FUSE_SET_ATTR_GID
static_assert(FUSE_FATTR_GID==FUSE_SET_ATTR_GID, "definition mismatch");
if (!err && (flags & FUSE_SET_ATTR_UID || flags & FUSE_SET_ATTR_GID)) {
gid = (flags & FUSE_SET_ATTR_GID) ? attr->st_gid : (gid_t)-1;