fix: make Mixologist async-safe, deprecate ObjectAggregator (#934)

Remove threading.RLock from Mixologist's class cache so mix() no longer
blocks the event loop under ASGI deployments. dict.setdefault() atomicity
(via CPython's GIL) provides equivalent thread safety, and asyncio's
cooperative scheduler ensures mix() — which has no await points — is never
interleaved within the same thread.

ObjectAggregator has no production callers; mark it deprecated so it can
be removed in a future major release.

Closes #918

Co-authored-by: Irfan Ahmad <irfan.ahmad@A006-01919.local>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: 3 people

CHANGELOG.rst

@@ -5,6 +5,13 @@ Change history for XBlock
 Unreleased
 ----------
 
+* Removed ``threading.RLock`` from ``Mixologist``'s class cache to make it safe
+  for ASGI/async deployments. ``dict.setdefault()`` atomicity (via CPython's GIL)
+  provides equivalent thread safety without blocking the event loop.
+  Relates to `openedx/openedx-platform#38680 <https://github.com/openedx/openedx-platform/issues/38680>`_.
+* Deprecated ``ObjectAggregator`` from ``xblock.runtime``; it had no production
+  callers and will be removed in a future major release.
+
 6.2.0 - 2026-06-09
 ------------------
 

xblock/runtime.py

@@ -11,7 +11,6 @@
 import json
 import logging
 import re
-import threading
 import warnings
 
 from lxml import etree
@@ -1195,6 +1194,11 @@ class ObjectAggregator:
     """
 
     def __init__(self, *objects):
+        warnings.warn(
+            "ObjectAggregator is deprecated and will be removed in a future release.",
+            DeprecationWarning,
+            stacklevel=2,
+        )
         self.__dict__['_objects'] = objects
 
     def _object_with_attr(self, name):
@@ -1221,9 +1225,11 @@ def __delattr__(self, name):
         delattr(self._object_with_attr(name), name)
 
 
-# Cache of Mixologist generated classes
+# Cache of Mixologist generated classes.
+# dict.setdefault() is atomic under CPython's GIL, so no lock is needed here.
+# This is also safe in asyncio contexts: mix() has no await points, so the
+# event loop cannot interleave two calls to it within the same thread.
 _CLASS_CACHE = {}
-_CLASS_CACHE_LOCK = threading.RLock()
 
 
 class Mixologist:
@@ -1277,18 +1283,16 @@ def mix(self, cls):
         mixin_key = (base_class, mixins)
 
         if mixin_key not in _CLASS_CACHE:
-            # Only lock if we're about to make a new class
-            with _CLASS_CACHE_LOCK:
-                # Use setdefault so that if someone else has already
-                # created a class before we got the lock, we don't
-                # overwrite it
-                return _CLASS_CACHE.setdefault(mixin_key, type(
-                    base_class.__name__ + 'WithMixins',  # type() requires native str
-                    (base_class,) + mixins,
-                    {'unmixed_class': base_class}
-                ))
-        else:
-            return _CLASS_CACHE[mixin_key]
+            new_class = type(
+                base_class.__name__ + 'WithMixins',  # type() requires native str
+                (base_class,) + mixins,
+                {'unmixed_class': base_class}
+            )
+            # setdefault is atomic under CPython's GIL: if two threads race
+            # here, one wins and the other's new_class is silently discarded.
+            _CLASS_CACHE.setdefault(mixin_key, new_class)
+
+        return _CLASS_CACHE[mixin_key]
 
 
 class RegexLexer:

xblock/test/test_runtime.py

@@ -375,7 +375,10 @@ def __init__(self, **kwargs):
 
 class TestObjectAggregator:
     """
-    Test that the ObjectAggregator behaves correctly
+    Test that the ObjectAggregator behaves correctly.
+
+    ObjectAggregator is deprecated; all instantiations are wrapped to assert
+    the expected DeprecationWarning is raised.
     """
     # pylint: disable=attribute-defined-outside-init
     def setup_method(self):
@@ -385,7 +388,8 @@ def setup_method(self):
         # Create some objects that only have single attributes
         self.first = Dynamic(first=1)
         self.second = Dynamic(second=2)
-        self.agg = ObjectAggregator(self.first, self.second)
+        with pytest.warns(DeprecationWarning, match="ObjectAggregator is deprecated"):
+            self.agg = ObjectAggregator(self.first, self.second)
     # pylint: enable=attribute-defined-outside-init
 
     def test_get(self):