[Async-sig] PEP: asynchronous generators

[Герасимов Михаил]

>     When an asynchronous generator is about to be garbage collected,
>     it calls its cached finalizer.  The assumption is that the finalizer
>     will schedule an ``aclose()`` call with the loop that was active
>     when the iteration started.
>
>     For instance, here is how asyncio can be modified to allow
>     safe finalization of asynchronous generators::
>
>         # asyncio/base_events.py
>
>         class BaseEventLoop:
>
>             def run_forever(self):
>                 ...
>                 old_finalizer = sys.get_asyncgen_finalizer()
>                 sys.set_asyncgen_finalizer(self._finalize_asyncgen)
>                 try:
>                     ...
>                 finally:
>                     sys.set_asyncgen_finalizer(old_finalizer)
>                     ...
>
>             def _finalize_asyncgen(self, gen):
>                 self.create_task(gen.aclose())
>
>     ``sys.set_asyncgen_finalizer`` is thread-specific, so several event
>     loops running in parallel threads can use it safely.

When asynchronous generator is about to be garbage collected event loop 
starts task to execute aclose, right? Can't such situation happen when 
this task is not finished at the moment event loop is closed? Something 
like:

    async def gen():
         try:
             yield 1
             yield 2
             yield 3
         except GeneratorExit as exc:
             await asyncio.sleep(100)
             raise exc


    async def main():
         async for i in gen():
             if i == 1:
                 break

         # main() and event loop is about to finish here,
         # while task to aclose will last much longer.


    if __name__ == "__main__":
         loop = asyncio.get_event_loop()
         loop.run_until_complete(main())

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