Clarify thread-safety of bulk_write context manager

common/lib/xmodule/xmodule/modulestore/__init__.py

@@ -646,7 +646,7 @@ class ModuleStoreWriteBase(ModuleStoreReadBase, ModuleStoreWrite):
     @contextmanager
     def bulk_write_operations(self, course_id):
         """
-        A context manager for notifying the store of bulk write events.
+        A context manager for notifying the store of bulk write events. This affects only the current thread.
 
         In the case of Mongo, it temporarily disables refreshing the metadata inheritance tree
         until the bulk operation is completed.

common/lib/xmodule/xmodule/modulestore/mongo/base.py

@@ -414,7 +414,7 @@ class MongoModuleStore(ModuleStoreDraftAndPublished, ModuleStoreWriteBase):
 
         # performance optimization to prevent updating the meta-data inheritance tree during
         # bulk write operations
-        self.ignore_write_events_on_courses = set()
+        self.ignore_write_events_on_courses = threading.local()
         self._course_run_cache = {}
 
     def close_connections(self):
@@ -439,13 +439,19 @@ class MongoModuleStore(ModuleStoreDraftAndPublished, ModuleStoreWriteBase):
         """
         Prevent updating the meta-data inheritance cache for the given course
         """
-        self.ignore_write_events_on_courses.add(course_id)
+        if not hasattr(self.ignore_write_events_on_courses.courses):
+            self.ignore_write_events_on_courses.courses = set()
+
+        self.ignore_write_events_on_courses.courses.add(course_id)
 
     def _end_bulk_write_operation(self, course_id):
         """
         Restart updating the meta-data inheritance cache for the given course.
         Refresh the meta-data inheritance cache now since it was temporarily disabled.
         """
+        if not hasattr(self.ignore_write_events_on_courses.courses):
+            return
+
         if course_id in self.ignore_write_events_on_courses:
             self.ignore_write_events_on_courses.remove(course_id)
             self.refresh_cached_metadata_inheritance_tree(course_id)
@@ -454,8 +460,11 @@ class MongoModuleStore(ModuleStoreDraftAndPublished, ModuleStoreWriteBase):
         """
         Returns whether a bulk write operation is in progress for the given course.
         """
+        if not hasattr(self.ignore_write_events_on_courses.courses):
+            return False
+
         course_id = course_id.for_branch(None)
-        return course_id in self.ignore_write_events_on_courses
+        return course_id in self.ignore_write_events_on_courses.courses
 
     def fill_in_run(self, course_key):
         """