// ntp is a minimal NTP client for gokrazy.
package main

import (
	"encoding/json"
	"flag"
	"io/ioutil"
	"log"
	"math/rand"
	"net"
	"os"
	"os/signal"
	"path/filepath"
	"syscall"
	"time"

	"github.com/beevik/ntp"
	"github.com/rtr7/dhcp4"
)

var server = "0.gokrazy.pool.ntp.org"

var permDir = flag.String("perm", "/perm", "path to replace /perm")

func set(rtc *os.File) error {
	r, err := ntp.Query(server)
	if err != nil {
		return err
	}

	tv := syscall.NsecToTimeval(r.Time.UnixNano())
	if err := syscall.Settimeofday(&tv); err != nil {
		return err
	}
	log.Printf("clock set to %v using %v", r.Time, server)

	if rtc == nil {
		return nil
	}
	return setRTC(rtc, r.Time.UTC())
}

func main() {
	flag.Parse()
	log.SetFlags(log.LstdFlags | log.Lshortfile)

	var rtc *os.File
	var err error
	if os.Getenv("NTP_PRIVILEGES_DROPPED") == "1" {
		if os.Getenv("NTP_RTC") == "1" {
			rtc = os.NewFile(3, "/dev/rtc0")
		}
	} else {
		rtc, err = os.Open("/dev/rtc0")
		if err != nil && !os.IsNotExist(err) {
			log.Fatal(err)
		}
	}

	mustDropPrivileges(rtc)

	go func() {
		ch := make(chan os.Signal, 1)
		signal.Notify(ch, syscall.SIGUSR1)
		for range ch {
			loadNTPServer()
		}
	}()
	loadNTPServer()

	for {
		if err := set(rtc); err != nil {
			log.Fatalf("setting time failed: %v", err)
		}
		time.Sleep(1*time.Hour + time.Duration(rand.Int63n(250))*time.Millisecond)
	}
}

func loadNTPServer() {
	var lease dhcp4.Lease
	file, err := ioutil.ReadFile(filepath.Join(*permDir, "dhcp4/lease.json"))
	if err != nil {
		log.Println(err)
		return
	}
	err = json.Unmarshal(file, &lease)
	if err != nil {
		log.Println(err)
		return
	}
	if len(lease.DNS) > 0 && !lease.DNS[0].To4().Equal(net.IPv4zero) {
		server = lease.DNS[0].String()
		log.Printf("Setting ntp server to: %s", server)
	}
}