io/fs: provide backwards-compatible migration path from ioutil.ReadFile to fs.ReadFile

[bouk]

Hi there, I'm enjoying the go1.16 RC, I'm trying to migrate some code from using ioutil.ReadFile to fs.ReadFile while staying backwards-compatible. I'm running into two things:

• It's not possible to make code that does something like ioutil.ReadFile("./path/to/file.txt") work with os.DirFS("."), because it rejects the leading ./
• There is no way to have absolute paths for the same reason, because 'Open should reject attempts to open names that do not satisfy ValidPath(name)' according to the fs.FS documentation

I can get around those things by doing something like this:

// osFS is an fs.FS implementation that just passes on to os.Open
type osFS struct{}

func (c osFS) Open(name string) (fs.File, error) {
	return os.Open(name)
}

Which is technically an invalid implementation, although it's not entirely clear why those restrictions are in place. It would be nice if something like the above osFS was built-in, so there's a clear migration path from ioutil.ReadFile to fs.ReadFile.

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

$ go version go1.16rc1 darwin/arm64

[D1CED]

You can just create a simple compatibility decorator like this:

package relfs

import (
	"io/fs"
	"os"
	"strings"
)

type allowRelativeFS struct{ fs.FS }

func (rfs allowRelativeFS) Open(name string) (fs.File, error) {
	if strings.HasPrefix(name, "./") {
		name = name[2:]
	}
	return rfs.FS.Open(name)
}

func DirAllowRelative(dir string) fs.FS {
	return allowRelativeFS{os.DirFS(dir)}
}

You can get a lot more fancy like allowing absolute paths or add logging to ease migration.

[josharian]

Related: #44279

[toothrot]

cc @rsc

[mpx]

Perhaps fs.ValidPath could accept paths starting with ./? There are (IMO) good reasons for disallowing /, but ./ is still clearly a relative path should be ok? This might be difficult given potential incompatibilty with existing fs.FS implementations tho'.