Multiprocessing module hangs on os.read() on Linux

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-12-13.23:37:58.808>
labels = ['type-bug', '3.9', 'expert-IO']
title = 'Multiprocessing module hangs on os.read() on Linux'
updated_at = <Date 2020-12-14.18:26:27.351>
user = 'https://bugs.python.org/to7m'

bugs.python.org fields:

activity = <Date 2020-12-14.18:26:27.351>
actor = 'to7m'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['IO']
creation = <Date 2020-12-13.23:37:58.808>
creator = 'to7m'
dependencies = []
files = []
hgrepos = []
issue_num = 42631
keywords = []
message_count = 1.0
messages = ['382945']
nosy_count = 1.0
nosy_names = ['to7m']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue42631'
versions = ['Python 3.9']