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
public static IndexShardSnapshotStatus newDone(
        final long startTime,
        final long totalTime,
        final int incrementalFileCount,
        final int fileCount,
        final long incrementalSize,
        final long size,
        ShardGeneration generation
    ) {
        
        return new IndexShardSnapshotStatus(
            Stage.DONE,
            startTime,
            totalTime,
            incrementalFileCount,
            fileCount,
            incrementalFileCount,
            incrementalSize,
            size,
            incrementalSize,
            null,
            generation
        );
    }
    /**
     * Returns an immutable state of {@link IndexShardSnapshotStatus} at a given point in time.
     */
    public static class Copy {
        private final Stage stage;
        private final long startTime;