cancel_goroutines.go

package main

import (
	"context"
	"log"
	"sync"
	"time"
)

var wg sync.WaitGroup
var stopEverything = make(chan bool)

func main() {
	//number of workers
	n := 4
	ctx, cancel := context.WithCancel(context.Background())

	//from 1 to n
	for i := 1; i <= n; i++ {
		wg.Add(1)
		go worker(ctx, i)
	}

mainLoop:
	for {
		select {
		case <-stopEverything:
			log.Printf("Callin cancel()...")
			cancel()
			break mainLoop
		}
	}
	wg.Wait()
}

//worker prints his ID every 2 seconds
func worker(ctx context.Context, id int) {
	//internal counter used to trigger an error condition
	x := 0
	for {
		select {
		default:
			x++
			//ERROR CONDITION!!!
			//WE SHOULD NEVER EVER EVER HAVE X GREATER THAN 3 WHEN ID=4 !!!!
			//STOP EVERYTHING
			if id == 4 && x > 3 {
				log.Printf("goroutine #%v: WE SHOULD NEVER EVER EVER HAVE X GREATER THAN 3 WHEN ID=4 !!!! STOP EVERYTHING", id)
				stopEverything <- true
			}

			time.Sleep(2 * time.Second)
			log.Printf("goroutine #%v", id)
		case <-ctx.Done():
			log.Printf("RETURNING from goroutine #%v", id)
			wg.Done()
			return
		}
	}
}