Golang: Creating temporary directory for testing a file reading function

I'm trying to write a test for a file reading function. The function accepts directory path & start scanning to categorize files with the sub-directories.

func FileScan(dir string) (map[string][]string, error) {
	var dataset = make(map[string][]string)

	files, err := ioutil.ReadDir(dir)
	if err != nil {
		return nil, err
	}

	for _, file := range files {
		if file.IsDir() {
			dirFiles, err := ioutil.ReadDir(fmt.Sprintf("%s/%s", dir, file.Name()))
			if err != nil {
				return nil, err
			}

			for _, dirFile := range dirFiles {
				if !dirFile.IsDir() {
					dataset[file.Name()] = append(dataset[file.Name()], dirFile.Name())
				}
			}
		}
	}
	return dataset, nil
}

For testing, I created a temporary directory next to the test file & put some sub-directories & files insides them, then start to test the function & call a cleaner function via defer to delete the directory after test.

Is this a good practice? 🤔
or maybe it's better to pass an array of []fs.FileInfo instead of directory path, so I can implement a mocked FileInfo in testing.

You can try to make your tests cleaner, faster and avoid touching the filesystem if you modify your code in a way it could be tested with https://pkg.go.dev/testing/fstest@go1.17.3#MapFS
Side note: it's recommended to use os.ReadDir instead of ioutil.ReadDir

Here is a working example with the minimal modification of your code (playground link):

package main

import (
	"fmt"
	"io/fs"
	"log"
	"path"
	"reflect"
	"testing/fstest"
)

func main() {
	var myFiles fstest.MapFS = make(map[string]*fstest.MapFile)
	myFiles["test_dir/i_am_a_file"] = &fstest.MapFile{}
	myFiles["test_dir/i_am_a_directory/add_me_to_map"] = &fstest.MapFile{}
	myFiles["test_dir/i_am_a_directory_too/add_me_to_map_too"] = &fstest.MapFile{}
	myFiles["test_dir/i_am_a_directory_too/and_me"] = &fstest.MapFile{}
	myFiles["test_dir/i_am_a_directory_too/do_not_add_me_to_map/"] = &fstest.MapFile{}
	m, err := FileScan(myFiles, "test_dir")
	if err != nil {
		log.Fatalln("Test failed:", err)
	}
	expectedMap := map[string][]string{
		"i_am_a_directory":     {"add_me_to_map"},
		"i_am_a_directory_too": {"add_me_to_map_too", "and_me"},
	}

	if !reflect.DeepEqual(expectedMap, m) {
		log.Fatalf("m: expected  = %v, observed = %v", 2, expectedMap, m)
	}
	fmt.Println("OK")
}

func FileScan(fileSystem fs.FS, dir string) (map[string][]string, error) {
	var dataset = make(map[string][]string)
	files, err := fs.ReadDir(fileSystem, dir)
	if err != nil {
		return nil, err
	}
	for _, file := range files {
		if !file.IsDir() {
			continue
		}
		dirFiles, err := fs.ReadDir(fileSystem, path.Join(dir, file.Name()))
		if err != nil {
			return nil, err
		}
		for _, dirFile := range dirFiles {
			if dirFile.IsDir() {
				continue
			}
			dataset[file.Name()] = append(dataset[file.Name()], dirFile.Name())
		}

	}
	return dataset, nil
}