macos-dgram-weirdness.py

import socket
import errno
import select

a, b = socket.socketpair(family=socket.AF_UNIX, type=socket.SOCK_DGRAM)
a.setblocking(False)

try:
    while True:
        a.send(b"hi")
except OSError as exc:
    print("send failed with errno {} ({})".format(exc.errno, errno.errorcode[exc.errno]))

print("checking for writability with select")
r, w, e = select.select([], [a], [])
if w:
    print("select says socket is writable")

print("checking for writability with kqueue")
kq = select.kqueue()
ke = select.kevent(a, select.KQ_FILTER_WRITE, select.KQ_EV_ADD)
events = kq.control([ke], 1)
if events:
    print("kqueue says socket is writable ({!r})".format(events))

print("trying to send again")
a.send(b"asdf")