package main

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

// docker 		run <container-name>	<command> <args>
// go main.go	run						<command> <args>

func main() {
	if len(os.Args) < 3 {
		panic("found too few arguments for the program.")
	}
	switch os.Args[1] {
	case "run":
		executeRun()
	case "child":
		executeChild()
	default:
		panic(fmt.Sprintf("found an unsupported command: %s", os.Args[0]))
	}
}

func executeRun() {
	fmt.Printf("starting: 'run' in PID: %d\n", os.Getpid())

	cmd := exec.Command("/proc/self/exe", append([]string{"child"}, os.Args[2:]...)...)
	cmd.Stdin = os.Stdin
	cmd.Stdout = os.Stdout
	cmd.Stderr = os.Stderr

	cmd.SysProcAttr = &syscall.SysProcAttr{
		Cloneflags:   syscall.CLONE_NEWUTS | syscall.CLONE_NEWPID | syscall.CLONE_NEWNS,
		Unshareflags: syscall.CLONE_NEWNS,
	}

	must(cmd.Run())
}

func executeChild() {
	fmt.Printf("starting: %s in PID: %d\n", os.Args[2], os.Getpid())

	setupCgroup()

	syscall.Sethostname([]byte("container"))
	syscall.Chroot("./container-root")
	os.Chdir("/")

	syscall.Mount("proc", "proc", "proc", 0, "")
	defer syscall.Unmount("/proc", 0)

	cmd := exec.Command(os.Args[2])
	cmd.Stdin = os.Stdin
	cmd.Stdout = os.Stdout
	cmd.Stderr = os.Stderr

	must(cmd.Run())
}

func setupCgroup() {
	cgroupDir := fmt.Sprintf("/sys/fs/cgroup/pids/%d", time.Now().Unix())
	must(os.Mkdir(cgroupDir, 0755))

	os.WriteFile(fmt.Sprintf("%s/pids.max", cgroupDir), []byte("20"), 0644)
	os.WriteFile(fmt.Sprintf("%s/notify_on_release", cgroupDir), []byte("1"), 0644)
	os.WriteFile(fmt.Sprintf("%s/cgroup.procs", cgroupDir), []byte(strconv.Itoa(os.Getpid())), 0644)
}

func must(err error) {
	if err != nil {
		panic(err)
	}
}