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# _siftdown(PyListObject *heap, Py_ssize_t startpos, Py_ssize_t pos)
# ...
# newitem = PyList_GET_ITEM(heap, pos);
# Py_INCREF(newitem);
# /* Follow the path to the root, moving parents down until finding
# a place newitem fits. */
# while (pos > startpos){
# parentpos = (pos - 1) >> 1;
# 1 parent = PyList_GET_ITEM(heap, parentpos);
# 2 cmp = PyObject_RichCompareBool(newitem, parent, Py_LT);
# if (cmp == -1) {
# Py_DECREF(newitem);
# return -1;
# }
# 3 if (size != PyList_GET_SIZE(heap)) {
# Py_DECREF(newitem);
# PyErr_SetString(PyExc_RuntimeError,
# "list changed size during iteration");
# return -1;
# }
# if (cmp == 0)
# break;
# 4 Py_INCREF(parent);
# ...
# 
# 1. parent isn't protected (refcnt==1)
# 2. custom compare function deletes all objects in "heap" and repopulates it with
# fresh instances. "parent" is freed
# 3. check is ineffective. Heap was mutated while preserving its size
# 4. use after free. Crash will manifest itself later.

New changeset 813854f49f9d by Raymond Hettinger in branch '3.4':
Issues #24099, #24100, and #24101: Fix free-after-use bug in heapq.
https://hg.python.org/cpython/rev/813854f49f9d 

New changeset d356e68de236 by Raymond Hettinger in branch '2.7':
Issues #24099, #24100, and #24101: Fix free-after-use bug in heapq.
https://hg.python.org/cpython/rev/d356e68de236