proposal: x/tools/txtar: add Extract function or method

[adonovan]

While developing tests, I've written this function at least four times this week:

package txtar

// Extract writes each file in the archive to the corresponding location beneath dir.
func Extract(ar *Archive, dir string) error {
	for _, file := range ar.Files {
		name := filepath.Join(dir, file.Name)
		if err := os.MkdirAll(filepath.Dir(name), 0777); err != nil {
			return err
		}
		if err := os.WriteFile(name, file.Data, 0666); err != nil {
			return err
		}
	}
	return nil
}

I propose that we add it to the txtar package, either as a function or a method of Archive.

[mvdan]

Written a while back, but it might be useful: https://pkg.go.dev/github.com/rogpeppe/go-internal/txtar#Write cc @rogpeppe @myitcv

Also, shouldn't we use #56219?

[adonovan]

Personally I only use txtar for tests, but rejecting up-level references seems prudent. I'm less convinced about O_EXCL: I expect an extract operation to write files, even over existing ones.

[seankhliao]

alternatively, txtar could implement fs.FS for #44158
and, os can get a WriteFS(dir string, fsys fs.FS) error that would be more flexible

[rsc]

I'm fine with this as txtar.Extract, but the implementation should use filepath.ToSlash and filepath.IsLocal, as @mvdan noted.

I would rather not introduce a method since right now the Archive type is trivial, pure data, and I'd like to keep it that way.

[rsc]

This proposal has been added to the active column of the proposals project
and will now be reviewed at the weekly proposal review meetings.
— rsc for the proposal review group

[rsc]

If we do #44158 and then add os.CopyFS from #45757 (comment), then like @seankhliao said above, that seems like a nicer outcome.

We would have to figure out what to do about txtar files that say ../ at the start, but archive/zip has to do that too and we can do whatever it does (drop them?).

[rsc]

#44158 is accepted, and #62484 is going to be accepted very soon, so probably we can close this one.

[rsc]

This proposal has been declined as retracted.
— rsc for the proposal review group