concurrency.go

package main

import (
	"fmt"
	"sync"
	"time"

	"errors"

	"math/rand"
)

func handler(a []string) error {
	errChan := make(chan error, len(a))
	sem := make(chan int, 4)
	var wg sync.WaitGroup
	wg.Add(len(a))
	for _, val := range a {
		go func(i string) {
			defer wg.Done()
			sem <- 1
			if err := fetch(i); err != nil {
				errChan <- err
			}
			<-sem
		}(val)
	}
	wg.Wait()
	close(errChan)
	return <-errChan
}

func fetch(a string) error {
	fmt.Println("fetch func ...")
	time.Sleep(1 * time.Second)
	if rand.Intn(10)%2 == 0 {
		return nil
	}
	return errors.New("error occurred")
}

func main() {
	input := []string{"a", "b", "c"}
	handler(input)
}