in play-v29/src/main/scala/com/gu/googleauth/auth.scala [107:111]
def ensureUserHasSessionId(t: String => Future[Result])(implicit request: RequestHeader, ec: ExecutionContext):Future[Result] = {
val sessionId = request.session.get(sessionIdKeyName).getOrElse(generateSessionId())
t(sessionId).map(_.addingToSession(sessionIdKeyName -> sessionId))
}