> What downsides do we see raising an exception?

Yeah, on second thought, that would probably be best. We still want PR #15641 as well, so the exception doesn't actually happen in practice.

Note that _PyEval_EvalCodeWithName (the main use of func_globals) already fails with an exception if globals is NULL.

> Either we remove tp_clear or somehow we have to protect all callers of the function. Technically, any field cleared from tp_cleared leaves the function in an inconsistent state

tp_clear has a purpose; it prevents memory leaks. That leaves protecting access to the cleared fields.

(But we don't need to clear func_name and func_qualname as these must be strings.)