presto-main/src/main/java/io/prestosql/metadata/MetadataManager.java/MetadataManager/listRoles
class MetadataManager:    @Override
    public Set<String> listRoles(Session session, String catalog)
    {
        Optional<CatalogMetadata> catalogMetadata = getOptionalCatalogMetadata(session, catalog);
        if (!catalogMetadata.isPresent()) {
            return ImmutableSet.of();
        }
        CatalogName catalogName = catalogMetadata.get().getCatalogName();
        ConnectorSession connectorSession = session.toConnectorSession(catalogName);
        ConnectorMetadata metadata = catalogMetadata.get().getMetadataFor(catalogName);
        return metadata.listRoles(connectorSession).stream()
                .map(role -> role.toLowerCase(ENGLISH))
                .collect(toImmutableSet());
    }