[issue44597] Poll returns POLLOUT on Pipe read endpoint on MacOS 10.14

[Sam Harding]

New submission from Sam Harding <samharding42 at gmail.com>:

The behaviour of select.poll() is inconsistent across MacOS versions, on MacOS Mojave (10.14.6) registering and polling the receiving channel of mp.Pipe(duplex=False) returns the event POLLOUT (ready to write to). This is verified by a colleagues setup.

Whereas on MacOS 11 the same scenario will not have poll() return that the receiving channel is ready for writing to (POLLOUT). 

Example:
###
import select
import multiprocessing as mp
recv_end, send_end = mp.Pipe(duplex=False)
poll = select.poll()
poll.register(recv_end)
print(poll.poll(1000))
###

MacOS 10.14.6 Result: 
> [(3,4)]

MacOS 11.0.1 Result:
> []

I am assuming that the MacOS 11 behaviour is should be the expected behaviour, and that the recv connection from a Pipe should never return that it is writable.

This was tested with Python 3.9.4, and 3.7.6.

----------
components: Extension Modules, macOS
messages: 397246
nosy: ned.deily, ronaldoussoren, samh42
priority: normal
severity: normal
status: open
title: Poll returns POLLOUT on Pipe read endpoint on MacOS 10.14
type: behavior
versions: Python 3.7, Python 3.9

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue44597>
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