[issue30985] Set closing variable in asyncore at close

[Nir Soffer]

New submission from Nir Soffer:

This is an old issue with asyncore - asyncore has a "closing" attribute,
but it was never used. Every user has to implement the closing once
logic in dispatcher subclasses.

Here is a typical fixes in user code:
- https://github.com/oVirt/vdsm/blob/master/lib/vdsm/storage/asyncevent.py#L497
- https://github.com/oVirt/vdsm/blob/master/lib/vdsm/storage/asyncevent.py#L540

Fixing closing attribute will allow fixing the races during
asyncore.poll() and asyncore.poll2(), see
https://github.com/python/cpython/pull/2764

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nosy: +Nir Soffer
versions: +Python 2.7, Python 3.5, Python 3.6, Python 3.7

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue30985>
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