Example how to use semaphore in Golang

main.go

package main

import (
	"context"
	"fmt"
	"os"
	"path/filepath"
	"strconv"
	"sync"

	"github.com/disintegration/imaging"
	"golang.org/x/sync/semaphore"
)

// Build with `go build -o thumbnail main.go`
func main() {
	args := os.Args
	if len(args) != 5 {
		fmt.Println("thumbnail converts images to thumbnail")
		fmt.Println("Usage: thumbnail src-folder dst-folder width height")
		return
	}

	src, dst := args[1], args[2]
	width, _ := strconv.Atoi(args[3])
	height, _ := strconv.Atoi(args[4])
	if src == "" || dst == "" || width <= 0 || height <= 0 {
		fmt.Println("invalid arguments")
		os.Exit(1)
	}

	os.MkdirAll(dst, 0777)

	fmt.Printf("resize images from '%s' to '%s'\n", src, dst)

	worker := newImageResizerWorker(10)

	srcFiles, _ := filepath.Glob(src + "/*")
	wg := sync.WaitGroup{}
	for _, in := range srcFiles {
		in := in
		_, out := filepath.Split(in)
		out = filepath.Join(dst, out)
		wg.Add(1)

		go func() {
			defer wg.Done()
			err := worker.ResizeFile(out, in, width, height)
			if err != nil {
				fmt.Printf("%s; %s\n", in, err)
			} else {
				fmt.Printf("%s; success\n", in)
			}
		}()
	}

	wg.Wait()
}

type imageResizerWorker struct {
	sem *semaphore.Weighted
}

func newImageResizerWorker(worker int64) *imageResizerWorker {
	return &imageResizerWorker{
		sem: semaphore.NewWeighted(worker),
	}
}

func (rz *imageResizerWorker) ResizeFile(dst, src string, width, height int) error {
	rz.sem.Acquire(context.Background(), 1)
	defer rz.sem.Release(1)

	format, err := imaging.FormatFromExtension(filepath.Ext(src))
	if err != nil {
		return err
	}

	r, err := os.Open(src)
	if err != nil {
		return err
	}
	defer r.Close()

	w, err := os.Create(dst)
	if err != nil {
		return err
	}
	defer w.Close()

	img, err := imaging.Decode(r)
	if err != nil {
		return err
	}

	img = imaging.Fill(img, width, height, imaging.Center, imaging.Lanczos)
	return imaging.Encode(w, img, format)
}