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
for (NodePath nodePath : nodePaths) {
            Path indicesPath = nodePath.indicesPath;
            if (Files.isDirectory(indicesPath)) {
                try (DirectoryStream<Path> indexStream = Files.newDirectoryStream(indicesPath)) {
                    for (Path indexPath : indexStream) {
                        if (Files.isDirectory(indexPath)) {
                            try (Stream<Path> shardStream = Files.list(indexPath)) {
                                shardStream.filter(subPathPredicate).map(Path::toAbsolutePath).forEach(indexSubPaths::add);
                            }
                        }
                    }
                }
            }
        }
        return indexSubPaths;
    }
    private static boolean isShardPath(Path path) {
        return Files.isDirectory(path) && path.getFileName().toString().chars().allMatch(Character::isDigit);
    }
    private static boolean isIndexMetadataPath(Path path) {
        return Files.isDirectory(path) && path.getFileName().toString().equals(MetadataStateFormat.STATE_DIR_NAME);
    }
    /**
     * Resolve the custom path for a index's shard.
     */
    public static Path resolveBaseCustomLocation(String customDataPath, Path sharedDataPath) {
        if (Strings.isNotEmpty(customDataPath)) {
            
            assert sharedDataPath != null;
            return sharedDataPath.resolve(customDataPath).resolve("0");