gh-138192: Fix Context initialization so that all subinterpreters are assigned the MISSING value.

[corona10]

• Issue: contextvars.Token.MISSING attribute not available in subinterpreters #138192

[corona10]

cc @ZeroIntensity @ericsnowcurrently

[corona10]

get_token_missing is using Singleton value so it is safe to use.

[corona10]

FYI, I locally checked that refleak tests are passed.

(.venv) ➜  cpython git:(gh-138192) ✗ ./python.exe -m test test_interpreters -R 3:3
Raised RLIMIT_NOFILE: 256 -> 1024
Using random seed: 1048920511
0:00:00 load avg: 4.18 Run 1 test sequentially in a single process
0:00:00 load avg: 4.18 [1/1] test_interpreters
beginning 6 repetitions. Showing number of leaks (. for 0 or less, X for 10 or more)
123:456
XX. 42.
test_interpreters leaked [4, 2, 0] references, sum=6 (this is fine)
test_interpreters leaked [3, 1, 0] memory blocks, sum=4 (this is fine)
0:00:40 load avg: 3.65 [1/1] test_interpreters passed in 40.9 sec

== Tests result: SUCCESS ==

1 test OK.

Total duration: 40.9 sec
Total tests: run=166 skipped=8
Total test files: run=1/1
Result: SUCCESS

[corona10]

@ZeroIntensity Would you like to take a look again :)

[ZeroIntensity]

Thanks, this looks good to me.

[miss-islington-app]

Thanks @corona10 for the PR 🌮🎉.. I'm working now to backport this PR to: 3.14.
🐍🍒⛏🤖

[bedevere-app]

GH-138505 is a backport of this pull request to the 3.14 branch.

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot AMD64 FreeBSD Refleaks 3.14 (tier-3) has failed when building commit cddb7e6.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/#/builders/1800/builds/445) and take a look at the build logs.
4. Check if the failure is related to this commit (cddb7e6) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/#/builders/1800/builds/445

Summary of the results of the build (if available):

==

Click to see traceback logs

Traceback (most recent call last):
  File "/buildbot/buildarea/3.14.ware-freebsd.refleak/build/Lib/test/support/__init__.py", line 835, in gc_collect
    gc.collect()
ResourceWarning: unclosed <socket.socket fd=8, family=2, type=1, proto=6, laddr=('127.0.0.1', 35910), raddr=('127.0.0.1', 35911)>
Task was destroyed but it is pending!
task: <Task pending name='Task-1257' coro=<BaseSelectorEventLoop._accept_connection2() done, defined at /buildbot/buildarea/3.14.ware-freebsd.refleak/build/Lib/asyncio/selector_events.py:217> wait_for=<Future pending cb=[Task.task_wakeup()]>>
Warning -- Unraisable exception
Exception ignored while calling deallocator <function _SelectorTransport.__del__ at 0x8409371d0>:
Traceback (most recent call last):
  File "/buildbot/buildarea/3.14.ware-freebsd.refleak/build/Lib/asyncio/selector_events.py", line 873, in __del__
    _warn(f"unclosed transport {self!r}", ResourceWarning, source=self)
ResourceWarning: unclosed transport <_SelectorSocketTransport closing fd=8>
k


Traceback (most recent call last):
  File "/buildbot/buildarea/3.14.ware-freebsd.refleak/build/Lib/test/support/__init__.py", line 835, in gc_collect
    gc.collect()
ResourceWarning: unclosed <socket.socket fd=9, family=2, type=1, proto=6, laddr=('127.0.0.1', 39409), raddr=('127.0.0.1', 39410)>
Task was destroyed but it is pending!
task: <Task pending name='Task-4079' coro=<BaseSelectorEventLoop._accept_connection2() done, defined at /buildbot/buildarea/3.14.ware-freebsd.refleak/build/Lib/asyncio/selector_events.py:217> wait_for=<Future pending cb=[Task.task_wakeup()]>>
Warning -- Unraisable exception
Exception ignored while calling deallocator <function _SelectorTransport.__del__ at 0x842b1b1d0>:
Traceback (most recent call last):
  File "/buildbot/buildarea/3.14.ware-freebsd.refleak/build/Lib/asyncio/selector_events.py", line 873, in __del__
    _warn(f"unclosed transport {self!r}", ResourceWarning, source=self)
ResourceWarning: unclosed transport <_SelectorSocketTransport closing fd=9>
k