discordgo zero downtime restart

main.go

package main

import (
	"flag"
	"fmt"
	"os"
	"os/signal"
	"reflect"
	"sync/atomic"
	"syscall"
	"unsafe"

	"github.com/bwmarrin/discordgo"
)

var (
	sid string
	seq int64
)

func init() {
	flag.StringVar(&sid, "sid", "", "")
	flag.Int64Var(&seq, "seq", 0, "")

	flag.Parse()
}

func main() {
	s, _ := discordgo.New("Bot " + os.Getenv("DISCORD_TOKEN"))

	if sid != "" && seq != 0 {
		setField(s, "sequence", &seq)
		setField(s, "sessionID", sid)
	}

	s.AddHandler(func(se *discordgo.Session, e *discordgo.Event) {
		fmt.Println(e.Type)
	})

	s.Open()

	sig := make(chan os.Signal)
	signal.Notify(sig, syscall.SIGINT)
	<-sig

	fmt.Printf("to resume: go run cmd/resume/main.go -seq %v -sid %v\n", getSequence(s), getField(s, "sessionID").String())
}

func setField(s interface{}, fieldName string, newVal interface{}) {
	val := reflect.ValueOf(s)
	field := reflect.Indirect(val).FieldByName(fieldName)
	ptrToField := unsafe.Pointer(field.UnsafeAddr())
	settableField := reflect.NewAt(field.Type(), ptrToField).Elem()

	n := reflect.ValueOf(newVal)
	settableField.Set(n)
}

func getField(s interface{}, fieldName string) reflect.Value {
	val := reflect.ValueOf(s)
	return reflect.Indirect(val).FieldByName(fieldName)
}

func getSequence(s *discordgo.Session) int64 {
	field := getField(s, "sequence")
	ptr := (*int64)(unsafe.Pointer(field.Pointer()))
	return atomic.LoadInt64(ptr)
}