udp-client.go

package main

import (
	"context"
	"fmt"
	"log"
	"net"
	"os"
	"time"

	"golang.org/x/sync/errgroup"
)

func main() {
	log.SetFlags(0)

	err := run(context.Background())
	if err != nil {
		log.Fatalln(err)
	}
}

func run(ctx context.Context) error {
	port, ok := os.LookupEnv("PORT")
	if !ok {
		return fmt.Errorf("PORT is required")
	}

	srcAddr, err := net.ResolveUDPAddr("udp", "127.0.0.1:0")
	if err != nil {
		return fmt.Errorf("error parsing source address: %w", err)
	}

	dstAddr, err := net.ResolveUDPAddr("udp", "127.0.0.1:"+port)
	if err != nil {
		return fmt.Errorf("error parsing destination address: %w", err)
	}

	log.Println("connect", "from", srcAddr, "to", dstAddr.String())
	conn, err := net.ListenUDP("udp", srcAddr)
	if err != nil {
		return fmt.Errorf("failed to start udp connection: %w", err)
	}

	eg, ctx := errgroup.WithContext(ctx)
	eg.Go(func() error {
		ticker := time.NewTicker(5 * time.Second)
		defer ticker.Stop()

		for {
			var tm time.Time
			select {
			case <-ctx.Done():
				return context.Cause(ctx)
			case tm = <-ticker.C:
			}

			str := tm.Format(time.RFC3339Nano)
			log.Println("send", str, "to", dstAddr.String())
			_, err := conn.WriteToUDP([]byte(str), dstAddr)
			if err != nil {
				return fmt.Errorf("error writing packet: %w", err)
			}
		}
	})
	eg.Go(func() error {
		for {
			var packet [65535]byte
			n, srcAddr, err := conn.ReadFromUDP(packet[:])
			if err != nil {
				return fmt.Errorf("error reading packet: %w", err)
			}
			log.Println("recv", string(packet[:n]), "from", srcAddr.String())
		}
	})
	return eg.Wait()
}