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<?

Lock::get('foo');

$foo = $api->get('foo');
$foo['bar'] = 'baz';
$api->put('foo', $foo);

Lock::release('foo');

?>

<?

class Lock {

	const TIMEOUT = 20;
	const SLEEP = 100000;

	/**
	 * Stores the expire time of the currently held lock
	 * @var int
	 */
	protected static $expire;

	/**
	 * Gets a lock or waits for it to become available
	 * @param  mixed	$key		Item to lock
	 * @param  int		$timeout	Time to wait for the key (seconds)
	 * @return mixed	The key
	 * @throws LockException If the key is invalid
	 * @throws LockTimeoutException If the lock is not acquired before the method times out
	 */
	public static function get($key, $timeout = null){
		if(!$key) throw new LockException("Invalid Key");

		$start = time();

		do{
			self::$expire = self::timeout();
			
			if($acquired = (Redis::setnx("Lock:{$key}", self::$expire))) break;
			if($acquired = (self::recover($key))) break;
			if($timeout === 0) break;

			usleep(self::SLEEP);
		} while(!is_numeric($timeout) || time() < $start + $timout);

		if(!$acquired) throw new LockTimeoutException("Timeout exceeded");

		return $key;
	}

	/**
	 * Releases the lock
	 * @param  mixed	$key	Item to lock
	 * @throws LockException If the key is invalid
	 */
	public static function release($key){
		if(!$key) throw new LockException("Invalid Key");

		// Only release the lock if it hasn't expired
		if(self::$expire > time()) Redis::del("Lock:{$key}");
	}

	/**
	 * Generates an expire time based on the current time
	 * @return int	timeout
	 */
	protected static function timeout(){
		return (int) (time() + self::TIMEOUT + 1);
	}

	/**
	 * Recover an abandoned lock
	 * @param  mixed	$key	Item to lock
	 * @return bool	Was the lock acquired?
	 */
	protected static function recover($key){
		if(($lockTimeout = Redis::get("Lock:{$key}")) > time()) return false;

		$timeout = self::timeout();
		$currentTimeout = Redis::getset("Lock:{$key}", $timeout);

		if($currentTimeout != $lockTimeout) return false;

		self::$expire = $timeout;
		return true;
	}

}

class LockException extends RuntimeException {}
class LockTimeoutException extends LockException {}

?>