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/internalClusterTest/java/org/elasticsearch/env/NodeEnvironmentIT.java
public Settings onNodeStopped(String nodeName) {
                    return nonDataNode();
                }
            })
        );
        assertThat(ex.getMessage(), containsString(indexUUID));
        assertThat(ex.getMessage(), startsWith("node does not have the data role but has shard data"));
    }
    private IllegalStateException expectThrowsOnRestart(CheckedConsumer<Path[], Exception> onNodeStopped) {
        internalCluster().startNode();
        final Path[] dataPaths = internalCluster().getInstance(NodeEnvironment.class).nodeDataPaths();
        return expectThrows(
            IllegalStateException.class,
            () -> internalCluster().restartRandomDataNode(new InternalTestCluster.RestartCallback() {
                @Override
                public Settings onNodeStopped(String nodeName) {
                    try {
                        onNodeStopped.accept(dataPaths);
                    } catch (Exception e) {
                        throw new AssertionError(e);
                    }
                    return Settings.EMPTY;
                }
            })
        );
    }
    public void testFailsToStartIfDowngraded() {
        final IllegalStateException illegalStateException = expectThrowsOnRestart(
            dataPaths -> PersistedClusterStateService.overrideVersion(NodeMetadataTests.tooNewVersion(), dataPaths)
        );