/etc/group file parsing doesn't support groups with multiple members

[joonas-fi]

Group List: It is a list of user names of users who are members of the group. The user names, must be separated by commas.

https://www.cyberciti.biz/faq/understanding-etcgroup-file/

[joonas-fi]

Should add tests

[joonas-fi]

Better implementation:

func resolveSupplementaryGids(username string) ([]uint32, error) {
	if err := ErrorIfUnset(username == "", "username"); err != nil {
		return nil,err
	}

	groups := []uint32{}

	groupFile, err := os.Open("/etc/group")
	if err != nil {
		return nil, err
	}
	defer groupFile.Close()

	groupLines := bufio.NewScanner(groupFile)

	for groupLines.Scan() {
		// docker:x:129:joonas
		// docker:x:129:joonas,anotheruser
		parts := strings.Split(groupLines.Text(), ":")
		if len(parts) < 4 {
			return nil, fmt.Errorf("/etc/group invalid parts number: %d", len(parts))
		}

		// "joonas,anotheruser" => ["joonas", "anotheruser"]
		lineUsernames := strings.Split(parts[3], ",")
		if sliceutil.ContainsString(lineUsernames, username) {
			gid, err := strconv.Atoi(parts[2])
			if err != nil {
				return nil, err
			}

			groups = append(groups, uint32(gid))
		}
	}
	if err := groupLines.Err(); err != nil {
		return nil, err
	}

	return groups, nil
}

[joonas-fi]

stdlib os/user seems to already have facilities for this