The `PyWeakref_IsDead()` function tests if a weak reference is dead
without any side effects. Although you can also detect if a weak
reference is dead using `PyWeakref_GetRef()`, that function returns a
strong reference that must be `Py_DECREF()`'d, which can introduce side
effects if the last reference is concurrently dropped (at least in the
free threading build).
This exposes `PyUnstable_Object_ClearWeakRefsNoCallbacks` as an unstable
C-API function to provide a thread-safe mechanism for clearing weakrefs
without executing callbacks.
Some C-API extensions need to clear weakrefs without calling callbacks,
such as after running finalizers like we do in subtype_dealloc.
Previously they could use `_PyWeakref_ClearRef` on each weakref, but
that's not thread-safe in the free-threaded build.
Co-authored-by: Petr Viktorin <encukou@gmail.com>
I think that none of these API calls can fail, but only few of them are
documented as such. Add the sentence "This function always succeeds" (which is
the same already used e.g. by PyNumber_Check) to all of them.
Sam Gross