You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: fileserver.go
    path of file: ./repos/bud/package/genfs
    
    the code of the file until where you have to start completion: package genfs

import (
	"io/fs"

	"github.com/livebud/bud/package/virtual"
)

type FileServer interface {
	ServeFile(fsys FS, file *File) error
}

type ServeFile func(fsys FS, file *File) error

func (fn ServeFile) ServeFile(fsys FS, file *File) error {
	return fn(fsys, file)
}

type fileServer struct {
	cache Cache
	fn    func(fsys FS, file *File) error
	genfs fs.FS

    
func (f *fileServer) Generate(target string) (fs.File, error) {
	if file, err := f.cache.Get(target); err == nil {
		return virtual.Open(file), nil
	}
	// Always return an empty directory if we request the root
	if f.path == target {
		return virtual.Open(&virtual.File{
			Path: f.path,
			Mode: fs.ModeDir,
		}), nil
	}
	scopedFS := &scopedFS{f.cache, f.genfs, target}
	file := &File{nil, f.path, target}
	// g.fsys.log.Fields(log.Fields{
	// 	"target": target,
	// 	"path":   g.node.Path(),
	// }).Debug("budfs: running file server function")
	if err := f.fn(scopedFS, file); err != nil {
		return nil, err
	}
	vfile := &virtual.File{
		Path: target,
		Mode: fs.FileMode(0),
		Data: file.Data,
	}
	if err := f.cache.Set(target, vfile); err != nil {
		return nil, err
	}
	return virtual.Open(vfile), nil
}