code: func (s *SessionStore) CloseSession(c context.Context, sessionID string) error {
  	c = defaultNS(c)
  	ent, err := s.fetchSession(c, sessionID)
  	switch {
  	case err != nil:
  		return err
  	case ent == nil:
  		return nil
  	default:
  		ent.IsClosed = true
  		ent.Closed = clock.Now(c).UTC()
  		return transient.Tag.Apply(ds.Put(ds.WithoutTransaction(c), ent))
  	}
  }
docs: // CloseSession closes a session given its ID. Does nothing if session is
  // already closed or doesn't exist. Returns only transient errors.