Internal error with recursive tasks and cancellation

I'm not really sure the cause of this nor whether we can fix this... Related to https://github.com/python-trio/trio/issues/3289

Reproducing program:

import trio

async def sub_tn():
    async with trio.open_nursery() as tn:
        tn.start_soon(trio.sleep_forever)
        tn.start_soon(sub_tn)
        try:
            await trio.sleep_forever()
        finally:
            await trio.lowlevel.checkpoint()
    

async def main():
    async with trio.open_nursery() as tn:
        tn.start_soon(trio.sleep_forever)
        tn.start_soon(sub_tn)
        await trio.sleep(2)
        tn.cancel_scope.cancel()

trio.run(main)

When I hit Ctrl+C a bunch while it runs, I get this error:

Traceback (most recent call last):
  File "C:\Users\A5rocks\Documents\trio\src\trio\_core\_run.py", line 2875, in unrolled_run
    runner.task_exited(task, final_outcome)
    ~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\A5rocks\Documents\trio\src\trio\_core\_run.py", line 2053, in task_exited
    task._parent_nursery._child_finished(task, outcome)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^
  File "C:\Users\A5rocks\Documents\trio\src\trio\_core\_run.py", line 1270, in _child_finished
    reason=f"child task raised exception {outcome.error!r}",
                                         ^^^^^^^^^^^^^^^^^
RecursionError: maximum recursion depth exceeded

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "C:\Users\A5rocks\Documents\trio\repro.py", line 23, in <module>
    trio.run(main)
    ~~~~~~~~^^^^^^
  File "C:\Users\A5rocks\Documents\trio\src\trio\_core\_run.py", line 2518, in run
    timeout = gen.send(next_send)
  File "C:\Users\A5rocks\Documents\trio\src\trio\_core\_run.py", line 2931, in unrolled_run
    raise TrioInternalError("internal error in Trio - please file a bug!") from exc
trio.TrioInternalError: internal error in Trio - please file a bug!

Start with the recursive cancellation reproducer and trace the reported path through src/trio/_core/_run.py, especially unrolled_run, task_exited, and _child_finished. Investigate why repeated Ctrl+C and nested nurseries produce a RecursionError while handling the child outcome; done means the reproducer no longer raises TrioInternalError or exceeds recursion depth.