simple static server who return static files in rotation for every request

i have this simple server writed in golang :

package main

import (
        "net/http"
)

func main() {
        http.Handle("/", http.FileServer(http.Dir("./static")))
        http.ListenAndServe(":3000", nil)
}

I want add new function :
every request GET /rotate return one file content in rotation from /static folder.
for example in /static folder exist 7 file, for every request server return : file1, file2, file3 ...

How i can do this in go ?

Below is a simple server that should get you started toward your goal. A couple of things to note about the implementation:

1. The file names are loaded only once during program startup. If a file disappears while the program is running, the server will return a 404 when it comes up in the rotation.
2. We need to use locking (in this case, via sync.Mutex) as each request will run in its own go routine.

<!-- -->

package main

import (
	"flag"
	"net/http"
	"os"
	"path/filepath"
	"sync"
)

func main() {
	dir := flag.String("dir", ".", "directory of files to serve")
	flag.Parse()

	f, err := os.Open(*dir)
	if err != nil {
		panic(err)
	}
	files, err := f.Readdir(0)
	if err != nil {
		panic(err)
	}

	filenames := make([]string, 0, len(files))
	for _, file := range files {
		if !file.IsDir() {
			filenames = append(filenames, file.Name())
		}
	}

	var (
		idxLock sync.Mutex
		idx     int
	)

	http.HandleFunc("/rotate", func(w http.ResponseWriter, r *http.Request) {
		if len(filenames) == 0 {
			http.NotFound(w, r)
			return
		}

		idxLock.Lock()
		i := idx
		idx++
		if idx >= len(filenames) {
			idx = 0
		}
		idxLock.Unlock()

		http.ServeFile(w, r, filepath.Join(*dir, filenames[i]))
	})

	if err := http.ListenAndServe(":3000", nil); err != nil {
		panic(err)
	}
}