During upgrades from 7.x to 8.x if a node was holding a frozen cache file, the system considers that as an unknown file and prevents starting the node:
```
java.lang.IllegalStateException: unexpected file/folder encountered during data folder upgrade: /app/data/nodes/0/shared_snapshot_cache
	at org.elasticsearch.env.NodeEnvironment.upgradeLegacyNodeFolders(NodeEnvironment.java:443) ~[elasticsearch-8.0.1.jar:8.0.1]
	at org.elasticsearch.env.NodeEnvironment.<init>(NodeEnvironment.java:290) ~[elasticsearch-8.0.1.jar:8.0.1]
	at org.elasticsearch.node.Node.<init>(Node.java:388) ~[elasticsearch-8.0.1.jar:8.0.1]
	at org.elasticsearch.node.Node.<init>(Node.java:277) ~[elasticsearch-8.0.1.jar:8.0.1]
	at org.elasticsearch.bootstrap.Bootstrap$5.<init>(Bootstrap.java:234) ~[elasticsearch-8.0.1.jar:8.0.1]
	at org.elasticsearch.bootstrap.Bootstrap.setup(Bootstrap.java:234) ~[elasticsearch-8.0.1.jar:8.0.1]
	at org.elasticsearch.bootstrap.Bootstrap.init(Bootstrap.java:358) [elasticsearch-8.0.1.jar:8.0.1]
	at org.elasticsearch.bootstrap.Elasticsearch.init(Elasticsearch.java:166) [elasticsearch-8.0.1.jar:8.0.1]
	at org.elasticsearch.bootstrap.Elasticsearch.execute(Elasticsearch.java:157) [elasticsearch-8.0.1.jar:8.0.1]
	at org.elasticsearch.cli.EnvironmentAwareCommand.execute(EnvironmentAwareCommand.java:77) [elasticsearch-8.0.1.jar:8.0.1]
	at org.elasticsearch.cli.Command.mainWithoutErrorHandling(Command.java:112) [elasticsearch-cli-8.0.1.jar:8.0.1]
	at org.elasticsearch.cli.Command.main(Command.java:77) [elasticsearch-cli-8.0.1.jar:8.0.1]
	at org.elasticsearch.bootstrap.Elasticsearch.main(Elasticsearch.java:122) [elasticsearch-8.0.1.jar:8.0.1]
	at org.elasticsearch.bootstrap.Elasticsearch.main(Elasticsearch.java:80) [elasticsearch-8.0.1.jar:8.0.1]
```
These files are expected files and should be ignored/accepted during the data folder upgrade.
server/src/main/java/org/elasticsearch/env/NodeEnvironment.java
shardLock(id, "checking if shard is locked").close();
            return false;
        } catch (ShardLockObtainFailedException ex) {
            return true;
        }
    }
    /**
     * Deletes an indexes data directory recursively iff all of the indexes
     * shards locks were successfully acquired. If any of the indexes shard directories can't be locked
     * non of the shards will be deleted
     *
     * @param index the index to delete
     * @param lockTimeoutMS how long to wait for acquiring the indices shard locks
     * @param indexSettings settings for the index being deleted
     * @throws IOException if any of the shards data directories can't be locked or deleted
     */
    public void deleteIndexDirectorySafe(Index index, long lockTimeoutMS, IndexSettings indexSettings, Consumer<Path[]> listener)
        throws IOException, ShardLockObtainFailedException {
        final List<ShardLock> locks = lockAllForIndex(index, indexSettings, "deleting index directory", lockTimeoutMS);
        try {
            deleteIndexDirectoryUnderLock(index, indexSettings, listener);
        } finally {
            IOUtils.closeWhileHandlingException(locks);
        }
    }
    /**
     * Deletes an indexes data directory recursively.
     * Note: this method assumes that the shard lock is acquired
     *
     * @param index the index to delete