Also, it appears that in earlier versions of CPython, it was okay to set tstate->frame = NULL. Some of Greenlet's tests *seem* to be doing this (although it also possible that my current scaffolding has just broken Greenlet).

Does anybody know off the top of their head what it means to set a NULL frame? I can dig around through the old code and see what the interpreter does when that happens, but it seems pretty nonsensical to me. Is it something that only happens during interpreter/thread initialization/shutdown? Or maybe it keeps that thread from becoming active?