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
/**
     * Returns a copy of the current {@link IndexShardSnapshotStatus}. This method is
     * intended to be used when a coherent state of {@link IndexShardSnapshotStatus} is needed.
     *
     * @return a  {@link IndexShardSnapshotStatus.Copy}
     */
    public synchronized IndexShardSnapshotStatus.Copy asCopy() {
        return new IndexShardSnapshotStatus.Copy(
            stage.get(),
            startTime,
            totalTime,
            incrementalFileCount,
            totalFileCount,
            processedFileCount,
            incrementalSize,
            totalSize,
            processedSize,
            failure
        );
    }
    public static IndexShardSnapshotStatus newInitializing(ShardGeneration generation) {
        return new IndexShardSnapshotStatus(Stage.INIT, 0L, 0L, 0, 0, 0, 0, 0, 0, null, generation);
    }
    public static IndexShardSnapshotStatus newFailed(final String failure) {
        assert failure != null : "expecting non null failure for a failed IndexShardSnapshotStatus";
        if (failure == null) {
            throw new IllegalArgumentException("A failure description is required for a failed IndexShardSnapshotStatus");
        }
        return new IndexShardSnapshotStatus(Stage.FAILURE, 0L, 0L, 0, 0, 0, 0, 0, 0, failure, null);
    }