fix: make Mixologist async-safe, deprecate ObjectAggregator

[irfanuddinahmad]

Summary

• Removes threading.RLock (_CLASS_CACHE_LOCK) from Mixologist's class cache, making mix() safe for ASGI/async deployments. dict.setdefault() atomicity (via CPython's GIL) provides equivalent thread safety without ever blocking the event loop.
• Deprecates ObjectAggregator with a DeprecationWarning — it has no production callers anywhere in XBlock or edx-platform and will be removed in a future major release.

Why the lock had to go

In ASGI (single event-loop thread), calling with threading.RLock(): stalls the entire event loop if another operation holds the lock, blocking every concurrent request. This is one of the threading hazards catalogued in the parent epic.

Two reasons no replacement lock is needed:

1. asyncio is cooperative — mix() has zero await points, so the scheduler can never interleave two calls on the same thread.
2. CPython's GIL makes dict.setdefault() atomic — under true multi-threading, two racing threads may both construct the type(...) object, but setdefault guarantees only one is stored. The other is discarded. This is a benign, one-time cost.

Test plan

• All existing TestMixologist tests pass unchanged (caching and mixin behaviour is identical)
• TestObjectAggregator tests pass and assert the new DeprecationWarning fires on instantiation
• grep threading xblock/runtime.py returns nothing

Closes #918
Part of openedx/openedx-platform#38680

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[Copilot]

Pull request overview

This PR updates XBlock’s runtime utilities to be safer in ASGI/async environments by removing a blocking thread lock from Mixologist’s class cache, and formally deprecates ObjectAggregator with a DeprecationWarning.

Changes:

• Removed threading.RLock usage around the Mixologist.mix() class cache and replaced it with a lock-free dict.setdefault()-based approach.
• Added a DeprecationWarning on ObjectAggregator instantiation and updated tests to assert the warning.
• Documented both changes in CHANGELOG.rst.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
xblock/runtime.py	Removes the lock from Mixologist class caching and adds a deprecation warning for ObjectAggregator.
xblock/test/test_runtime.py	Updates TestObjectAggregator to assert the new DeprecationWarning.
CHANGELOG.rst	Notes the async-safety change and the ObjectAggregator deprecation in the Unreleased section.

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[farhan]

@ormsbee PR looks good to me. It would be great if you could take a look

[ormsbee]

This looks fine to me as well, though I'm not sure if @kdmccormick wants to look at it.

This all happens at startup initialization before we start serving requests anyway, right? We're not actually mixing classes together during the request/response lifecycle, are we?

[kdmccormick]

Thank you both for reviewing.

@ormsbee , blocks are actually mixed on-demand, not at startup. If you throw a breakpoint here and start up a fresh CMS, it won't be hit until you load up a course, at which point you'll see it hit as part of the request handler once per loaded block type. Does that change your review at all?

[ormsbee]

It doesn't change my review, but it just seems like something that should happen at one time and place during startup (triggered from an AppConfig somewhere), unless there's a really compelling reason to delay it.

[ormsbee]

(With the justification that the Mixologist engages in weird magic, and weird magic should ideally happen only once and in obvious place and time.) To be clear, I'm not asking for this PR to change this behavior, especially since that's likely to be risky.