Fix uncaught exception in pre_spawn_start

jupyterhub/handlers/pages.py

@@ -172,7 +172,19 @@ class SpawnHandler(BaseHandler):
                 spawner._spawn_future = None
             # not running, no form. Trigger spawn and redirect back to /user/:name
             f = asyncio.ensure_future(self.spawn_single_user(user, server_name))
-            await asyncio.wait([f], timeout=1)
+            done, pending = await asyncio.wait([f], timeout=1)
+            # If spawn_single_user throws an exception, raise a 500 error
+            # otherwise it may cause a redirect loop
+            if f in done:
+                future, = done
+                exc = future.exception()
+                if exc:
+                    raise web.HTTPError(
+                        500,
+                        "Error in Authenticator.pre_spawn_start: %s %s"
+                        % (type(exc).__name__, str(exc)),
+                    )
+                    return
             self.redirect(pending_url)
 
     @web.authenticated

jupyterhub/user.py

@@ -529,7 +529,10 @@ class User:
 
         # trigger pre-spawn hook on authenticator
         authenticator = self.authenticator
+        try:
             if authenticator:
+                # pre_spawn_start can thow errors that can lead to a redirect loop
+                # if left uncaught (see https://github.com/jupyterhub/jupyterhub/issues/2683)
                 await maybe_future(authenticator.pre_spawn_start(self, spawner))
 
             spawner._start_pending = True
@@ -539,7 +542,6 @@ class User:
             ) = spawner.orm_spawner.last_activity = datetime.utcnow()
             db.commit()
             # wait for spawner.start to return
-        try:
             # run optional preparation work to bootstrap the notebook
             await maybe_future(spawner.run_pre_spawn_hook())
             if self.settings.get('internal_ssl'):