embed: modification time not set for embedded files

[eudore]

What version of Go are you using (go version)?

$ go version
go version go1.16 linux/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env GOARCH GOOS
amd64
linux

What did you do?

package main

import (
	"embed"
	"fmt"
)

//go:embed static
var f embed.FS

func main() {
	f, err := f.Open("static/server.html")
	if err != nil {
		fmt.Println(err)
		return
	}
	stat, err := f.Stat()
	if err != nil {
		fmt.Println(err)
		return
	}
	fmt.Println(stat.ModTime())
}

What did you expect to see?

2021-03-08 11:03:38 +0800 CST

What did you see instead?

0001-01-01 00:00:00 +0000 UTC

[jayconrod]

Closing as infeasible, unfortunately.

When modules are downloaded, their files are stripped of all metadata including timestamps and permission bits. That means embedded files from a module don't have that information. The only case where it would be feasible is when the embedded files are part of the main module, but then a program's behavior would subtly depend on how it was built.

This was discussed a bit in the embed proposal (#41191) (expand all comments, search for "timestamp"). The discussion there seems to recommend hashing and ETag for cache invalidation. However, ETag wasn't implemented in embed.FS or io/fs, and the proposal (#43223) was declined. It sounds like that may be reopened at some point when there's a clearer picture of how to implement that.

[jypelle]

I'm using this workaround to serve static files (css, js, ...):

type StaticFSWrapper struct {
	http.FileSystem
	FixedModTime time.Time
}

func (f *StaticFSWrapper) Open(name string) (http.File, error) {
	file, err := f.FileSystem.Open(name)

	return &StaticFileWrapper{File: file, fixedModTime: f.FixedModTime}, err
}

type StaticFileWrapper struct {
	http.File
	fixedModTime time.Time
}

func (f *StaticFileWrapper) Stat() (os.FileInfo, error) {

	fileInfo, err := f.File.Stat()

	return &StaticFileInfoWrapper{FileInfo: fileInfo, fixedModTime: f.fixedModTime}, err
}

type StaticFileInfoWrapper struct {
	os.FileInfo
	fixedModTime time.Time
}

func (f *StaticFileInfoWrapper) ModTime() time.Time {
	return f.fixedModTime
}

...

	// Static file handler
	//
	var staticFs     fs.FS
	// TODO: initialize staticFS with embed.FS var
	
	staticFileHandler := http.StripPrefix(
		"/static",
		http.FileServer(
			&StaticFSWrapper{
				FileSystem: http.FS(staticFs),
				FixedModTime: time.Now(),
			},
		),
	)
	d.router.PathPrefix("/static").HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		w.Header().Set("Expires", "")
		w.Header().Set("Cache-Control", "no-cache, max-age=0")
		w.Header().Set("Pragma", "")
		staticFileHandler.ServeHTTP(w, r)
	})