Don't assume empty state means not running

Some Spawners may not need state,
and they should be allowed to resume on Hub restart as well.

Adds some detail about when .poll may be called and how it should behave in less obvious circumstances

jupyterhub/app.py

@@ -1058,11 +1058,6 @@ class JupyterHub(Application):
 
         for orm_user in db.query(orm.User):
             self.users[orm_user.id] = user = User(orm_user, self.tornado_settings)
-            if not user.state:
-                # without spawner state, server isn't valid
-                user.server = None
-                user_summaries.append(_user_summary(user))
-                continue
             self.log.debug("Loading state for %s from db", user.name)
             spawner = user.spawner
             status = yield spawner.poll()

jupyterhub/spawner.py

@@ -191,7 +191,7 @@ class Spawner(LoggingConfigurable):
     def load_state(self, state):
         """load state from the database
         
-        This is the extensible part of state
+        This is the extensible part of state.
         
         Override in a subclass if there is state to load.
         Should call `super`.
@@ -351,6 +351,20 @@ class Spawner(LoggingConfigurable):
         """Check if the single-user process is running
 
         return None if it is, an exit status (0 if unknown) if it is not.
+        If the Spawner has not been initialized (neither loaded state, nor called start),
+        it should behave as if it is not running (status=0).
+        If the Spawner has not finished starting,
+        it should behave as if it is running (status=None).
+
+        Assumptions about poll and when it can be called:
+
+        - poll may be called before start when state is loaded on Hub restart.
+          It should return 0 if the Spawner has not been initialized
+          via either load_state or start.
+        - If `.start()` is async, poll may be called during any yielded
+          portions of start. It should return None in this condition,
+          indicating that the process has not yet exited.
+
         """
         raise NotImplementedError("Override in subclass. Must be a Tornado gen.coroutine.")