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Just as they do today, all externally created threads will still go to *one* interpreter when they hit PyGILState_Ensure(). It's just that interpreter won't be the main one.
Since the whole point of the PyGILState API is to support threads that don't have a previously created thread state, there's no getting around the requirement to have a single blessed interpreter that handles all externally created threads in a given process.
It will be up to mod_wsgi (and any other embedding application that uses the new function) to make sure it calls this at a time when there aren't any existing calls to PyGILState that would be disrupted. (Assuming we can't figure out a locking scheme that *ensures* no such threads are running when the switch occurs)