For username not found in the list of users returned by Google, the user poller should:

• If the username belongs to an external user
  • If there is a username conflict for this username, mark it as resolved (as it indicates the user has been deleted from the Google domain).
  • If there is no username conflict for this username, do nothing.
• If the username belongs to a Google user, disable the user and make it an external user (remove the OpenID attribute) and remove the user from the 'staff' group. This serves two purposes:
  1. When a Google user is created in the future, a new username conflict will be detected so that the new Google user will not automatically inherit the existing username.
  2. If the username is migrated to be an external user, user admin just need to add it to the 'users' group.