add User.proxy_pending

flag for waiting for the proxy to be updated

avoids User.running being True when the user's server has not yet been added to the proxy,
causing potential redirect loops.

jupyterhub/handlers/base.py

@@ -321,6 +321,7 @@ class BaseHandler(RequestHandler):
         tic = IOLoop.current().time()
 
         f = user.spawn(options)
+        user.proxy_pending = True
 
         @gen.coroutine
         def finish_user_spawn(f=None):
@@ -335,8 +336,16 @@ class BaseHandler(RequestHandler):
             toc = IOLoop.current().time()
             self.log.info("User %s server took %.3f seconds to start", user.name, toc-tic)
             self.statsd.timing('spawner.success', (toc - tic) * 1000)
-            yield self.proxy.add_user(user)
-            user.spawner.add_poll_callback(self.user_stopped, user)
+            try:
+                yield self.proxy.add_user(user)
+            except Exception:
+                self.log.exception("Failed to add user %s to proxy!", user)
+                self.log.error("Stopping user %s to avoid inconsistent state")
+                yield user.stop()
+            else:
+                user.spawner.add_poll_callback(self.user_stopped, user)
+            finally:
+                user.proxy_pending = False
 
         try:
             yield gen.with_timeout(timedelta(seconds=self.slow_spawn_timeout), f)
@@ -532,7 +541,7 @@ class UserSpawnHandler(BaseHandler):
 
             # logged in as correct user, spawn the server
             if current_user.spawner:
-                if current_user.spawn_pending:
+                if current_user.spawn_pending or current_user.proxy_pending:
                     # spawn has started, but not finished
                     self.statsd.incr('redirects.user_spawn_pending', 1)
                     html = self.render_template("spawn_pending.html", user=current_user)