You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: lookup_unix.go
    path of file: ./repos/kubernetes/vendor/github.com/opencontainers/runc/libcontainer/user
    
    the code of the file until where you have to start completion: //go:build darwin || dragonfly || freebsd || linux || netbsd || openbsd || solaris
// +build darwin dragonfly freebsd linux netbsd openbsd solaris

package user

import (
	"io"
	"os"
	"strconv"

	"golang.org/x/sys/unix"
)

// Unix-specific path to the passwd and group formatted files.
const (
	unixPasswdPath = "/etc/p
    
func lookupGroupFunc(filter func(g Group) bool) (Group, error) {
	// Get operating system-specific group reader-closer.
	group, err := GetGroup()
	if err != nil {
		return Group{}, err
	}
	defer group.Close()

	// Get the users.
	groups, err := ParseGroupFilter(group, filter)
	if err != nil {
		return Group{}, err
	}

	// No user entries found.
	if len(groups) == 0 {
		return Group{}, ErrNoGroupEntries
	}

	// Assume the first entry is the "correct" one.
	return groups[0], nil
}