Bug report

I think the selectors.KqueueSelector, and selectors.PollSelector behave differently in Mac OS X.
I tested the following code in Python 3.10.4, Mac OS X (12.5) Apple Sillicon.

Below is a simple code that creates a named pipe, opens two descriptors for read (non-block) and write(non-block).
I register a EVENT_READ event on the read file descriptor, and write some message bytes to the pipe.
When the message is small (<8192 b), both Kqueue and Poll invokes a selector event.
However, for large messages, Poll works, but hangs with Kqueue.

import os
import selectors

def test_should_return(selector: selectors.BaseSelector, msg: bytes):
    pipe = "pipe.fifo"

    os.mkfifo(pipe)
    f1 = os.open(pipe, os.O_RDONLY | os.O_NONBLOCK)
    f2 = os.open(pipe, os.O_WRONLY | os.O_NONBLOCK)

    try:
        selector.register(f1, selectors.EVENT_READ)

        os.write(f2, msg)
        assert selector.select()  # should return as pipe can be read now

        print("Passed!")

    finally:
        os.close(f1)
        os.close(f2)
        os.remove(pipe)


test_should_return(selectors.PollSelector(), b"m")                       # Passes
test_should_return(selectors.KqueueSelector(), b"m")                 # Passes
test_should_return(selectors.PollSelector(), b"m" * 10000)          # Passes
test_should_return(selectors.KqueueSelector(), b"m" * 10000)    # Hangs

I expected that for all four cases, the function should return normally (without the selector.select function blocking). However, for Kqueue selector and full-buffer writes to the named pipe leads to an infinite block.

Your environment

Python 3.10.4, Mac OS X (12.5) Apple Sillicon.