Path mismatch when using `dev` tag

[wonderhoss]

I'm trying to follow the example in the README on how to handle local filesystem vs embedded assets with a dev build tag.

My project looks like this:

├── Makefile
├── cmd
│   └── main.go
├── frontend
│   └── index.html
├── go.mod
├── go.sum
└── pkg
    └── data
        ├── assets_generate.go
        ├── data.go
        └── dev.go

In my data.go I have:

// +build ignore

package main

import (
	"log"
	"github.com/shurcooL/vfsgen"

	"github.com/gargath/vfstest/pkg/data"
)

func main() {
	err := vfsgen.Generate(data.Assets, vfsgen.Options{
		PackageName:  "data",
		BuildTags:    "!dev",
		VariableName: "Assets",
	})

	if err != nil {
		log.Fatalln(err)
	}
}

while in dev.go I have:

// +build dev

package data

import (
	"net/http"
)

var Assets http.FileSystem = http.Dir("../../frontend/")

My cmd/main.go is a simple

package main

import (
	"net/http"
	"log"

	"github.com/gargath/vfstest/pkg/data"
)

func main() {
	fs := http.FileServer(data.Assets)
	http.Handle("/", fs)

	err := http.ListenAndServe(":3000", nil)
	if err != nil {
		log.Fatal(err)
	}
}

When I run go generate ./pkg/... and then go build -o vfserver github.com/gargath/vfstest/cmd, I get a working binary that serves the embedded content.

However, when running go build -tags dev -o vfserver github.com/gargath/vfstest/cmd, I get 404 errors from the HTTP server because it is now trying to find the path ../../frontend/ relative to the binary in the project root.
I can fix this by changing the path in data.Assets to frontend/, but then go generate no longer works:

go generate ./pkg/...
2020/03/17 11:22:14 open frontend: no such file or directory
exit status 1
pkg/data/data.go:3: running "go": exit status 1
make: *** [generate] Error 1

How can I structure a project so that the same static assets directory can be used to both generate and serve locally?

[DHowett]

I'm curious about this as well.

There are a few other folks who have put together examples of vfsgen's usage (https://github.com/ahrtr/vfsgenDemo and https://github.com/artificerpi/vfsgen-sample), but they all seem to have the same issue: they place assets in a subdirectory of a subpackage, but use a path relative to the root of the subpackage in their http.FileSystem.

I'm not certain either of these samples have been tested in development mode 😄

[dmitshur]

Sorry I missed this earlier.

Your definition of Assets in dev.go specifies a relative path, which is prone to this kind of problem.

When go generate runs data.go, the working directory is one thing, but when you run your project's binary via go build -tags dev, the working directory may be different. So a relative path is not a reliable way of specifying a directory.

I suggest fixing this problem by defining Assets using http.Dir with an absolute path. There are various ways to do this. I've been using a helper like this:

func importPathToDir(importPath string) string {
	p, err := build.Import(importPath, "", build.FindOnly)
	if err != nil {
		log.Fatalln(err)
	}
	return p.Dir
}

Then you can do something like:

var Assets http.FileSystem = http.Dir(filepath.Join(importPathToDir("github.com/gargath/vfstest"), "frontend"))

You can see an example of me using vfsgen in one of my projects here.

[DHowett]

This is excellent. Thanks!

[fopina]

I literally just started using vfsgen (30min ago) and bumped into this immediately, no idea how so many projects use relative paths, pretty sure they have never executed with -tags=dev ....

I decided to go with a generate.go such as:

// +build ignore

package main

import (
	"log"
	"os"

	"github.com/owner/example/assets"
	"github.com/shurcooL/vfsgen"
)

func main() {
	// need to change directory so assets.Assets "static" works for both dev and vfsgen...
	// gap in vfsgen or misuse?
	err := os.Chdir("..")
	if err != nil {
		log.Fatalln(err)
	}
	err = vfsgen.Generate(assets.Assets, vfsgen.Options{
		PackageName:  "assets",
		BuildTags:    "!dev",
		VariableName: "Assets",
		Filename:     "assets/assets_vfsdata.go",
	})
	if err != nil {
		log.Fatalln(err)
	}
}

And defining http.Dir to just static (instead of ../static)

When executing go run main.go static is in that folder. When doing go generate ./assets, regardless of where I call it, current directory will always be the one of generate.go, so Chdir("..") should work every time.

But even though I agree that relative paths are prone to errors, maybe the helper mentioned in this issue should be part of the package officially?