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
} else if (ignoredFileNames.contains(fileName) == false) {
                            throw new IllegalStateException(
                                "unexpected file/folder encountered during data folder upgrade: " + subFolderPath
                            );
                        }
                    }
                }
                assert Sets.difference(folderNames, expectedFolderNames).isEmpty()
                    : "expected indices and/or state dir folder but was " + folderNames;
                upgradeActions.add(() -> {
                    for (String folderName : folderNames) {
                        final Path sourceSubFolderPath = legacyNodePath.path.resolve(folderName);
                        final Path targetSubFolderPath = nodePath.path.resolve(folderName);
                        Files.move(sourceSubFolderPath, targetSubFolderPath, StandardCopyOption.ATOMIC_MOVE);
                        logger.info("data folder upgrade: moved from [{}] to [{}]", sourceSubFolderPath, targetSubFolderPath);
                    }
                    IOUtils.fsync(nodePath.path, true);
                });
            }
            
            for (CheckedRunnable<IOException> upgradeAction : upgradeActions) {
                upgradeAction.run();
            }
        } finally {
            legacyNodeLock.close();
        }
        
        IOUtils.rm(Stream.of(environment.dataFiles()).map(path -> path.resolve("nodes")).toArray(Path[]::new));
        return true;
    }