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Here's a first stab at a patch for this. A linked list is maintained which maps pthread_key_t instances to an int "key_id" that is used for the PyThread API. Each function needs to map a given key_id to the actual pthread_key_t. This adds a little bit of overhead of course, but in typical usage I don't think there are many entries in this list to loop over.
This also reverts the change from #22206 which is no longer relevant. No synchronization is provided for PyThread_create_key and PyThread_delete_key, but in typical usage that would be the user's responsibility anyways. Otherwise every PyThread_*_key function would have to have a mutex around it.