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At this point you might be thinking that Py_NewInterpreter/
Py_EndInterpreter loops are not the best tested things in the world.  You'd 
be right.

Neil's fix *does* fix the crash, but also shares PyObject's between 
interpreter states, which is a bit of a no-no, but otoh, this problem is far 
from unique to gcmodule.c.

> besides: the incref is only in gcinit, but the list can be
> created in gcinit OR in handle_finalizers (and while i was
> looking at it in the debugger it always was handle_finalizers).

That slightly misses the point -- the reason you got a crash was because 
the PyModule_AddObject initgc stole a reference to garbage, so when the 
module got cleaned up, it got deallocated despite there still being a 
reference to it in the module level global.  If initgc is never run, the 
refcount of garbage stays at 1.

> i guess the proper way to fix it would be a cleanup function
> for the gcmodule that is called by Py_Finalize.

Yeah, but quite a lot of modules could do with that, I think.  Do you have 
the energy to work on a 'proper' solution?  I don't.