web._cancel_all_tasks causes spurious errors on Python <3.7

[sgentle]

Long story short

Python 3.5's asyncio.Task.all_tasks differs from Python 3.7's asyncio.all_tasks in that it includes completed tasks. However, the all_tasks helper uses them interchangeably.

This means that when you shutdown a server started with web.run_app on Python 3.5 or 3.6, failed requests are rethrown as exceptions in web._cancel_all_tasks, whether they were handled or not.

Steps to reproduce

1. python3.5 -c 'from aiohttp import web; web.run_app(web.Application())'
2. visit http://localhost:8080
3. ctrl+c the server

Expected behaviour

Actual behaviour

unhandled exception during asyncio.run() shutdown
task: <Task finished coro=<Application._handle() done, defined at /usr/lib/python3.5/site-packages/aiohttp/web_app.py:405> exception=<HTTPNotFound Not Found eof>
  File "/usr/lib/python3.5/site-packages/aiohttp/web_protocol.py", line 406, in start
    resp = await task
  File "/usr/lib/python3.5/asyncio/futures.py", line 380, in __iter__
    yield self  # This tells Task to wait for completion.
  File "/usr/lib/python3.5/asyncio/tasks.py", line 304, in _wakeup
    future.result()
  File "/usr/lib/python3.5/asyncio/futures.py", line 293, in result
    raise self._exception
  File "/usr/lib/python3.5/asyncio/tasks.py", line 239, in _step
    result = coro.send(None)
  File "/usr/lib/python3.5/site-packages/aiohttp/web_app.py", line 435, in _handle
    resp = await handler(request)
  File "/usr/lib/python3.5/site-packages/aiohttp/web_urldispatcher.py", line 848, in _handle
    raise self._http_exception
aiohttp.web_exceptions.HTTPNotFound: Not Found

[aio-libs-bot]

GitMate.io thinks the contributor most likely able to help you is @asvetlov.

Possibly related issues are #3242 (Error), #1745 (Call https website through proxy will cause error), #816 (Logging error), #1839 (aiohttp webserver with python 3.7 broken), and #429 (Websocket error).

[asvetlov]

Thanks for the report.
I've committed the difference into asyncio but forgot about the behavior change on working on aiohttp.

[lock]

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