temp: switch downstream_customized from Set to List

TODO details

FAILED xmodule/tests/test_library_tools.py::ContentLibraryToolsTest::test_update_children_for_v2_lib -
bson.errors.InvalidDocument: cannot encode object: {'display_name'}, of type: <class 'set'>

cms/lib/xblock/test/test_upstream_sync.py

@@ -64,7 +64,7 @@ def test_sync_to_unmodified_content(self):
         """
         downstream = self.store.get_item(self.downstream_key)
         assert downstream.upstream_display_name == "original upstream title"
-        assert downstream.downstream_customized == set()
+        assert downstream.downstream_customized == []
         assert downstream.display_name == "original upstream title"
         assert downstream.data == "\n<p>original upstream content</p>\n"  # @@TODO newlines??
 
@@ -75,7 +75,7 @@ def test_sync_to_unmodified_content(self):
 
         self.sync_service.sync_from_upstream(downstream, apply_updates=True)
         assert downstream.upstream_display_name == "NEW upstream title"
-        assert downstream.downstream_customized == set()
+        assert downstream.downstream_customized == []
         assert downstream.display_name == "NEW upstream title"
         assert downstream.data == "\n<p>NEW upstream content</p>\n"  # @@TODO newlines??
 
@@ -95,6 +95,6 @@ def test_sync_to_modified_contenet(self):
 
         self.sync_service.sync_from_upstream(downstream, apply_updates=True)
         assert downstream.upstream_display_name == "NEW upstream title"
-        assert downstream.downstream_customized == {"display_name"}
+        assert downstream.downstream_customized == ["display_name"]
         assert downstream.display_name == "downstream OVERRIDE of the title"
         assert downstream.data == "\n<p>NEW upstream content</p>\n"  # @@TODO newlines??

cms/lib/xblock/upstream_sync.py

@@ -18,7 +18,7 @@
 from opaque_keys.edx.locator import LibraryUsageLocatorV2
 from rest_framework.exceptions import NotFound
 from xblock.exceptions import XBlockNotFoundError
-from xblock.fields import Scope, String, Integer, Set
+from xblock.fields import Scope, String, Integer, List
 from xblock.core import XBlockMixin, XBlock
 
 import openedx.core.djangoapps.xblock.api as xblock_api
@@ -168,13 +168,13 @@ class UpstreamSyncMixin(XBlockMixin):
         ),
         default=None, scope=Scope.settings, hidden=True, enforce_type=True,
     )
-    downstream_customized = Set(
+    downstream_customized = List(
         help=(
             "Names of the fields which have values set on the upstream block yet have been explicitly "
             "overridden on this downstream block. Unless explicitly cleared by the user, these customizations "
             "will persist even when updates are synced from the upstream."
         ),
-        default=set(), scope=Scope.settings, hidden=True, enforce_type=True,
+        default=[], scope=Scope.settings, hidden=True, enforce_type=True,
     )
 
     # Store upstream defaults for customizable fields.
@@ -214,7 +214,7 @@ def save(self, *args, **kwargs):
             # If this field's value doesn't match the synced upstream value, then mark the field
             # as customized so that we don't clobber it later when syncing.
             if getattr(self, field_name) != getattr(self, restore_field_name):
-                self.downstream_customized.add(field_name)
+                self.downstream_customized.append(field_name)
 
         super().save(*args, **kwargs)