when is os.posix_spawn(setsid=True) safe?

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-04-12.17:58:10.185>
labels = ['type-feature', '3.8', 'expert-IO']
title = 'when is os.posix_spawn(setsid=True) safe?'
updated_at = <Date 2019-04-15.09:42:02.043>
user = 'https://github.com/cagney'

bugs.python.org fields:

activity = <Date 2019-04-15.09:42:02.043>
actor = 'vstinner'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['IO']
creation = <Date 2019-04-12.17:58:10.185>
creator = 'cagney'
dependencies = []
files = []
hgrepos = []
issue_num = 36619
keywords = []
message_count = 3.0
messages = ['340091', '340248', '340249']
nosy_count = 6.0
nosy_names = ['vstinner', 'methane', 'serhiy.storchaka', 'cagney', 'pablogsal', 'nanjekyejoannah']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue36619'
versions = ['Python 3.8']