class LockBackendAbstract

abstract class LockBackendAbstract implements LockBackendInterface {
    
    /**
     * Current page lock token identifier.
     *
     * @var string
     */
    protected $lockId;
    
    /**
     * Existing locks for this page.
     *
     * @var array
     */
    protected $locks = [];
    
    /**
     * {@inheritdoc}
     */
    public function wait($name, $delay = 30) {
        // Pause the process for short periods between calling
        // lock_may_be_available(). This prevents hitting the database with constant
        // database queries while waiting, which could lead to performance issues.
        // However, if the wait period is too long, there is the potential for a
        // large number of processes to be blocked waiting for a lock, especially
        // if the item being rebuilt is commonly requested. To address both of these
        // concerns, begin waiting for 25ms, then add 25ms to the wait period each
        // time until it reaches 500ms. After this point polling will continue every
        // 500ms until $delay is reached.
        // $delay is passed in seconds, but we will be using usleep(), which takes
        // microseconds as a parameter. Multiply it by 1 million so that all
        // further numbers are equivalent.
        $delay = (int) $delay * 1000000;
        // Begin sleeping at 25ms.
        $sleep = 25000;
        while ($delay > 0) {
            // This function should only be called by a request that failed to get a
            // lock, so we sleep first to give the parallel request a chance to finish
            // and release the lock.
            usleep($sleep);
            // After each sleep, increase the value of $sleep until it reaches
            // 500ms, to reduce the potential for a lock stampede.
            $delay = $delay - $sleep;
            $sleep = min(500000, $sleep + 25000, $delay);
            if ($this->lockMayBeAvailable($name)) {
                // No longer need to wait.
                return FALSE;
            }
        }
        // The caller must still wait longer to get the lock.
        return TRUE;
    }
    
    /**
     * {@inheritdoc}
     */
    public function getLockId() {
        if (!isset($this->lockId)) {
            $this->lockId = uniqid(mt_rand(), TRUE);
        }
        return $this->lockId;
    }

}