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> by the time the relevant C stack frame goes away, ReleaseBuffer should 
> already have been called.
Hmm. I'm not sure I understand how/when that would happen. Looking at the current py3k code, in Objects/memoryobject.c at line 92, we have:
PyObject *
PyMemoryView_FromObject(PyObject *base)
{
 PyMemoryViewObject *mview;
 Py_buffer view;
 if (!PyObject_CheckBuffer(base)) {
 PyErr_SetString(PyExc_TypeError,
 "cannot make memory view because object does "
 "not have the buffer interface");
 return NULL;
 }
 if (PyObject_GetBuffer(base, &view, PyBUF_FULL_RO) < 0)
 return NULL;
 mview = (PyMemoryViewObject *)PyMemoryView_FromBuffer(&view);
 if (mview == NULL) {
 PyBuffer_Release(&view);
 return NULL;
 }
 return (PyObject *)mview;
}
So here 'view' is being allocated on the stack, and its address passed to PyObject_GetBuffer; PyBuffer_Release isn't called (except when an error happens) before the function exits and the stack frame becomes invalid.
Sorry for the odd questions; it's clear to me that I'm misunderstanding something fundamental, but I'm struggling to figure out what.