class MetadataBag

class MetadataBag extends SymfonyMetadataBag {
    
    /**
     * The key used to store the CSRF token seed in the session.
     */
    const CSRF_TOKEN_SEED = 's';
    
    /**
     * Constructs a new metadata bag instance.
     *
     * @param \Drupal\Core\Site\Settings $settings
     *   The settings instance.
     */
    public function __construct(Settings $settings) {
        $update_threshold = $settings->get('session_write_interval', 180);
        parent::__construct('_sf2_meta', $update_threshold);
    }
    
    /**
     * Set the CSRF token seed.
     *
     * @param string $csrf_token_seed
     *   The per-session CSRF token seed.
     */
    public function setCsrfTokenSeed($csrf_token_seed) {
        $this->meta[static::CSRF_TOKEN_SEED] = $csrf_token_seed;
    }
    
    /**
     * Get the CSRF token seed.
     *
     * @return string|null
     *   The per-session CSRF token seed or null when no value is set.
     */
    public function getCsrfTokenSeed() {
        if (isset($this->meta[static::CSRF_TOKEN_SEED])) {
            return $this->meta[static::CSRF_TOKEN_SEED];
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function stampNew($lifetime = NULL) : void {
        parent::stampNew($lifetime);
        // Set the token seed immediately to avoid a race condition between two
        // simultaneous requests without a seed.
        $this->setCsrfTokenSeed(Crypt::randomBytesBase64());
    }

}