gh-144295: Fix data race in dict method lookup and global load

[colesbury]

In _PyDict_GetMethodStackRef, only use the fast-path unicode lookup when the dict is owned by the current thread or already marked as shared. This prevents a race between the lookup and concurrent dict resizes, which may free the PyDictKeysObject (i.e., it ensures that the resize uses QSBR).

Address a similar issue in _Py_dict_lookup_threadsafe_stackref by calling ensure_shared_on_read().

• Issue: Possible data race in new_keys_object in dictobject.c #144295

[miss-islington-app]

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

[miss-islington-app]

Sorry, @colesbury, I could not cleanly backport this to 3.14 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker e666a01ef42939f77f4c22ca47a610df5ef8b7ab 3.14

[bedevere-app]

GH-144346 is a backport of this pull request to the 3.14 branch.

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot x86 Debian Installed with X 3.14 (no tier) has failed when building commit 06f9c8c.

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/1793/builds/902) and take a look at the build logs.
4. Check if the failure is related to this commit (06f9c8c) 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/1793/builds/902

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

==

Click to see traceback logs

Traceback (most recent call last):
  File �[35m"/buildbot/buildarea/3.14.ware-debian-x86.installed/build/target/lib/python3.14/test/test_annotationlib.py"�[0m, line �[35m161�[0m, in �[35mtest_nonexistent_attribute�[0m
    �[31mself.assertEqual�[0m�[1;31m(epsilon_anno, support.EqualToForwardRef("some | {obj, module}", owner=f))�[0m
    �[31m~~~~~~~~~~~~~~~~�[0m�[1;31m^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^�[0m
�[1;35mAssertionError�[0m: �[35mForwardRef('some | {module, obj}', owner=[79 chars]1e0>) != EqualToForwardRef('some | {obj, module}',[86 chars]1e0>)�[0m


Traceback (most recent call last):
  File �[35m"/buildbot/buildarea/3.14.ware-debian-x86.installed/build/target/lib/python3.14/test/test_annotationlib.py"�[0m, line �[35m161�[0m, in �[35mtest_nonexistent_attribute�[0m
    �[31mself.assertEqual�[0m�[1;31m(epsilon_anno, support.EqualToForwardRef("some | {obj, module}", owner=f))�[0m
    �[31m~~~~~~~~~~~~~~~~�[0m�[1;31m^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^�[0m
�[1;35mAssertionError�[0m: �[35mForwardRef('some | {module, obj}', owner=[79 chars]868>) != EqualToForwardRef('some | {obj, module}',[86 chars]868>)�[0m