test.go

package main

import (
	"fmt"
	"math/rand"
	"time"
)

// Message ...
type Message struct {
	cmd       string
	timestamp time.Time
}

func randint(min int, max int) int {
	return rand.Intn(max-min) + min
}

func ack(ch chan string, cmd string) {
	num := randint(3, 10)
	time.Sleep(time.Duration(num) * time.Second)
	ch <- fmt.Sprintf("ACK: %s", cmd)
}

func send(ch chan Message) {
	for {
		num := randint(5, 10)
		time.Sleep(time.Duration(num) * time.Second)
		timestamp := time.Now()
		msg := new(Message)
		msg.cmd = fmt.Sprintf("cmd%d", num)
		msg.timestamp = timestamp
		ch <- *msg
	}
}

func retry(sendChan chan Message, ackChan chan string, msg Message) {
	now := time.Now()
	if now.Sub(msg.timestamp) < time.Duration(2*time.Second) {
		newMsg := new(Message)
		newMsg.cmd = fmt.Sprintf("Retry: %s", msg.cmd)
		newMsg.timestamp = msg.timestamp
		sendChan <- *newMsg
		ack(ackChan, msg.cmd)
	} else {
		fmt.Printf("Timeout: %s", msg.cmd)
	}
}

func main() {
	ackChan := make(chan string, 10)
	sendChan := make(chan Message, 10)

	var msg *Message

	go send(sendChan)

	for {
		select {
		case res := <-ackChan:
			fmt.Println(res)
		case res := <-sendChan:
			msg = new(Message)
			msg.cmd = res.cmd
			msg.timestamp = res.timestamp
			fmt.Println(res.cmd)
			go ack(ackChan, res.cmd)
		case <-time.After(1 * time.Second):
			if msg != nil {
				go retry(sendChan, ackChan, *msg)
				msg = nil
			}
		}
	}
}