Ok, I fixed test_writer_callback() in the tulip repo.

Indeed the error I reported first is gone now.

I logged the value of kev on failure, and got this output:

ERROR:root:kev = <select.kevent ident=83 filter=-2 flags=0x1 fflags=0x0 data=0x131750 udata=0x0>
Traceback (most recent call last):
  File "/Users/guido/tulip/tulip/selectors.py", line 178, in _key_from_fd
    return self._fd_to_key[fd]
KeyError: 83

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/guido/tulip/tulip/selectors.py", line 330, in select
    key = self._key_from_fd(fd)
  File "/Users/guido/tulip/tulip/selectors.py", line 180, in _key_from_fd
    raise RuntimeError("No key found for fd {}".format(fd))
RuntimeError: No key found for fd 83



The full traceback printed by the unittest framework was:

ERROR: test_sock_client_ops (tulip.events_test.KqueueEventLoopTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/guido/tulip/tulip/selectors.py", line 178, in _key_from_fd
    return self._fd_to_key[fd]
KeyError: 83

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/guido/tulip/tulip/events_test.py", line 168, in test_sock_client_ops
    el.run_until_complete(el.sock_sendall(sock, b'GET / HTTP/1.0\r\n\r\n'))
  File "/Users/guido/tulip/tulip/unix_events.py", line 146, in run_until_complete
    self.run()
  File "/Users/guido/tulip/tulip/unix_events.py", line 110, in run
    self._run_once()
  File "/Users/guido/tulip/tulip/unix_events.py", line 582, in _run_once
    event_list = self._selector.select(timeout)
  File "/Users/guido/tulip/tulip/selectors.py", line 330, in select
    key = self._key_from_fd(fd)
  File "/Users/guido/tulip/tulip/selectors.py", line 180, in _key_from_fd
    raise RuntimeError("No key found for fd {}".format(fd))
RuntimeError: No key found for fd 83