Semaphore on channel

main.go

package main

import (
	"bufio"
	"flag"
	"fmt"
	"io/ioutil"
	"net/http"
	"os"
	"strings"
	"sync"
	"sync/atomic"
)

func main() {
	parallelism := flag.Int("n", 4, "Number of go-routines")
	flag.Parse()
	if *parallelism < 1 {
		fmt.Fprintf(os.Stderr, "invalid parallelism number %d, should be a positive number", *parallelism)
		os.Exit(2)
	}
	err := work(*parallelism)
	if err != nil {
		fmt.Fprintf(os.Stderr, "%+v", err)
		os.Exit(1)
	}
}

func work(parallelism int) error {
	semaphore := make(chan struct{}, parallelism)
	wg := new(sync.WaitGroup)
	scanner := bufio.NewScanner(os.Stdin)
	var total int64
	for scanner.Scan() {
		if err := scanner.Err(); err != nil {
			return err
		}
		line := scanner.Text()
		semaphore <- struct{}{}
		wg.Add(1)
		go func() {
			n, err := processURL(line)
			if err != nil {
				fmt.Fprintf(os.Stderr, "Error occurred on loading %s: %+v\n", line, err)
			}
			fmt.Printf("Count for %s: %d\n", line, n)
			atomic.AddInt64(&total, int64(n))
			<-semaphore
			wg.Done()
		}()
	}
	wg.Wait()
	fmt.Printf("Total: %d\n", total)

	return nil
}

func processURL(url string) (int, error) {
	res, err := http.Get(url)
	if err != nil {
		return 0, err
	}
	defer func() {
		_ = res.Body.Close()
	}()
	body, err := ioutil.ReadAll(res.Body)
	if err != nil {
		return 0, err
	}
	return strings.Count(string(body), "Go"), nil
}