private List<DeploymentSpec.ChangeBlocker> readChangeBlockers(Element parent, Element globalBlockersParent) {
        List<DeploymentSpec.ChangeBlocker> changeBlockers = new ArrayList<>();
        if (globalBlockersParent != parent) {
            for (Element tag : XML.getChildren(globalBlockersParent, blockChangeTag))
                changeBlockers.add(readChangeBlocker(tag));
        }
        for (Element tag : XML.getChildren(parent, blockChangeTag))
            changeBlockers.add(readChangeBlocker(tag));
        return Collections.unmodifiableList(changeBlockers);
    }
Returns the given non-blank attribute of tag or throw
config-model-api/src/main/java/com/yahoo/config/application/api/xml/DeploymentSpecXmlReader.java