Provide more detailed error message in case of version mismatch

this is the most likely cause of redirect loops when using docker,
so record the spawner version and check it when a redirect is detected.

In the event of a redirect and mismatch, fail with a message explaining the version mismatch and how to fix it.

jupyterhub/handlers/base.py

@@ -20,7 +20,7 @@ from .. import __version__
 from .. import orm
 from ..objects import Server
 from ..spawner import LocalProcessSpawner
-from ..utils import default_server_name, url_path_join, exponential_backoff
+from ..utils import default_server_name, url_path_join
 
 # pattern for the authentication token header
 auth_header_pat = re.compile(r'^(?:token|bearer)\s+([^\s]+)$', flags=re.IGNORECASE)
@@ -675,7 +675,10 @@ class UserSpawnHandler(BaseHandler):
                 return
 
             # spawn has supposedly finished, check on the status
-            status = yield spawner.poll()
+            if spawner.ready:
+                status = yield spawner.poll()
+            else:
+                status = 0
             if status is not None:
                 if spawner.options_form:
                     self.redirect(url_concat(url_path_join(self.hub.base_url, 'spawn'),
@@ -693,9 +696,23 @@ class UserSpawnHandler(BaseHandler):
                 self.log.warning("Invalid redirects argument %r", self.get_argument('redirects'))
                 redirects = 0
 
-            if redirects >= self.settings.get('user_redirect_limit', 5):
+            # check redirect limit to prevent browser-enforced limits.
+            # In case of version mismatch, raise on only two redirects.
+            if redirects >= self.settings.get(
+                'user_redirect_limit', 4
+            ) or (redirects >= 2 and spawner._jupyterhub_version != __version__):
                 # We stop if we've been redirected too many times.
-                raise web.HTTPError(500, "Redirect loop detected.")
+                msg = "Redirect loop detected."
+                if spawner._jupyterhub_version != __version__:
+                    msg += (
+                        " Notebook has jupyterhub version {singleuser}, but the Hub expects {hub}."
+                        " Try installing jupyterhub=={hub} in the user environment"
+                        " if you continue to have problems."
+                    ).format(
+                        singleuser=spawner._jupyterhub_version or 'unknown (likely < 0.8)',
+                        hub=__version__,
+                    )
+                raise web.HTTPError(500, msg)
 
             # set login cookie anew
             self.set_login_cookie(current_user)