[Async-sig] asyncio.timeout() is not portable

[Nathaniel Smith]

On Jun 6, 2016 10:19 AM, "Yury Selivanov" <yselivanov at gmail.com> wrote:
>
>
> > On Jun 5, 2016, at 11:56 AM, Ben Darnell <ben at bendarnell.com> wrote:
> >
> > It has come to my attention (
https://github.com/KeepSafe/aiohttp/issues/877) that Python 3.5.2 (!)
introduced a new context manager asyncio.timeout, which attaches a timeout
to the current asyncio.Task. Any library or application that uses this
feature will not be portable to other coroutine runners.
> >
> >     with asyncio.timeout(1):
> >         result = await aiohttp.get('http://www.example.com')
> >
> > It's difficult to make this interface portable. We'd need to introduce
a global thread-specific object that all of the coroutine runners could
register the current task on, and define an interface that asyncio.timeout
would call on that object to set the timeout. We could get rid of the
global if the context manager used `async with` instead of `with`, since
that would give us a way to communicate with the coroutine runner directly,
but the net result is still that each runner needs to implement hooks for
this feature (the hooks may or may not be generalizable to other features
in the future).
>
> We’ve discussed this and decided that we should revert asyncio.Timeout
from 3.5.2.  I’ll do that later today.

This is tangential and somewhat irrelevant given the above, but since I'm
not sure where else to put it: having thought about this issue a bit more
last night, I also think that it's important that the timeout manager
(if/when it's reintroduced) should inject a meaningful exception like
TimeoutError, and it should be legal for the coroutine to catch this and
continue. This is in contrast to the one that's currently being reverted,
which uses the task cancellation feature and is indistinguishable from any
other use of the task cancellation feature.

-n
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