`contextvars.Token.MISSING` attribute not available in subinterpreters

[mariojonke]

Bug report

Bug description:

Accessing the Token.MISSING class var from the contextvars module within a subinterpreter raises an AttributeError. E.g. the following script:

import contextvars
from concurrent.interpreters import create, get_current

print(get_current(), contextvars.Token, getattr(contextvars.Token, "MISSING", None))

ip1 = create()
ip1.exec("""
import contextvars
from concurrent.interpreters import get_current

print(get_current(), contextvars.Token, getattr(contextvars.Token, "MISSING", "'doesn't exist'"))
""")

outputs

Interpreter(0) <class '_contextvars.Token'> <Token.MISSING>
Interpreter(1) <class '_contextvars.Token'> 'doesn't exist'

CPython versions tested on:

3.14

Operating systems tested on:

Linux

Linked PRs

• gh-138192: Fix Context initialization so that all subinterpreters are assigned the MISSING value. #138503
• [3.14] gh-138192: Fix Context initialization so that all subinterpreters are assigned the MISSING value. (gh-138503) #138505

[ZeroIntensity]

I think we need to remove these lines:

https://github.com/python/cpython/blob/main/Python/context.c#L1363-L1365

The type's attribute dictionary is per-interpreter, so we need to set MISSING for all interpreters rather than just the main one.