avoid calling Spawner.poll during Spawner.start

moves `spawn_pending` flag to only around start, not the HTTP wait.

Some Spawners may not know how to poll until start has finished (DockerSpawner).
Let's not require that they do.

jupyterhub/handlers/base.py

@@ -296,16 +296,23 @@ class BaseHandler(RequestHandler):
             yield gen.with_timeout(timedelta(seconds=self.slow_spawn_timeout), f)
         except gen.TimeoutError:
             if user.spawn_pending:
+                # still in Spawner.start, which is taking a long time
+                # we shouldn't poll while spawn_pending is True
+                self.log.warn("User %s server is slow to start", user.name)
+                # schedule finish for when the user finishes spawning
+                IOLoop.current().add_future(f, finish_user_spawn)
+            else:
+                # start has finished, but the server hasn't come up
+                # check if the server died while we were waiting
                 status = yield user.spawner.poll()
                 if status is None:
-                    # hit timeout, but spawn is still pending
-                    self.log.warn("User %s server is slow to start", user.name)
+                    # hit timeout, but server's running. Hope that it'll show up soon enough,
+                    # though it's possible that it started at the wrong URL
+                    self.log.warn("User %s server is slow to become responsive", user.name)
                     # schedule finish for when the user finishes spawning
                     IOLoop.current().add_future(f, finish_user_spawn)
                 else:
                     raise web.HTTPError(500, "Spawner failed to start [status=%s]" % status)
-            else:
-                raise
         else:
             yield finish_user_spawn()
     

jupyterhub/user.py

@@ -206,6 +206,7 @@ class User(HasTraits):
         self.state = spawner.get_state()
         self.last_activity = datetime.utcnow()
         db.commit()
+        self.spawn_pending = False
         try:
             yield self.server.wait_up(http=True, timeout=spawner.http_timeout)
         except Exception as e:
@@ -232,7 +233,6 @@ class User(HasTraits):
                 ), exc_info=True)
             # raise original TimeoutError
             raise e
-        self.spawn_pending = False
         return self
 
     @gen.coroutine