archive/tar: add (*Writer).AddFS

[c9845]

This proposal is similar to #54898, except for archive/tar.

The idea would be to provide an AddFS method that would tar an entire directory tree provided by fs.FS.

Why implement this:

1. If similar functionality will be implemented for archive/zip, it makes sense to implement it for archive/tar since the use case is similar.
2. This would prevent the need for developers to manually write a "tar this directory" function which would more likely have bugs than a standard-lib implementation.
3. Taring a directory is a pretty common task, and since fs.FS exists, we might as well use it.

[ianlancetaylor]

CC @dsnet

[rsc]

Generally speaking we keep archive/tar and archive/zip with comparable API, so I think it makes sense to accept this at the same time we accept the archive/zip change.

[rsc]

I thought I needed this function in a program I was writing, so I wrote it (I've since changed my mind about needing it in this case). Should be something like this.

func tarAddFS(w *tar.Writer, fsys fs.FS) error {
	return fs.WalkDir(fsys, ".", func(name string, d fs.DirEntry, err error) error {
		if err != nil {
			return err
		}
		if d.IsDir() {
			return nil
		}
		info, err := d.Info()
		if err != nil {
			return err
		}
		h, err := tar.FileInfoHeader(info, "")
		if err != nil {
			return err
		}
		h.Name = name
		if err := w.WriteHeader(h); err != nil {
			return err
		}
		f, err := fsys.Open(name)
		if err != nil {
			return err
		}
		defer f.Close()
		_, err = io.Copy(w, f)
		return err
	})
}

[rsc]

Based on the discussion above, this proposal seems like a likely accept.
— rsc for the proposal review group

[rsc]

No change in consensus, so accepted. 🎉
This issue now tracks the work of implementing the proposal.
— rsc for the proposal review group