set_exception_handler is not handling exception as expected.

• uvloop version: 0.14.0
• Python version: 3.6.9
• Platform: Linux c965e3d4adf2 5.3.0-1032-gcp #34~18.04.1-Ubuntu SMP
• Can you reproduce the bug with PYTHONASYNCIODEBUG in env?: yes
• Does uvloop behave differently from vanilla asyncio? How?:

Yes asyncio default loop catches TimeoutError: [Errno 110] Connection timed out whereas uvloop closes the loop and throws RuntimeError: Event loop stopped before Future completed.

I ran across this bug when dealing with websockets (you could see the issue - https://github.com/aaugustin/websockets/issues/823)

To log this error :

def handle_async_exception(loop, ctx):
    logger.error("Loop error occured", ctx=ctx)
loop = asyncio.get_event_loop()
loop.set_exception_handler(handle_async_exception)

Python websockets throw a TimeoutError when a client is not found in a certain time limit (for ping/ pongs). In asyncio, the error is bubbled to set_exception_handler, and I could log it there as expected. However in uvloop, instead of having the exception being bubbled to exception_handler, it shuts down the loop. I get the following error:

Process Process-9:
Traceback (most recent call last):
  File "/usr/lib/python3.6/multiprocessing/process.py", line 258, in _bootstrap
    self.run()
  File "/usr/lib/python3.6/multiprocessing/process.py", line 93, in run
    self._target(*self._args, **self._kwargs)
  File "/home/streaming/app/emitter_app/emitter.py", line 60, in start_emitter
    loop.run_until_complete(asyncio.gather(server.serve(), push_payload(queue)))
  File "uvloop/loop.pyx", line 1454, in uvloop.loop.Loop.run_until_complete
RuntimeError: Event loop stopped before Future completed.

Thanks for your time! Appreciate such a neat package to make asyncio faster in python.

首先，将 uvloop/loop.pyx 中 run_until_complete 附近的代码（第 1454 行）与 vanilla asyncio 对 set_exception_handler 的处理进行比较。在启用 PYTHONASYNCIODEBUG 的 Python 3.6.9 下复现 websockets 超时场景；当 TimeoutError 到达已配置的异常处理器且不会意外停止事件循环时，即表示完成。