I tried to track this down, but ran out of time.  Here's
the little that I discovered;  maybe someone else with
access to OS X (which I have) and an understanding of
poll (which I lack) can build on this.

The failure has to do with select.poll differences between Linux
and OS X.  It seems that OS X can return POLLIN | POLLPRI | POLLHUP
in situations where Linux just returns POLLIN, and this combination
of flags causes the code in asyncore to first close the socket and
then try to read from it.  The relevant code is at around line 440
of Lib/asyncore.py.