auth0: Allow redirecting to custom URL after logout

This can't be just done by setting logout_redirect_url
unfortunately, as client_id is also required to be passed in
here (see https://auth0.com/docs/authenticate/login/logout/redirect-users-after-logout).

While that could be done via a callable, this is much cleaner
to do.

oauthenticator/auth0.py

@@ -2,6 +2,7 @@
 A JupyterHub authenticator class for use with Auth0 as an identity provider.
 """
 import os
+from urllib.parse import urlencode
 
 from jupyterhub.auth import LocalAuthenticator
 from traitlets import Unicode, default
@@ -41,6 +42,17 @@ def _auth0_domain_default(self):
             "Configuring either auth0_domain or auth0_subdomain is required"
         )
 
+    logout_redirect_to_url = Unicode(
+        config=True,
+        help="""
+        Redirect to this URL after the user is logged out.
+
+        Must be explicitly added to the "Allowed Logout URLs" in the configuration
+        for this Auth0 application. See https://auth0.com/docs/authenticate/login/logout/redirect-users-after-logout
+        for more information.
+        """
+    )
+
     auth0_subdomain = Unicode(
         config=True,
         help="""
@@ -57,7 +69,13 @@ def _auth0_subdomain_default(self):
 
     @default("logout_redirect_url")
     def _logout_redirect_url_default(self):
-        return f"https://{self.auth0_domain}/v2/logout"
+        url = f"https://{self.auth0_domain}/v2/logout"
+        if self.logout_redirect_to_url:
+            # If a redirectTo is set, we must also include the `client_id`
+            # Auth0 expects `client_id` to be snake cased while `redirectTo` is camel cased
+            params = urlencode({"client_id": self.client_id, "redirectTo": self.logout_redirect_to_url})
+            url = f"{url}?{params}"
+        return url
 
     @default("authorize_url")
     def _authorize_url_default(self):

oauthenticator/tests/test_auth0.py

@@ -96,8 +96,13 @@ async def test_auth0(
         assert auth_model == None
 
 
-async def test_custom_logout(monkeypatch):
+@mark.parametrize(("logout_redirect_to_url", "redirect_url"), [
+    ("", f"https://{AUTH0_DOMAIN}/v2/logout"),
+    ("https://hub-url.com", f"https://{AUTH0_DOMAIN}/v2/logout?client_id=&redirectTo=https%3A%2F%2Fhub-url.com")
+])
+async def test_custom_logout(monkeypatch, logout_redirect_to_url, redirect_url):
     authenticator = Auth0OAuthenticator()
+    authenticator.logout_redirect_to_url = logout_redirect_to_url
     logout_handler = mock_handler(OAuthLogoutHandler, authenticator=authenticator)
     monkeypatch.setattr(web.RequestHandler, 'redirect', Mock())
 
@@ -114,8 +119,7 @@ async def test_custom_logout(monkeypatch):
     # Check redirection to the custom logout url
     authenticator.auth0_domain = AUTH0_DOMAIN
     await logout_handler.get()
-    custom_logout_url = f'https://{AUTH0_DOMAIN}/v2/logout'
-    logout_handler.redirect.assert_called_with(custom_logout_url)
+    logout_handler.redirect.assert_called_with(redirect_url)
 
 
 @mark.parametrize(