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
throw e;
            }
        });
    }
    /**
     * Generates an identifier from the current state of a shard that can be used to detect whether a shard's contents
     * have changed between two snapshots.
     * A shard is assumed to have unchanged contents if its global- and local checkpoint are equal, its maximum
     * sequence number has not changed and its history- and force-merge-uuid have not changed.
     * The method returns {@code null} if global and local checkpoint are different for a shard since no safe unique
     * shard state id can be used in this case because of the possibility of a primary failover leading to different
     * shard content for the same sequence number on a subsequent snapshot.
     *
     * @param indexShard          Shard
     * @param snapshotIndexCommit IndexCommit for shard
     * @return shard state id or {@code null} if none can be used
     */
    @Nullable
    public static String getShardStateId(IndexShard indexShard, IndexCommit snapshotIndexCommit) throws IOException {
        final Map<String, String> userCommitData = snapshotIndexCommit.getUserData();
        final SequenceNumbers.CommitInfo seqNumInfo = SequenceNumbers.loadSeqNoInfoFromLuceneCommit(userCommitData.entrySet());
        final long maxSeqNo = seqNumInfo.maxSeqNo;
        if (maxSeqNo != seqNumInfo.localCheckpoint || maxSeqNo != indexShard.getLastSyncedGlobalCheckpoint()) {
            return null;
        }
        return userCommitData.get(Engine.HISTORY_UUID_KEY)
            + "-"
            + userCommitData.getOrDefault(Engine.FORCE_MERGE_UUID_KEY, "na")
            + "-"
            + maxSeqNo;