package main

import (
    "fmt"
    "os"
    "os/exec"
    "time"
)

func main() {
    prom := exec.Command("tail", "-f /dev/null")
    done := make(chan error)
    go func() {
        done <- prom.Run()
    }()

    prom.Process.Signal(os.Interrupt)
    <-done

    pid, e := os.FindProcess(prom.Process.Pid)

    fmt.Println(pid.Pid) // Here it always returns some PID as if the process hasn't stopped yet.
    fmt.Println(e)
}