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
assertEnvIsLocked();
        final NodePath[] nodePaths = nodePaths();
        final Path[] shardLocations = new Path[nodePaths.length];
        for (int i = 0; i < nodePaths.length; i++) {
            shardLocations[i] = nodePaths[i].resolve(shardId);
        }
        return shardLocations;
    }
    /**
     * Returns all folder names in ${data.paths}/indices folder
     */
    public Set<String> availableIndexFolders() throws IOException {
        return availableIndexFolders(p -> false);
    }
    /**
     * Returns folder names in ${data.paths}/indices folder that don't match the given predicate.
     * @param excludeIndexPathIdsPredicate folder names to exclude
     */
    public Set<String> availableIndexFolders(Predicate<String> excludeIndexPathIdsPredicate) throws IOException {
        if (nodePaths == null || locks == null) {
            throw new IllegalStateException("node is not configured to store local location");
        }
        assertEnvIsLocked();
        Set<String> indexFolders = new HashSet<>();
        for (NodePath nodePath : nodePaths) {
            indexFolders.addAll(availableIndexFoldersForPath(nodePath, excludeIndexPathIdsPredicate));
        }
        return indexFolders;
    }
    /**