Embed: file.ModTime returns time.Time{} instead of the file's modification time at the time it was embedded

#1

I embedded a file like this:

package main

import (
	"fmt"
	"embed"
	"net/http"
)

//go:embed test.txt
var fs embed.FS

func main() {
	f, _ := fs.Open("test.txt")
	fi, _ := f.Stat()
	fmt.Println(fi.ModTime()) // 0001-01-01 00:00:00 +0000 UTC

	// With Last-Modified header
	http.Handle("/dir/", http.StripPrefix("/dir/", http.FileServer(http.Dir("."))))
	
	// Without Last-Modified header -> easy to circumvent, but not suitable as drop-in replacement
	http.Handle("/embed/", http.StripPrefix("/embed/", http.FileServer(http.FS(fs))))

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

I expected fi.ModTime to return the file’s modification time at the time it was embedded.

time.Time{} is returned instead (https://golang.org/src/embed/embed.go#229). The above program includes an example of when this becomes relevant. Please help me understand this design decision.

#2

If I’m reading the git blame correctly, it looks like it was committed this way in the initial implementation. Based on that, I suspect it wasn’t considered important to include it. You could open an issue requesting the functionality, maybe add a PR with an implementation submitting a patch that implements this functionality.

I’m curious, what do you want the embedded file’s original modification time for?

EDIT: You could probably wrap the embedded fs.FS to implement this yourself with a go:generate command.

#3

Oh, I found it:

It’s about build reproducibility. Embedded files with different modification times but the same contents shouldn’t result in different builds.

2 Likes
#4

Excellent find, thanks. I now understand the rationale behind this design, but this makes the documentation (https://golang.org/pkg/embed/#hdr-File_Systems) misleading. At the very least this justifies a warning. Do you know how to properly report this?