This is Neil's fix for SF bug 535905 (Evil Trashcan and GC interaction).

The fix makes it possible to call PyObject_GC_UnTrack() more than once
on the same object, and then move the PyObject_GC_UnTrack() call to
*before* the trashcan code is invoked.

BUGFIX CANDIDATE!

Modules/gcmodule.c

@@ -819,7 +819,9 @@ _PyObject_GC_Track(PyObject *op)
 void
 _PyObject_GC_UnTrack(PyObject *op)
 {
-	_PyObject_GC_UNTRACK(op);
+	PyGC_Head *gc = AS_GC(op);
+	if (gc->gc.gc_next != NULL)
+		_PyObject_GC_UNTRACK(op);
 }
 
 PyObject *

Objects/dictobject.c

@@ -694,8 +694,8 @@ dict_dealloc(register dictobject *mp)
 {
 	register dictentry *ep;
 	int fill = mp->ma_fill;
+ 	PyObject_GC_UnTrack(mp);
 	Py_TRASHCAN_SAFE_BEGIN(mp)
- 	_PyObject_GC_UNTRACK(mp);
 	for (ep = mp->ma_table; fill > 0; ep++) {
 		if (ep->me_key) {
 			--fill;

Objects/frameobject.c

@@ -67,8 +67,8 @@ frame_dealloc(PyFrameObject *f)
 	PyObject **fastlocals;
 	PyObject **p;
 
+ 	PyObject_GC_UnTrack(f);
 	Py_TRASHCAN_SAFE_BEGIN(f)
-	_PyObject_GC_UNTRACK(f);
 	/* Kill all local variables */
 	slots = f->f_nlocals + f->f_ncells + f->f_nfreevars;
 	fastlocals = f->f_localsplus;

Objects/listobject.c

@@ -195,8 +195,8 @@ static void
 list_dealloc(PyListObject *op)
 {
 	int i;
+	PyObject_GC_UnTrack(op);
 	Py_TRASHCAN_SAFE_BEGIN(op)
-	_PyObject_GC_UNTRACK(op);
 	if (op->ob_item != NULL) {
 		/* Do it backwards, for Christian Tismer.
 		   There's a simple test case where somehow this reduces

Objects/tupleobject.c

@@ -139,8 +139,8 @@ tupledealloc(register PyTupleObject *op)
 {
 	register int i;
 	register int len =  op->ob_size;
+	PyObject_GC_UnTrack(op);
 	Py_TRASHCAN_SAFE_BEGIN(op)
-	_PyObject_GC_UNTRACK(op);
 	if (len > 0) {
 		i = len;
 		while (--i >= 0)