Tim Peters added the comment:
Sorry for the earlier noise. I'm fighting a flu and my head is mush :-(
Anyway, this doesn't look obvious. We get to this point:
if (Py_REFCNT(op) == 1) {
/* op will be destroyed */
gc = gc->gc.gc_prev;
}
and op is the type object for class B. gc gets set to the previous object, a
list. Everything looks fine at this point. But when we get back from:
Py_DECREF(op);
the list's gc.gc_next field has been overwritten with NULL. That's why gc gets
set to NULL on the next trip through the loop.
I spaced out stepping through all the type deallocation code, and didn't find
exactly when the list's gc_next is overwritten. The list's gc_prev is still
fine. Perhaps some code called _PyObject_GC_UNTRACK on the list object (which
NULLs out the gc_next pointer but not the gc_prev pointer).
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Python tracker <[email protected]>
<http://bugs.python.org/issue21435>
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