Problem:

If the loop() function of asyncore is invoked with poll
rather than select, the function readwrite() is used
when I/O is available on a socket.  However, this
function does not check for hangup - provided by POLLHUP.

If a socket is attempting to write, then POLLOUT never
gets set, so the socket hangs.  

Because poll() is returning immediately, but the return
value is never used, asyncore busy-loops, consuming all
available CPU.


Possible solutions:

The easy solution is to check for POLLHUP in the
readwrite() function:

        if flags & (select.POLLOUT | select.POLLHUP):
            obj.handle_write_event()

This makes the poll work exactly like the select - the
application raises a socket.error set to EPIPE.

An alternate solution - possibly more graceful - is to
invoke the handle_close() method of the object:

        if flags & select.POLLHUP:
            obj.handle_close()
        else:
            if flags & select.POLLIN:
                obj.handle_read_event()
            if flags & select.pollout:
                obj.handle_write_event()

This is incompatible with the select model, but it
means that the read and write logic is now the same for
socket hangups - handle_close() is invoked.