embed: unable to access embed openFile Seek method

[nekinie]

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

go version go1.16rc1 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
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/nekinie/.cache/go-build"
GOENV="/home/nekinie/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/nekinie/Go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/nekinie/Go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/home/nekinie/sdk/go1.16rc1"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/nekinie/sdk/go1.16rc1/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.16rc1"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/nekinie/Projects/cthulhu/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build3754598342=/tmp/go-build -gno-record-gcc-switches"

What did you do?

//go:embed html/* css/compiled.css
var static embed.FS

func serveFile(name string) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		file, err := static.Open(name)
		if err != nil {
			http.Error(w, "file not found", http.StatusNotFound)
			return
		}

		defer file.Close()
		http.ServeContent(w, r, file.Name(), file.ModTime(), file)
	}
}

What did you expect to see?

The file to be served.

What did you see instead?

./main.go:32:31: file.Name undefined (type fs.File has no field or method Name)
./main.go:32:44: file.ModTime undefined (type fs.File has no field or method ModTime)
./main.go:32:52: cannot use file (type fs.File) as type io.ReadSeeker in argument to http.ServeContent:
        fs.File does not implement io.ReadSeeker (missing Seek method)

Extra

embed.openFile implements io.ReadSeeker:
https://github.com/golang/go/blob/master/src/embed/embed.go#L357

but we return fs.File interface when opening a file:
https://github.com/golang/go/blob/master/src/embed/embed.go#L295

and the openFile type is not public:
https://github.com/golang/go/blob/master/src/embed/embed.go#L337

My opinion is openFile type should be public and we should return it instead of fs.FS
Edit: On second thought, I think the openFile and openDir types being public would be enough

[nekinie]

https://go-review.googlesource.com/c/go/+/290175

[ianlancetaylor]

The intent is that you write the code to work with any file system, perhaps like this:

func serveFile(name string) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		file, err := static.Open(name)
		if err != nil {
			http.Error(w, "file not found", http.StatusNotFound)
			return
		}

		defer file.Close()
		st, err := file.Stat()
		if err != nil {
			http.Error(w, err.Error(), http.StatusForbidden)
			return
		}
		http.ServeContent(w, r, name, st.ModTime(), file.(io.ReadSeeker))
	}
}

[nekinie]

I can confirm this resolves my issue.
I assume file.(io.ReadSeeker) works because the underlying Seeker method is already publicly exposed.
Thanks for taking the time to look at this.

[kamranjon]

Does this work with the renderer? I'm getting:
no such file or directory
When my TMX file references a png tileset.

looking at this line here and wondering if it works with the embedded FS impl: https://github.com/lafriks/go-tiled/blob/master/render/renderer.go#L89