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
throw new IllegalArgumentException("settings must contain a non-null > 0 number of shards");
        }
        logger.trace("locking all shards for index {} - [{}]", index, numShards);
        List<ShardLock> allLocks = new ArrayList<>(numShards);
        boolean success = false;
        long startTimeNS = System.nanoTime();
        try {
            for (int i = 0; i < numShards; i++) {
                long timeoutLeftMS = Math.max(0, lockTimeoutMS - TimeValue.nsecToMSec((System.nanoTime() - startTimeNS)));
                allLocks.add(shardLock(new ShardId(index, i), lockDetails, timeoutLeftMS));
            }
            success = true;
        } finally {
            if (success == false) {
                logger.trace("unable to lock all shards for index {}", index);
                IOUtils.closeWhileHandlingException(allLocks);
            }
        }
        return allLocks;
    }
    /**
     * Tries to lock the given shards ID. A shard lock is required to perform any kind of
     * write operation on a shards data directory like deleting files, creating a new index writer
     * or recover from a different shard instance into it. If the shard lock can not be acquired
     * a {@link ShardLockObtainFailedException} is thrown.
     *
     * Note: this method will return immediately if the lock can't be acquired.
     *
     * @param id the shard ID to lock
     * @param details information about why the shard is being locked