@Override
    public void set(File file, String view, String attribute, Object value, boolean create) {
        switch (attribute) {
            case "uid" -> {
                file.setAttribute("owner", "owner", new BasicUserPrincipal(String.valueOf(value)));
                return;
            }
            case "gid" -> {
                file.setAttribute("posix", "group", new BasicGroupPrincipal(String.valueOf(value)));
                return;
            }
        }
        throw unsettable(view, attribute, create);
    }
Same as {@code com.google.common.jimfs.UnixAttributeProvider} except that getUniqueId() will return user
if user is a numerical string.
testutil/src/main/java/com/yahoo/vespa/test/file/UnixUidGidAttributeProvider.java