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/snapshots/SnapshotShardsService.java
}
                }
            }
        }
    }
    private Runnable newShardSnapshotTask(
        final ShardId shardId,
        final Snapshot snapshot,
        final IndexId indexId,
        final IndexShardSnapshotStatus snapshotStatus,
        final Version entryVersion,
        final long entryStartTime
    ) {
        
        return () -> snapshot(shardId, snapshot, indexId, snapshotStatus, entryVersion, entryStartTime, new ActionListener<>() {
            @Override
            public void onResponse(ShardSnapshotResult shardSnapshotResult) {
                final ShardGeneration newGeneration = shardSnapshotResult.getGeneration();
                assert newGeneration != null;
                assert newGeneration.equals(snapshotStatus.generation());
                if (logger.isDebugEnabled()) {
                    final IndexShardSnapshotStatus.Copy lastSnapshotStatus = snapshotStatus.asCopy();
                    logger.debug(
                        "[{}][{}] completed snapshot to [{}] with status [{}] at generation [{}]",
                        shardId,
                        snapshot,
                        snapshot.getRepository(),
                        lastSnapshotStatus,
                        snapshotStatus.generation()
                    );