auth: Improve Confirmation link error handling in social auth codepath.

render_confirmation_key_error provides better error responses than
simply rendering the "link expired" template.

zerver/tests/test_auth_backends.py

@@ -1526,7 +1526,7 @@ class SocialAuthBase(DesktopFlowTestingLib, ZulipTestCase):
         result = self.client_get(result.url)
 
         self.assertEqual(result.status_code, 404)
-        self.assert_in_response("The registration link has expired or is not valid.", result)
+        self.assert_in_response("Whoops. The confirmation link is malformed.", result)
 
     @override_settings(TERMS_OF_SERVICE=None)
     def test_social_auth_registration_using_multiuse_invite(self) -> None:

zerver/views/auth.py

@@ -34,6 +34,7 @@ from confirmation.models import (
     ConfirmationKeyException,
     create_confirmation_link,
     get_object_from_key,
+    render_confirmation_key_error,
 )
 from version import API_FEATURE_LEVEL, ZULIP_MERGE_BASE, ZULIP_VERSION
 from zerver.context_processors import get_realm_from_request, login_context, zulip_default_context
@@ -187,8 +188,8 @@ def maybe_send_to_registration(
         from_multiuse_invite = True
         try:
             multiuse_obj = get_object_from_key(multiuse_object_key, [Confirmation.MULTIUSE_INVITE])
-        except ConfirmationKeyException:
-            return render(request, "zerver/confirmation_link_expired_error.html", status=404)
+        except ConfirmationKeyException as exception:
+            return render_confirmation_key_error(request, exception)
 
         assert multiuse_obj is not None
         realm = multiuse_obj.realm