expose cookie options and pass them down to spawners

enables forcing all-session cookies with:

```python
c.JupyterHub.tornado_settings['cookie_options'] = {
    'expires_days': None,
}
```

jupyterhub/services/auth.py

@@ -30,7 +30,10 @@ from tornado.httputil import url_concat
 from tornado.web import HTTPError, RequestHandler
 
 from traitlets.config import SingletonConfigurable
-from traitlets import Unicode, Integer, Instance, default, observe, validate
+from traitlets import (
+    Unicode, Integer, Instance, Dict,
+    default, observe, validate,
+)
 
 from ..utils import url_path_join
 
@@ -197,6 +200,24 @@ class HubAuth(SingletonConfigurable):
         help="""The name of the cookie I should be looking for"""
     ).tag(config=True)
 
+    cookie_options = Dict(
+        help="""Additional options to pass when setting cookies.
+
+        Can include things like `expires_days=None` for session-expiry
+        or `secure=True` if served on HTTPS and default HTTPS discovery fails
+        (e.g. behind some proxies).
+        """
+    ).tag(config=True)
+
+    @default('cookie_options')
+    def _default_cookie_options(self):
+        # load default from env
+        options_env = os.environ.get('JUPYTERHUB_COOKIE_OPTIONS')
+        if options_env:
+            return json.loads(options_env)
+        else:
+            return {}
+
     cookie_cache_max_age = Integer(help="DEPRECATED. Use cache_max_age")
     @observe('cookie_cache_max_age')
     def _deprecated_cookie_cache(self, change):
@@ -580,6 +601,8 @@ class HubOAuth(HubAuth):
         }
         if handler.request.protocol == 'https':
             kwargs['secure'] = True
+        # load user cookie overrides
+        kwargs.update(self.cookie_options)
         handler.set_secure_cookie(
             cookie_name,
             b64_state,
@@ -627,6 +650,8 @@ class HubOAuth(HubAuth):
         }
         if handler.request.protocol == 'https':
             kwargs['secure'] = True
+        # load user cookie overrides
+        kwargs.update(self.cookie_options)
         app_log.debug("Setting oauth cookie for %s: %s, %s",
             handler.request.remote_ip, self.cookie_name, kwargs)
         handler.set_secure_cookie(

jupyterhub/services/service.py

@@ -218,6 +218,7 @@ class Service(LoggingConfigurable):
     base_url = Unicode()
     db = Any()
     orm = Any()
+    cookie_options = Dict()
 
     oauth_provider = Any()
 
@@ -299,6 +300,7 @@ class Service(LoggingConfigurable):
             environment=env,
             api_token=self.api_token,
             oauth_client_id=self.oauth_client_id,
+            cookie_options=self.cookie_options,
             cwd=self.cwd,
             hub=self.hub,
             user=_MockUser(

jupyterhub/spawner.py

@@ -6,6 +6,7 @@ Contains base Spawner class & default implementation
 # Distributed under the terms of the Modified BSD License.
 
 import errno
+import json
 import os
 import pipes
 import shutil
@@ -99,11 +100,12 @@ class Spawner(LoggingConfigurable):
         """
         return bool(self.pending or self.ready)
 
-
+    # options passed by constructor
     authenticator = Any()
     hub = Any()
     orm_spawner = Any()
     db = Any()
+    cookie_options = Dict()
 
     @observe('orm_spawner')
     def _orm_spawner_changed(self, change):
@@ -587,6 +589,8 @@ class Spawner(LoggingConfigurable):
             env['JUPYTERHUB_ADMIN_ACCESS'] = '1'
         # OAuth settings
         env['JUPYTERHUB_CLIENT_ID'] = self.oauth_client_id
+        if self.cookie_options:
+            env['JUPYTERHUB_COOKIE_OPTIONS'] = json.dumps(self.cookie_options)
         env['JUPYTERHUB_HOST'] = self.hub.public_host
         env['JUPYTERHUB_OAUTH_CALLBACK_URL'] = \
             url_path_join(self.user.url, self.name, 'oauth_callback')

jupyterhub/user.py

@@ -215,6 +215,7 @@ class User:
             proxy_spec=url_path_join(self.proxy_spec, name, '/'),
             db=self.db,
             oauth_client_id=client_id,
+            cookie_options = self.settings.get('cookie_options', {}),
         )
         # update with kwargs. Mainly for testing.
         spawn_kwargs.update(kwargs)