Adding more info after discussion in github.

After polling readable/writeable dispatchers, asyncore.poll(2) receive a list of ready file descriptors, and invoke callbacks on the dispatcher objects.

If a dispatchers is closed and and a new dispatcher is created, the new dispatcher may get the same file descriptor. If the file descriptor was in the ready list returned from poll()/select(), asyncore may try to invoke one of the callbacks (e.g. handle_write) on the new dispatcher.

Here is an example in asycore.poll()

        r, w, e = select.select(r, w, e, timeout)

        for fd in r:
            obj = map.get(fd)
            if obj is None:
                continue
            read(obj)

read close obj, removing fd from map, then creates a new one
getting the same fd...

        for fd in w:
            obj = map.get(fd)

this get the new object from the map, instead of the closed one.

            if obj is None:
                continue
            write(obj)

invoke write on the wrong socket, which is not writable

        for fd in e:
            obj = map.get(fd)

same issue here

            if obj is None:
                continue
            _exception(obj)


asyncore.poll2() has same issue:

        r = pollster.poll(timeout)
        for fd, flags in r:
            obj = map.get(fd)
            if obj is None:
                continue
            readwrite(obj, flags)

fd may have been closed and recreated by in a previous iteration of the loop.

This issue is demonstrated in the failing test:
https://github.com/python/cpython/pull/2707/commits/5aeb0098d2347984f3a89cf036c305edd2b8382b