presto-iceberg/src/main/java/io/prestosql/plugin/iceberg/IcebergTransactionManager.java/IcebergTransactionManager/put
class IcebergTransactionManager:    public void put(ConnectorTransactionHandle transaction, IcebergMetadata metadata)
    {
        ConnectorMetadata existing = transactions.putIfAbsent(transaction, metadata);
        checkState(existing == null, "transaction already exists: %s", existing);
    }