class WriteSafeSessionHandler

class WriteSafeSessionHandler implements \SessionHandlerInterface, WriteSafeSessionHandlerInterface {
    
    /**
     * @var \SessionHandlerInterface
     */
    protected $wrappedSessionHandler;
    
    /**
     * Whether or not the session is enabled for writing.
     *
     * @var bool
     */
    protected $sessionWritable;
    
    /**
     * The read sessions.
     *
     * @var array
     *   Session data keyed by the session ID.
     */
    private $readSessions;
    
    /**
     * Constructs a new write safe session handler.
     *
     * @param \SessionHandlerInterface $wrapped_session_handler
     *   The underlying session handler.
     * @param bool $session_writable
     *   Whether or not the session should be initially writable.
     */
    public function __construct(\SessionHandlerInterface $wrapped_session_handler, $session_writable = TRUE) {
        $this->wrappedSessionHandler = $wrapped_session_handler;
        $this->sessionWritable = $session_writable;
    }
    
    /**
     * {@inheritdoc}
     */
    public function close() {
        return $this->wrappedSessionHandler
            ->close();
    }
    
    /**
     * {@inheritdoc}
     */
    public function destroy($session_id) {
        return $this->wrappedSessionHandler
            ->destroy($session_id);
    }
    
    /**
     * {@inheritdoc}
     */
    public function gc($max_lifetime) {
        return $this->wrappedSessionHandler
            ->gc($max_lifetime);
    }
    
    /**
     * {@inheritdoc}
     */
    public function open($save_path, $session_id) {
        return $this->wrappedSessionHandler
            ->open($save_path, $session_id);
    }
    
    /**
     * {@inheritdoc}
     */
    public function read($session_id) {
        $value = $this->wrappedSessionHandler
            ->read($session_id);
        $this->readSessions[$session_id] = $value;
        return $value;
    }
    
    /**
     * {@inheritdoc}
     */
    public function write($session_id, $session_data) {
        // Only write the session when it has been modified.
        if (isset($this->readSessions[$session_id]) && $this->readSessions[$session_id] === $session_data) {
            return TRUE;
        }
        if ($this->isSessionWritable()) {
            return $this->wrappedSessionHandler
                ->write($session_id, $session_data);
        }
        return TRUE;
    }
    
    /**
     * {@inheritdoc}
     */
    public function setSessionWritable($flag) {
        $this->sessionWritable = (bool) $flag;
    }
    
    /**
     * {@inheritdoc}
     */
    public function isSessionWritable() {
        return $this->sessionWritable;
    }

}