Today the `ReservedClusterStateService` decides whether or not to submit an update-error task depending on the content of the last-applied state as exposed by `ClusterService#state()`. This may happen concurrently with another state being published, so does not give a fully consistent view of the latest state. Instead it should check the state within the cluster state task executor, triggering a no-op update as appropriate.
server/src/main/java/org/elasticsearch/reservedstate/service/ReservedStateUpdateTask.java
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 * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
 * or more contributor license agreements. Licensed under the Elastic License
 * 2.0 and the Server Side Public License, v 1; you may not use this file except
 * in compliance with, at your election, the Elastic License 2.0 or the Server
 * Side Public License, v 1.
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package org.elasticsearch.reservedstate.service;
import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
import org.elasticsearch.Version;
import org.elasticsearch.action.ActionListener;
import org.elasticsearch.action.ActionResponse;
import org.elasticsearch.cluster.ClusterState;
import org.elasticsearch.cluster.ClusterStateTaskListener;
import org.elasticsearch.cluster.metadata.Metadata;
import org.elasticsearch.cluster.metadata.ReservedStateErrorMetadata;
import org.elasticsearch.cluster.metadata.ReservedStateHandlerMetadata;