code: def oauth_authenticate(client_id, user, require_existing_link=False):
      
      # Authenticate via the access token (access token used to get user_id)
      if not requires_confirmation(user):
          after_this_request(_commit)
          if login_user(user, remember=False):
              if require_existing_link:
                  account = RemoteAccount.get(user.id, client_id)
                  if account is None:
                      logout_user()
                      return False
              return True
      return False
docs: Authenticate an oauth authorized callback.
  
      :param client_id: The client id.
      :param user: A user instance.
      :param require_existing_link: If ``True``, check if remote account exists.
          (Default: ``False``)
      :returns: ``True`` if the user is successfully authenticated.