Tim Peters added the comment: OK! This has nothing to do with the trashcan mechanism.
The list object whose gc_next gets stomped on is not itself in a cycle. It's an empty list, and just happens to be a value in a dict, which in turn is a value in another dict. Its refcount falls to 0 as an ordinary part of its containing dict getting deallocated, and that's why the list becomes untracked. This was confusing me because the memory for the list object was apparently not deallocated: if it had been, pymalloc would have sprayed 0xdb into most of it, and gc_next would have appeared to me as 0xdbdbdbdb, not as 0. But after calling PyObject_GC_UnTrack on it (which sets gc_next to NULL), list_dealloc() just pushed the list object onto a free list, so no other kind of list destruction got done. That pretty much explains everything. Cute: it so happens that the _entire_ `collectable` list gets cleared out as a side effect of a single finalize(op); call. The iteration approach in the patch is robust against that, but it's hard to imagine that anything simpler could be. ---------- _______________________________________ Python tracker <rep...@bugs.python.org> <http://bugs.python.org/issue21435> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com