cache get_current_user result

avoids raising an error rendering templates, etc.

jupyterhub/handlers/base.py

@@ -265,10 +265,17 @@ class BaseHandler(RequestHandler):
 
     def get_current_user(self):
         """get current username"""
+        if not hasattr(self, '_jupyterhub_user'):
+            try:
                 user = self.get_current_user_token()
-        if user is not None:
-            return user
-        return self.get_current_user_cookie()
+                if user is None:
+                    user = self.get_current_user_cookie()
+                self._jupyterhub_user = user
+            except Exception:
+                # don't let errors here raise more than once
+                self._jupyterhub_user = None
+                raise
+        return self._jupyterhub_user
 
     def find_user(self, name):
         """Get a user by name