);
     Py_DECREF(PyInstanceMethod_GET_FUNCTION(self));
     PyObject_GC_Del(self);
 }

Objects/descrobject.c

@@ -18,7 +18,6 @@ class property "propertyobject *" "&PyProperty_Type"
 static void
 descr_dealloc(PyDescrObject *descr)
 {
-    _PyObject_GC_UNTRACK(descr);
     Py_XDECREF(descr->d_type);
     Py_XDECREF(descr->d_name);
     Py_XDECREF(descr->d_qualname);
@@ -1154,7 +1153,6 @@ static PyMethodDef mappingproxy_methods[] = {
 static void
 mappingproxy_dealloc(mappingproxyobject *pp)
 {
-    _PyObject_GC_UNTRACK(pp);
     Py_DECREF(pp->mapping);
     PyObject_GC_Del(pp);
 }
@@ -1265,12 +1263,9 @@ typedef struct {
 static void
 wrapper_dealloc(wrapperobject *wp)
 {
-    PyObject_GC_UnTrack(wp);
-    Py_TRASHCAN_BEGIN(wp, wrapper_dealloc)
     Py_XDECREF(wp->descr);
     Py_XDECREF(wp->self);
     PyObject_GC_Del(wp);
-    Py_TRASHCAN_END
 }
 
 static PyObject *
@@ -1577,7 +1572,6 @@ property_dealloc(PyObject *self)
 {
     propertyobject *gs = (propertyobject *)self;
 
-    _PyObject_GC_UNTRACK(self);
     Py_XDECREF(gs->prop_get);
     Py_XDECREF(gs->prop_set);
     Py_XDECREF(gs->prop_del);

Objects/dictobject.c

@@ -1928,9 +1928,6 @@ dict_dealloc(PyDictObject *mp)
     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_BEGIN(mp, dict_dealloc)
     if (values != NULL) {
         if (values != empty_values) {
             for (i = 0, n = mp->ma_keys->dk_nentries; i < n; i++) {
@@ -1955,7 +1952,6 @@ dict_dealloc(PyDictObject *mp)
     else {
         Py_TYPE(mp)->tp_free((PyObject *)mp);
     }
-    Py_TRASHCAN_END
 }
 
 
@@ -3570,8 +3566,6 @@ 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);
@@ -4066,8 +4060,6 @@ PyTypeObject PyDictRevIterValue_Type = {
 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);
 }

Objects/exceptions.c

@@ -89,7 +89,6 @@ BaseException_clear(PyBaseExceptionObject *self)
 static void
 BaseException_dealloc(PyBaseExceptionObject *self)
 {
-    _PyObject_GC_UNTRACK(self);
     BaseException_clear(self);
     Py_TYPE(self)->tp_free((PyObject *)self);
 }
@@ -535,7 +534,6 @@ StopIteration_clear(PyStopIterationObject *self)
 static void
 StopIteration_dealloc(PyStopIterationObject *self)
 {
-    _PyObject_GC_UNTRACK(self);
     StopIteration_clear(self);
     Py_TYPE(self)->tp_free((PyObject *)self);
 }
@@ -602,7 +600,6 @@ SystemExit_clear(PySystemExitObject *self)
 static void
 SystemExit_dealloc(PySystemExitObject *self)
 {
-    _PyObject_GC_UNTRACK(self);
     SystemExit_clear(self);
     Py_TYPE(self)->tp_free((PyObject *)self);
 }
@@ -684,7 +681,6 @@ ImportError_clear(PyImportErrorObject *self)
 static void
 ImportError_dealloc(PyImportErrorObject *self)
 {
-    _PyObject_GC_UNTRACK(self);
     ImportError_clear(self);
     Py_TYPE(self)->tp_free((PyObject *)self);
 }
@@ -1087,7 +1083,6 @@ OSError_clear(PyOSErrorObject *self)
 static void
 OSError_dealloc(PyOSErrorObject *self)
 {
-    _PyObject_GC_UNTRACK(self);
     OSError_clear(self);
     Py_TYPE(self)->tp_free((PyObject *)self);
 }
@@ -1364,7 +1359,6 @@ NameError_clear(PyNameErrorObject *self)
 static void
 NameError_dealloc(PyNameErrorObject *self)
 {
-    _PyObject_GC_UNTRACK(self);
     NameError_clear(self);
     Py_TYPE(self)->tp_free((PyObject *)self);
 }
@@ -1443,7 +1437,6 @@ AttributeError_clear(PyAttributeErrorObject *self)
 static void
 AttributeError_dealloc(PyAttributeErrorObject *self)
 {
-    _PyObject_GC_UNTRACK(self);
     AttributeError_clear(self);
     Py_TYPE(self)->tp_free((PyObject *)self);
 }
@@ -1538,7 +1531,6 @@ SyntaxError_clear(PySyntaxErrorObject *self)
 static void
 SyntaxError_dealloc(PySyntaxErrorObject *self)
 {
-    _PyObject_GC_UNTRACK(self);
     SyntaxError_clear(self);
     Py_TYPE(self)->tp_free((PyObject *)self);
 }
@@ -1988,7 +1980,6 @@ UnicodeError_clear(PyUnicodeErrorObject *self)
 static void
 UnicodeError_dealloc(PyUnicodeErrorObject *self)
 {
-    _PyObject_GC_UNTRACK(self);
     UnicodeError_clear(self);
     Py_TYPE(self)->tp_free((PyObject *)self);
 }
@@ -2429,8 +2420,6 @@ MemoryError_dealloc(PyBaseExceptionObject *self)
         return;
     }
 
-    _PyObject_GC_UNTRACK(self);
-
     struct _Py_exc_state *state = get_exc_state();
     if (state->memerrors_numfree >= MEMERRORS_SAVE) {
         Py_TYPE(self)->tp_free((PyObject *)self);

Objects/frameobject.c

@@ -615,11 +615,6 @@ static PyGetSetDef frame_getsetlist[] = {
 static void _Py_HOT_FUNCTION
 frame_dealloc(PyFrameObject *f)
 {
-    if (_PyObject_GC_IS_TRACKED(f)) {
-        _PyObject_GC_UNTRACK(f);
-    }
-
-    Py_TRASHCAN_BEGIN(f, frame_dealloc);
     PyCodeObject *co = NULL;
 
     /* Kill all local variables including specials, if we own them */
@@ -659,7 +654,6 @@ frame_dealloc(PyFrameObject *f)
     }
 
     Py_XDECREF(co);
-    Py_TRASHCAN_END;
 }
 
 static int