Two folders in the file system as one virtual folder in the path of the site address (web server in Go)

I have two folders in the file system "files1" and "files2".

I can host one folder in the file system as one virtual folder in the path of the site address like this:

http.Handle("/public/", http.StripPrefix("/public/", http.FileServer(http.Dir("./files1"))))

How can I host the contents of the "files1" and "files2" folders on the same path of the site address "/public/" ?

A simple solution is to implement the http.FileSystem interface.

Here is the demo:

package main

import (
	"errors"
	"io/fs"
	"net/http"
)

func main() {
	http.Handle("/public/", http.StripPrefix("/public/", http.FileServer(mergedDir{
		Dir1: "./files1",
		Dir2: "./files2",
	})))

	http.ListenAndServe(":8080", nil)
}

type mergedDir struct {
	Dir1 http.Dir // Dir1 will be tried first so it has higher priority.
	Dir2 http.Dir
}

func (d mergedDir) Open(name string) (http.File, error) {
	f, err := d.Dir1.Open(name)

	if err != nil {
		if errors.Is(err, fs.ErrNotExist) {
			return d.Dir2.Open(name)
		}
	}

	return f, err
}

I have tested with this directory structure:

├── files1
│   ├── f1-1.txt
│   └── f1-sub
│       └── f1-s.txt
└── files2
    ├── f1-1.txt
    ├── f2-1.txt
    └── f2-sub
        └── f2-s.txt

There are two f1-1.txt, the one in files1 is served because files1 is tried first.

Update:

Another version of mergedDir to support multiple directories as requested by the author:

type mergedDir struct {
	Dirs []http.Dir
}

func (d mergedDir) Open(name string) (http.File, error) {
	for _, dir := range d.Dirs {
		f, err := dir.Open(name)
		if err == nil {
			return f, nil
		}
		if !errors.Is(err, fs.ErrNotExist) {
			return f, err
		}
	}

	return nil, fs.ErrNotExist
}