bpo-35983: skip trashcan for subclasses (GH-11841)

Add new trashcan macros to deal with a double deallocation that could occur when the `tp_dealloc` of a subclass calls the `tp_dealloc` of a base class and that base class uses the trashcan mechanism.

Patch by Jeroen Demeyer.

Objects/descrobject.c

@@ -1021,11 +1021,11 @@ static void
 wrapper_dealloc(wrapperobject *wp)
 {
     PyObject_GC_UnTrack(wp);
-    Py_TRASHCAN_SAFE_BEGIN(wp)
+    Py_TRASHCAN_BEGIN(wp, wrapper_dealloc)
     Py_XDECREF(wp->descr);
     Py_XDECREF(wp->self);
     PyObject_GC_Del(wp);
-    Py_TRASHCAN_SAFE_END(wp)
+    Py_TRASHCAN_END
 }
 
 static PyObject *

Objects/dictobject.c

@@ -1978,7 +1978,7 @@ dict_dealloc(PyDictObject *mp)
 
     /* bpo-31095: UnTrack is needed before calling any callbacks */
     PyObject_GC_UnTrack(mp);
-    Py_TRASHCAN_SAFE_BEGIN(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++) {
@@ -1996,7 +1996,7 @@ dict_dealloc(PyDictObject *mp)
         free_list[numfree++] = mp;
     else
         Py_TYPE(mp)->tp_free((PyObject *)mp);
-    Py_TRASHCAN_SAFE_END(mp)
+    Py_TRASHCAN_END
 }
 
 

Objects/listobject.c

@@ -361,7 +361,7 @@ list_dealloc(PyListObject *op)
 {
     Py_ssize_t i;
     PyObject_GC_UnTrack(op);
-    Py_TRASHCAN_SAFE_BEGIN(op)
+    Py_TRASHCAN_BEGIN(op, list_dealloc)
     if (op->ob_item != NULL) {
         /* Do it backwards, for Christian Tismer.
            There's a simple test case where somehow this reduces
@@ -377,7 +377,7 @@ list_dealloc(PyListObject *op)
         free_list[numfree++] = op;
     else
         Py_TYPE(op)->tp_free((PyObject *)op);
-    Py_TRASHCAN_SAFE_END(op)
+    Py_TRASHCAN_END
 }
 
 static PyObject *

Objects/odictobject.c

@@ -1356,28 +1356,17 @@ static PyGetSetDef odict_getset[] = {
 static void
 odict_dealloc(PyODictObject *self)
 {
-    PyThreadState *tstate = _PyThreadState_GET();
-
     PyObject_GC_UnTrack(self);
-    Py_TRASHCAN_SAFE_BEGIN(self)
+    Py_TRASHCAN_BEGIN(self, odict_dealloc)
 
     Py_XDECREF(self->od_inst_dict);
     if (self->od_weakreflist != NULL)
         PyObject_ClearWeakRefs((PyObject *)self);
 
     _odict_clear_nodes(self);
-
-    /* Call the base tp_dealloc().  Since it too uses the trashcan mechanism,
-     * temporarily decrement trash_delete_nesting to prevent triggering it
-     * and putting the partially deallocated object on the trashcan's
-     * to-be-deleted-later list.
-     */
-    --tstate->trash_delete_nesting;
-    assert(_tstate->trash_delete_nesting < PyTrash_UNWIND_LEVEL);
     PyDict_Type.tp_dealloc((PyObject *)self);
-    ++tstate->trash_delete_nesting;
 
-    Py_TRASHCAN_SAFE_END(self)
+    Py_TRASHCAN_END
 }
 
 /* tp_repr */

Objects/setobject.c

@@ -559,7 +559,7 @@ set_dealloc(PySetObject *so)
 
     /* bpo-31095: UnTrack is needed before calling any callbacks */
     PyObject_GC_UnTrack(so);
-    Py_TRASHCAN_SAFE_BEGIN(so)
+    Py_TRASHCAN_BEGIN(so, set_dealloc)
     if (so->weakreflist != NULL)
         PyObject_ClearWeakRefs((PyObject *) so);
 
@@ -572,7 +572,7 @@ set_dealloc(PySetObject *so)
     if (so->table != so->smalltable)
         PyMem_DEL(so->table);
     Py_TYPE(so)->tp_free(so);
-    Py_TRASHCAN_SAFE_END(so)
+    Py_TRASHCAN_END
 }
 
 static PyObject *

Objects/tupleobject.c

@@ -240,7 +240,7 @@ tupledealloc(PyTupleObject *op)
     Py_ssize_t i;
     Py_ssize_t len =  Py_SIZE(op);
     PyObject_GC_UnTrack(op);
-    Py_TRASHCAN_SAFE_BEGIN(op)
+    Py_TRASHCAN_BEGIN(op, tupledealloc)
     if (len > 0) {
         i = len;
         while (--i >= 0)
@@ -259,7 +259,7 @@ tupledealloc(PyTupleObject *op)
     }
     Py_TYPE(op)->tp_free((PyObject *)op);
 done:
-    Py_TRASHCAN_SAFE_END(op)
+    Py_TRASHCAN_END
 }
 
 static PyObject *

Objects/typeobject.c

@@ -1120,7 +1120,6 @@ subtype_dealloc(PyObject *self)
 {
     PyTypeObject *type, *base;
     destructor basedealloc;
-    PyThreadState *tstate = _PyThreadState_GET();
     int has_finalizer;
 
     /* Extract the type; we expect it to be a heap type */
@@ -1174,11 +1173,7 @@ subtype_dealloc(PyObject *self)
     /* UnTrack and re-Track around the trashcan macro, alas */
     /* See explanation at end of function for full disclosure */
     PyObject_GC_UnTrack(self);
-    ++_PyRuntime.gc.trash_delete_nesting;
-    ++ tstate->trash_delete_nesting;
-    Py_TRASHCAN_SAFE_BEGIN(self);
-    --_PyRuntime.gc.trash_delete_nesting;
-    -- tstate->trash_delete_nesting;
+    Py_TRASHCAN_BEGIN(self, subtype_dealloc);
 
     /* Find the nearest base with a different tp_dealloc */
     base = type;
@@ -1271,11 +1266,7 @@ subtype_dealloc(PyObject *self)
       Py_DECREF(type);
 
   endlabel:
-    ++_PyRuntime.gc.trash_delete_nesting;
-    ++ tstate->trash_delete_nesting;
-    Py_TRASHCAN_SAFE_END(self);
-    --_PyRuntime.gc.trash_delete_nesting;
-    -- tstate->trash_delete_nesting;
+    Py_TRASHCAN_END
 
     /* Explanation of the weirdness around the trashcan macros:
 
@@ -1312,67 +1303,6 @@ subtype_dealloc(PyObject *self)
           looks like trash to gc too, and gc also tries to delete self
           then.  But we're already deleting self.  Double deallocation is
           a subtle disaster.
-
-       Q. Why the bizarre (net-zero) manipulation of
-          _PyRuntime.trash_delete_nesting around the trashcan macros?
-
-       A. Some base classes (e.g. list) also use the trashcan mechanism.
-          The following scenario used to be possible:
-
-          - suppose the trashcan level is one below the trashcan limit
-
-          - subtype_dealloc() is called
-
-          - the trashcan limit is not yet reached, so the trashcan level
-        is incremented and the code between trashcan begin and end is
-        executed
-
-          - this destroys much of the object's contents, including its
-        slots and __dict__
-
-          - basedealloc() is called; this is really list_dealloc(), or
-        some other type which also uses the trashcan macros
-
-          - the trashcan limit is now reached, so the object is put on the
-        trashcan's to-be-deleted-later list
-
-          - basedealloc() returns
-
-          - subtype_dealloc() decrefs the object's type
-
-          - subtype_dealloc() returns
-
-          - later, the trashcan code starts deleting the objects from its
-        to-be-deleted-later list
-
-          - subtype_dealloc() is called *AGAIN* for the same object
-
-          - at the very least (if the destroyed slots and __dict__ don't
-        cause problems) the object's type gets decref'ed a second
-        time, which is *BAD*!!!
-
-          The remedy is to make sure that if the code between trashcan
-          begin and end in subtype_dealloc() is called, the code between
-          trashcan begin and end in basedealloc() will also be called.
-          This is done by decrementing the level after passing into the
-          trashcan block, and incrementing it just before leaving the
-          block.
-
-          But now it's possible that a chain of objects consisting solely
-          of objects whose deallocator is subtype_dealloc() will defeat
-          the trashcan mechanism completely: the decremented level means
-          that the effective level never reaches the limit.      Therefore, we
-          *increment* the level *before* entering the trashcan block, and
-          matchingly decrement it after leaving.  This means the trashcan
-          code will trigger a little early, but that's no big deal.
-
-       Q. Are there any live examples of code in need of all this
-          complexity?
-
-       A. Yes.  See SF bug 668433 for code that crashed (when Python was
-          compiled in debug mode) before the trashcan level manipulations
-          were added.  For more discussion, see SF patches 581742, 575073
-          and bug 574207.
     */
 }