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
if (genToLoad > generation) {
            // 
            // 
            // 
            logger.debug(
                "Determined repository's generation from its contents to ["
                    + generation
                    + "] but "
                    + " generation is at least ["
                    + genToLoad
                    + "]"
            );
        }
        if (genToLoad != repositoryStateId) {
            throw new RepositoryException(
                metadata.name(),
                "con modification of the index-N file, expected  generation ["
                    + repositoryStateId
                    + "], actual  generation ["
                    + genToLoad
                    + "]"
            );
        }
        if (cached != null && cached.getGenId() == genToLoad) {
            return cached;
        }
        return getRepositoryData(genToLoad);
    }
    /**
     * After updating the {@link RepositoryData} each of the shards directories is individually first moved to the next shard generation