check that notebook_dir and root_dir are the same

jupyter_server/serverapp.py

@@ -1059,10 +1059,14 @@ def _default_notebook_dir(self):
         self.log.warning(_("\n    notebook_dir is deprecated, use root_dir.\n"))
         return self.root_dir
 
-    @validate('notebook_dir')
+    @observe('notebook_dir')
     def _update_notebook_dir(self, change):
         self.log.warning(_("\n    notebook_dir is deprecated, use root_dir.\n"))
-        self.root_dir = change['value']
+        """Do a bit of validation of the notebook dir."""
+        new = change['new']
+        # Check that notebook_dir and root_dir are equal
+        if new != self.root_dir:
+            self.root_dir = new
 
     root_dir = Unicode(config=True,
         help=_("The directory to use for notebooks and kernels.")
@@ -1098,6 +1102,9 @@ def _update_root_dir(self, change):
         new = change['new']
         self.config.FileContentsManager.root_dir = new
         self.config.MappingKernelManager.root_dir = new
+        # Check that notebook_dir and root_dir are equal
+        if self.notebook_dir != new:
+            self.notebook_dir = new
 
     @observe('server_extensions')
     def _update_server_extensions(self, change):