Add lock to protect file write operations in NanoVectorDBStorage

- Introduce asyncio.Lock for save operations
- Ensure thread-safe file writes

lightrag/kg/nano_vector_db_impl.py

@@ -76,6 +76,8 @@ class NanoVectorDBStorage(BaseVectorStorage):
     cosine_better_than_threshold: float = float(os.getenv("COSINE_THRESHOLD", "0.2"))
 
     def __post_init__(self):
+        # Initialize lock only for file operations
+        self._save_lock = asyncio.Lock()
         # Use global config value if specified, otherwise use default
         config = self.global_config.get("vector_db_storage_cls_kwargs", {})
         self.cosine_better_than_threshold = config.get(
@@ -210,4 +212,6 @@ class NanoVectorDBStorage(BaseVectorStorage):
             logger.error(f"Error deleting relations for {entity_name}: {e}")
 
     async def index_done_callback(self):
+        # Protect file write operation
+        async with self._save_lock:
             self._client.save()