golang StartProcess with setuid

main.go

package main

import (
    "flag"
    "os"
    "syscall"
)

func main() {
    var uid uint
    flag.UintVar(&uid, "uid", 0, "user id")
    flag.Parse()

    attr := &os.ProcAttr{
        Files: []*os.File{os.Stdin, os.Stdout, os.Stderr},
        Sys: &syscall.SysProcAttr{
            Credential: &syscall.Credential{
                Uid: uint32(uid),
            },
        },
    }
    args := []string{
        "/usr/bin/whoami",
    }
    process, err := os.StartProcess("/usr/bin/whoami", args, attr)
    if err != nil {
        panic(err)
    }
    _, err = process.Wait()
    if err != nil {
        panic(err)
    }
}

example session

$ grep '^_uucp' /etc/passwd
_uucp:*:4:4:Unix to Unix Copy Protocol:/var/spool/uucp:/usr/sbin/uucico
$ sudo go run main.go -uid 4
_uucp
$ go version
go version go1.5.1 darwin/amd64