client/python: make LazyBrowseNode.expand concurrency-safe

clients/python/src/zb_mom_ww_mxgateway/galaxy.py

@@ -292,6 +292,7 @@ class LazyBrowseNode:
         self._options = options
         self._children: list[LazyBrowseNode] = []
         self._is_expanded = False
+        self._expand_lock = asyncio.Lock()
 
     @property
     def object(self) -> galaxy_pb.GalaxyObject:
@@ -317,14 +318,17 @@ class LazyBrowseNode:
         """Fetch direct children of this node; no-op on subsequent calls."""
         if self._is_expanded:
             return
-        new_children: list[LazyBrowseNode] = []
-        async for child in self._client._iter_browse_children(
-            parent_gobject_id=self._object.gobject_id,
-            options=self._options,
-        ):
-            new_children.append(child)
-        self._children.extend(new_children)
-        self._is_expanded = True
+        async with self._expand_lock:
+            if self._is_expanded:
+                return
+            new_children: list[LazyBrowseNode] = []
+            async for child in self._client._iter_browse_children(
+                parent_gobject_id=self._object.gobject_id,
+                options=self._options,
+            ):
+                new_children.append(child)
+            self._children.extend(new_children)
+            self._is_expanded = True
 
 
 async def _canceling_iterator(call: Any) -> AsyncIterator[galaxy_pb.DeployEvent]: