Immortalize Py_EMPTY_KEYS

[ericsnowcurrently]

The every dict has a keys "object", of type PyDictKeysObject. While it isn't actually a Python object, it does have a refcount, which is used to know when to free it. PyDictKeysObject (and the helpers, dictkeys_incref() and dictkeys_decref()) was not updated to be immortal when the other singletons were. When it comes to interpreter isolation, that's a problem for empty dicts.

Every empty dict shares a global, statically allocated singleton for its keys: Py_EMPTY_KEYS (AKA static PyDictKeysObject empty_keys_struct). This singleton is defined and used internally in dictobject.c, so we don't have the same ABI compatibility concerns that we have with object ref counts generally,

One way or another, we need to isolate Py_EMPTY_KEYS. Otherwise we end up with races on the refcount.

cc @eduardo-elizondo @markshannon

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Possible solutions:

1. update the code in dictobject.c to make Py_EMPTY_KEYS immortal
2. move Py_EMPTY_KEYS to PyInterpreterState

The first one seems simpler.

Linked PRs

• gh-104252: Immortalize Py_EMPTY_KEYS #104253