src/MockStorage.php
public function lock(string $sessionIdentifier): bool
    {
        if (in_array($sessionIdentifier, self::$lockedIdentifiers)) {
            return true;
        }

        self::$lockedIdentifiers[] = $sessionIdentifier;

        return true;
    }
public function lock(string $sessionIdentifier): bool
    {
        if (in_array($sessionIdentifier, self::$lockedIdentifiers)) {
            return true;
        }

        self::$lockedIdentifiers[] = $sessionIdentifier;

        return true;
    }
Asks the drive to lock the session storage

@param string $sessionIdentifier The session identifier to be locked
@return bool