Poll returns POLLOUT on Pipe read endpoint on MacOS 10.14

[samh42]

BPO	44597
Nosy	@ronaldoussoren, @ned-deily

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-07-10.11:02:26.984>
labels = ['extension-modules', 'OS-mac', 'type-bug', '3.7', '3.9']
title = 'Poll returns POLLOUT on Pipe read endpoint on MacOS 10.14'
updated_at = <Date 2021-07-13.06:35:52.184>
user = 'https://bugs.python.org/samh42'

bugs.python.org fields:

activity = <Date 2021-07-13.06:35:52.184>
actor = 'ronaldoussoren'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Extension Modules', 'macOS']
creation = <Date 2021-07-10.11:02:26.984>
creator = 'samh42'
dependencies = []
files = []
hgrepos = []
issue_num = 44597
keywords = []
message_count = 2.0
messages = ['397246', '397386']
nosy_count = 3.0
nosy_names = ['ronaldoussoren', 'ned.deily', 'samh42']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue44597'
versions = ['Python 3.7', 'Python 3.9']

[samh42]

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.

[ronaldoussoren]

Without having looked at this issue in detail...

This looks like an issue with macOS and not Python. In general API's providing access to system calls are just thin wrappers around those system calls. If this is a bug in macOS there's nothing we can do about this, other than perhaps working around the bug in stdlib usages of select.poll.

[ronaldoussoren]

I'm closing this issue because this is a platform bug, and one that's fixed in later versions of the OS at that.

@samh42