in play-v29/src/main/scala/com/gu/googleauth/actions.scala [190:198]
def processOauth2Callback(requiredGoogleGroups: Set[String], groupChecker: GoogleGroupChecker)
(implicit request: RequestHeader, ec: ExecutionContext): Future[Result] = {
(for {
identity <- checkIdentity()
_ <- enforceGoogleGroups(identity, requiredGoogleGroups, groupChecker)
} yield {
setupSessionWhenSuccessful(identity)
}).merge
}