private TreeConfigProducer<AnyConfigProducer> getDocProc(ApplicationConfigProducerRoot root) {
            AnyConfigProducer current = root.getChildren().get(DOCPROC_RESERVED_NAME);
            if (current == null) {
                return new SimpleConfigProducer(root, DOCPROC_RESERVED_NAME);
            }
            if (current instanceof TreeConfigProducer t) {
                return t;
            }
            throw new IllegalStateException("ApplicationConfigProducerRoot " + root + " with bad type for " + DOCPROC_RESERVED_NAME + ": " + current.getClass());
        }
Select/creates and initializes the indexing cluster coupled to this
config-model/src/main/java/com/yahoo/vespa/model/content/Content.java