Unexpected CancelledError

[mheppner]

I get errors coming from the aiomcache connection and I'm honestly not sure what the issue is. The aiohttp server sets up a connection when the app starts up:

# app.py
app = web.Application()
app.on_startup.append(tasks.start_memcached)
app.on_cleanup.append(tasks.stop_memcached)

# tasks.py
async def start_memcached(app):
    app['memcached'] = aiomcache.Client(host, port, loop=app.loop)

async def stop_memcached(app):
    app['memcached'].cancel()

Views that use the Client() instance will sometimes fail. I have a /health endpoint that is being checked every 30 seconds or so:

# views/health.py
async def health(request):
    is_healthy = False
    try:
        await request.app['memcached'].set(key, token)
        cache_response = await request.app['memcached'].get(key)
    except Exception:
        logger.exception('cannot connect to memcache')
    else:
        is_healthy = cache_response == token

    return web.json_response({'healthy': is_healthy})

The other requests that fail have the same traceback, but this is specifically the one from the health endpoint:

Traceback (most recent call last):
  File "/app/src/views/health.py", line 35, in health
    await cache.set(key, token)
  File "/usr/local/lib/python3.6/site-packages/aiomcache/client.py", line 20, in wrapper
    return (yield from func(self, conn, *args, **kwargs))
  File "/usr/local/lib/python3.6/site-packages/aiomcache/client.py", line 252, in set
    conn, b'set', key, value, flags, exptime)
  File "/usr/local/lib/python3.6/site-packages/aiomcache/client.py", line 232, in _storage_command
    resp = yield from self._execute_simple_command(conn, cmd)
  File "/usr/local/lib/python3.6/site-packages/aiomcache/client.py", line 68, in _execute_simple_command
    line = yield from conn.reader.readline()
  File "/usr/local/lib/python3.6/asyncio/streams.py", line 488, in readline
    line = yield from self.readuntil(sep)
  File "/usr/local/lib/python3.6/asyncio/streams.py", line 581, in readuntil
    yield from self._wait_for_data('readuntil')
  File "/usr/local/lib/python3.6/asyncio/streams.py", line 464, in _wait_for_data
    yield from self._waiter
concurrent.futures._base.CancelledError

There is nothing obvious in the memcached logs.

[asvetlov]

Is it https://docs.aiohttp.org/en/stable/web_advanced.html#web-handler-cancellation ?

[mheppner]

Thank you, I used your aiojobs and it seems to have helped. Maybe a note about that section should go in the quickstart, I missed it in the advanced section. I'll reopen if I figure out if there actually is an issue with aiomcache.