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/repositories/blobstore/BlobStoreRepository.java
*/
    private Tuple<BlobStoreIndexShardSnapshots, Long> buildBlobStoreIndexShardSnapshots(Set<String> blobs, BlobContainer shardContainer)
        throws IOException {
        long latest = latestGeneration(blobs);
        if (latest >= 0) {
            final BlobStoreIndexShardSnapshots shardSnapshots = INDEX_SHARD_SNAPSHOTS_FORMAT.read(
                metadata.name(),
                shardContainer,
                Long.toString(latest),
                namedXContentRegistry
            );
            return new Tuple<>(shardSnapshots, latest);
        } else if (blobs.stream()
            .anyMatch(b -> b.startsWith(SNAPSHOT_PREFIX) || b.startsWith(INDEX_FILE_PREFIX) || b.startsWith(UPLOADED_DATA_BLOB_PREFIX))) {
                logger.warn(
                    "Could not find a readable index-N file in a non-empty shard snapshot directory [" + shardContainer.path() + "]"
                );
            }
        return new Tuple<>(BlobStoreIndexShardSnapshots.EMPTY, latest);
    }
    /**
     * Snapshot individual file
     * @param fileInfo file to snapshot
     */
    protected void snapshotFile(SnapshotShardContext context, FileInfo fileInfo) throws IOException {
        final IndexId indexId = context.indexId();
        final Store store = context.store();
        final ShardId shardId = store.shardId();
        final IndexShardSnapshotStatus snapshotStatus = context.status();
        final SnapshotId snapshotId = context.snapshotId();