MainThread association logic is fragile

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2017-09-19.13:00:11.242>
labels = ['expert-subinterpreters', 'type-feature', '3.8', '3.9', 'docs']
title = 'MainThread association logic is fragile'
updated_at = <Date 2020-06-03.16:42:33.468>
user = 'https://github.com/pitrou'

bugs.python.org fields:

activity = <Date 2020-06-03.16:42:33.468>
actor = 'vstinner'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation', 'Subinterpreters']
creation = <Date 2017-09-19.13:00:11.242>
creator = 'pitrou'
dependencies = []
files = ['47153']
hgrepos = []
issue_num = 31517
keywords = []
message_count = 11.0
messages = ['302521', '302523', '302583', '302589', '302601', '302602', '302603', '302606', '347264', '347266', '358744']
nosy_count = 10.0
nosy_names = ['tim.peters', 'ncoghlan', 'pitrou', 'vstinner', 'fabioz', 'asvetlov', 'docs@python', 'eric.snow', 'aldwinaldwin', 'int19h']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue31517'
versions = ['Python 3.8', 'Python 3.9']