Chris Jerdonek <chris.jerdo...@gmail.com> added the comment:
To start out sharing what I found in the Django code: Here inside BaseHandler._get_response_async(): https://github.com/django/django/blob/3460ea49e839fd6bb924c48eaa1cd3d6dc888035/django/core/handlers/base.py#L226-L232 try: response = await wrapped_callback(request, *callback_args, **callback_kwargs) except Exception as e: response = await sync_to_async( # This line hangs. self.process_exception_by_middleware, thread_sensitive=True, )(e, request) you can see an exception being handled, which is then passed to process_exception_by_middleware(). Process_exception_by_middleware() can wind up re-raising that same exception, which causes __context__ to be set circularly inside the except block: https://github.com/django/django/blob/3460ea49e839fd6bb924c48eaa1cd3d6dc888035/django/core/handlers/base.py#L323-L332 If you boil this down, you get the following as a simple reproducer. This doesn't hang, but you can tell the difference by comparing exc2 to exc2.__context as indicated below: import asyncio async def process_exc(exc): raise exc async def run(): try: raise RuntimeError except Exception as exc: task = asyncio.create_task(process_exc(exc)) try: await task except BaseException as exc2: # Prints True in 3.9.0b1 and False in 3.9.0a6. print(exc2 is exc2.__context__) loop = asyncio.new_event_loop() try: loop.run_until_complete(run()) finally: loop.close() The cause is probably the following PR, which enabled exception chaining for gen.throw() in the yield from case: https://github.com/python/cpython/pull/19858 So the answer might be to do some cycle detection when chaining the exception, which apparently _PyErr_ChainExceptions() doesn't do. ---------- _______________________________________ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue40696> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
