Exception conniption - TrioInternalError from async generator.

[clopmasterc]

I don't know whether or not you need this, as it seems to be occurring because of known problematic generator usage, the original code was faulty and the advice given in the traceback does indeed fix the issue. However, trio said to report it, so here it is.

import trio
import trio_websocket                               # pip
import httpx                                        # pip

async def _websocket():
    async with trio_websocket.open_websocket_url('wss://echo.websocket.org') as w:
        await w.send_message('hi !')
        msg=await w.get_message()
        print('Websocket responded', flush=True)
        yield msg

async def main2():
    async with httpx.AsyncClient() as httpclient:
        await httpclient.get("https://example.com/")
        print('Page responded', flush=True)
        async for msg in _websocket():
            raise NameError

trio.run(main2)

Python 3.9, Trio 0.18.0

When echo.websocket.org can be bothered to reply at all, there's a > 50% chance that the following traceback will occur:

[neil@localhost ~]$ python ./trio-error.py
/home/neil/.local/lib/python3.9/site-packages/trio/_core/_multierror.py:502: RuntimeWarning: You seem to already have a custom sys.excepthook handler installed. I'll skip installing Trio's custom handler, but this means MultiErrors will not show full tracebacks.
  warnings.warn(
Page responded
Websocket responded
Traceback (most recent call last):
  File "/home/neil/.local/lib/python3.9/site-packages/trio/_core/_run.py", line 2185, in unrolled_run
    runner.task_exited(task, final_outcome)
  File "/home/neil/.local/lib/python3.9/site-packages/trio/_core/_run.py", line 1506, in task_exited
    self.tasks.remove(task)
KeyError: <Task '__main__.main2' at 0x7f1fa8d23970>

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

Traceback (most recent call last):
  File "/home/neil/./trio-error.py", line 21, in <module>
    trio.run(main2)
  File "/home/neil/.local/lib/python3.9/site-packages/trio/_core/_run.py", line 1923, in run
    timeout = gen.send(next_send)
  File "/home/neil/.local/lib/python3.9/site-packages/trio/_core/_run.py", line 2230, 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!
Exception ignored in: <function Nursery.__del__ at 0x7f1fa991d940>
Traceback (most recent call last):
  File "/home/neil/.local/lib/python3.9/site-packages/trio/_core/_run.py", line 1055, in __del__
AssertionError: 
Exception ignored in: <coroutine object Runner.init at 0x7f1fa8cf09c0>
Traceback (most recent call last):
  File "/home/neil/.local/lib/python3.9/site-packages/trio/_core/_run.py", line 1613, in init
  File "/home/neil/.local/lib/python3.9/site-packages/trio/_core/_run.py", line 802, in __aexit__
  File "/home/neil/.local/lib/python3.9/site-packages/trio/_core/_run.py", line 914, in _nested_child_finished
  File "/home/neil/.local/lib/python3.9/site-packages/trio/_core/_run.py", line 896, in _add_exc
  File "/home/neil/.local/lib/python3.9/site-packages/trio/_core/_ki.py", line 159, in wrapper
  File "/home/neil/.local/lib/python3.9/site-packages/trio/_core/_run.py", line 681, in cancel
  File "/home/neil/.local/lib/python3.9/site-packages/trio/_core/_run.py", line 357, in recalculate
  File "/home/neil/.local/lib/python3.9/site-packages/trio/_core/_run.py", line 1174, in _attempt_delivery_of_any_pending_cancel
  File "/home/neil/.local/lib/python3.9/site-packages/trio/_core/_run.py", line 1156, in _attempt_abort
  File "/home/neil/.local/lib/python3.9/site-packages/trio/_core/_io_epoll.py", line 292, in abort
  File "/home/neil/.local/lib/python3.9/site-packages/trio/_core/_io_epoll.py", line 261, in _update_registrations
ValueError: I/O operation on closed epoll object
Exception ignored in: <function Nursery.__del__ at 0x7f1fa991d940>
Traceback (most recent call last):
  File "/home/neil/.local/lib/python3.9/site-packages/trio/_core/_run.py", line 1055, in __del__
AssertionError: 
Exception ignored during finalization of async generator '__main__._websocket' -- surround your use of the generator in 'async with aclosing(...):' to raise exceptions like this in the context where they're generated
Traceback (most recent call last):
  File "/home/neil/.local/lib/python3.9/site-packages/trio/_core/_asyncgens.py", line 186, in _finalize_one
    await agen.aclose()
  File "/home/neil/.local/lib/python3.9/site-packages/trio/_core/_ki.py", line 159, in wrapper
    return fn(*args, **kwargs)
  File "/home/neil/.local/lib/python3.9/site-packages/trio/_core/_run.py", line 539, in __exit__
    remaining_error_after_cancel_scope = self._close(exc)
  File "/home/neil/.local/lib/python3.9/site-packages/trio/_core/_run.py", line 476, in _close
    scope_task = current_task()
  File "/home/neil/.local/lib/python3.9/site-packages/trio/_core/_run.py", line 2271, in current_task
    raise RuntimeError("must be called from async context") from None
RuntimeError: must be called from async context
[neil@localhost ~]$ 

[Zac-HD]

Thanks for the report! This is indeed a case of #264, which we're aiming to turn into a reliable and immediate error (python/peps#3782).