presto-resource-group-managers/src/main/java/io/prestosql/plugin/resourcegroups/db/DbResourceGroupConfigurationManager.java/DbResourceGroupConfigurationManager/configuredGroups
class DbResourceGroupConfigurationManager:    private List<ResourceGroupId> configuredGroups(ResourceGroupIdTemplate idTemplate)
    {
        List<ResourceGroupId> groups = configuredGroups.get(idTemplate);
        if (groups == null) {
            return ImmutableList.of();
        }
        synchronized (groups) { // configuredGroups values are synchronized lists
            return ImmutableList.copyOf(groups);
        }
    }