When a shard is to be snapshotted, we acquire a store ref (wrapped in an index commit) and then enqueue the shard snapshot task for execution by the repository:

https://github.com/elastic/elasticsearch/blob/b124763772748d8993911e4a7ba94b17472a9539/server/src/main/java/org/elasticsearch/snapshots/SnapshotShardsService.java#L369-L384

If the shard is closed while its snapshot is in the queue then it looks like we do not release the store ref promptly. Instead, we keep hold of it until it reaches the front of the queue, which could be many minutes later. During this time the shard cannot fully shut down and release the shard lock, leading to a `ShardLockObtainFailedException` being thrown on attempts to reinitialise the shard.
server/src/main/java/org/elasticsearch/index/snapshots/IndexShardSnapshotStatus.java
/*
 * 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.
 */
package org.elasticsearch.index.snapshots;
import org.elasticsearch.common.Strings;
import org.elasticsearch.repositories.ShardGeneration;
import org.elasticsearch.repositories.ShardSnapshotResult;
import org.elasticsearch.snapshots.AbortedSnapshotException;
import java.util.Objects;
import java.util.concurrent.atomic.AtomicReference;
/**
 * Represent shard snapshot status
 */
public class IndexShardSnapshotStatus {
    /**
     * Snapshot stage
     */
    public enum Stage {
        /**
         * Snapshot hasn't started yet