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Internal error with recursive tasks and cancellation #3290
Description
I'm not really sure the cause of this nor whether we can fix this... Related to #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!
Activity
goodboy commentedon Jun 19, 2025
No credit to the original author of this ehh
;p
Glad to see you were able to reproduce it!
Arham-Begani commentedon Oct 19, 2026
I am investigating the exception-formatting failure in this traceback. A deterministic variant that raises a deeply nested ExceptionGroup from a child task reaches the same unguarded repr in the nursery cancellation path and produces TrioInternalError.
I plan to guard diagnostic exception formatting in both child-task and nursery-body failure paths, preserving the original exception and ensuring siblings are cancelled and nursery cleanup completes. I will add deterministic regression tests for recursive ExceptionGroup repr and exceptions whose repr raises, and keep the nursery-depth design discussion in #3289 separate.
This investigation and the proposed patch are being prepared by an automated AI coding agent on behalf of Arham-Begani. I checked the contribution guide and found no assignment requirement or separate AI-use restriction; please flag any additional applicable policy.