feat: bridge FEATURES proxy reads to @override_settings overrides

Lets @override_settings(KEY=val) be seen through both settings.KEY
and settings.FEATURES['KEY']/.get('KEY'), so tests can migrate to
the flat setting without first updating every production reader
that still does settings.FEATURES['KEY'].

We walk only the UserSettingsHolder chain that @override_settings
pushes onto django.conf.settings._wrapped, NOT the bottom Settings
layer. Django's bottom Settings is a *snapshot* of the settings
module's globals taken at init time, so it doesn't reflect runtime
mutations of the module's globals — which is exactly what legacy
patch.dict(settings.FEATURES, ...) does via proxy.ns. Reading the
snapshot would mask those mutations and break legacy tests; walking
only the explicit override layers lets both paths coexist during
the migration.

ns kept as-is. A stateless redesign (drop ns, route everything
through django.conf.settings, set FEATURES = FeaturesProxy() in
each env file) was considered but rejected for this step because
FEATURES.copy() and __iter__ are widely used to snapshot the full
set of feature flags; without ns there is no clean source of truth
for "which keys are features". Once all callers have moved off the
dict pattern the proxy can be deleted outright (every
settings.FEATURES['X'] becomes settings.X), making the stateless
refactor unnecessary.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: feanil

openedx/core/lib/features_setting_proxy.py

@@ -24,8 +24,37 @@ def __init__(self, namespace=None):
         """Store the namespace (as a dict)"""
         self.ns = namespace or {}
 
+    _NOT_OVERRIDDEN = object()
+
+    def _resolve(self, key):
+        """Return key's value if it's been overridden via @override_settings, else self._NOT_OVERRIDDEN.
+
+        We deliberately walk only the UserSettingsHolder chain (the layers that
+        @override_settings pushes onto django.conf.settings._wrapped) rather
+        than reading django.conf.settings.X directly. Django's bottom Settings
+        layer is a *snapshot* taken at init time, so it doesn't reflect runtime
+        mutations of the settings module's globals (which is what
+        proxy.ns mutations and legacy patch.dict(settings.FEATURES, ...) do).
+        Walking only the explicit override layers lets the new
+        @override_settings(X=Y) path work while leaving the legacy patch.dict
+        path untouched.
+        """
+        from django.conf import settings as django_settings
+        wrapped = django_settings._wrapped  # pylint: disable=protected-access
+        # UserSettingsHolder has a `default_settings` attribute and stores
+        # explicit overrides in its own __dict__; the bottom Settings has no
+        # default_settings, so the loop terminates there.
+        while hasattr(wrapped, 'default_settings'):
+            if key in wrapped.__dict__:
+                return wrapped.__dict__[key]
+            wrapped = wrapped.default_settings
+        return self._NOT_OVERRIDDEN
+
     def __getitem__(self, key):
-        """Retrieve a feature flag by key"""
+        """Retrieve a feature flag by key, preferring @override_settings overrides."""
+        value = self._resolve(key)
+        if value is not self._NOT_OVERRIDDEN:
+            return value
         return self.ns[key]
 
     def __setitem__(self, key, value):
@@ -49,14 +78,17 @@ def __len__(self):
         return len(self.ns)
 
     def __contains__(self, key):
-        return key in self.ns
+        return self._resolve(key) is not self._NOT_OVERRIDDEN or key in self.ns
 
     def clear(self):
         """Remove all feature flags from the namespace."""
         self.ns.clear()
 
     def get(self, key, default=None):
-        """Standard dict-style get with default"""
+        """Standard dict-style get with default; prefers @override_settings overrides."""
+        value = self._resolve(key)
+        if value is not self._NOT_OVERRIDDEN:
+            return value
         return self.ns.get(key, default)
 
     def update(self, other=(), /, **kwds):