package reportbuf

import (
	"errors"
	"io/fs"
	"os"
	"os/user"
	"path/filepath"
	"strconv"
)

// ErrDataDirOutsideVolume is returned by PrepareDataDir for a DATA_DIR
// that is not the volume or a path below it, written in full.
var ErrDataDirOutsideVolume = errors.New(
	"must be /data or a path below it, with no '.', '..' or extra '/'")

// PrepareDataDir gets dir, the server's DATA_DIR, ready for owner, the
// user the server runs as, so that a host directory mounted at volume,
// /data in the image, needs no preparing: it creates dir, gives volume
// and everything in it to owner, and gives volume and dir the mode the
// server gives a directory it creates. dir must be volume or a path
// below it, with no '.', '..', empty part or '/' at the end.
//
// bin/entrypoint.sh runs this as root, which would follow a symbolic
// link anywhere, so every step goes through an os.Root opened on
// volume: it follows a link only when it is written as a relative
// path that stays inside volume, and refuses any other. A process on
// the host can swap a link onto a path in volume at any moment while
// this runs, but the os.Root checks each link as it reaches it, so the
// most that can do is make a step fail or act on something else
// inside volume.
func PrepareDataDir(volume, dir string, owner *user.User) error {
	// rel is dir as a path from volume; IsLocal is false for one that
	// leads out of it.
	rel, err := filepath.Rel(volume, dir)
	if err != nil || dir != filepath.Clean(dir) || !filepath.IsLocal(rel) {
		return ErrDataDirOutsideVolume
	}

	uid, err := strconv.Atoi(owner.Uid)
	if err != nil {
		return err
	}

	gid, err := strconv.Atoi(owner.Gid)
	if err != nil {
		return err
	}

	root, err := os.OpenRoot(volume)
	if err != nil {
		return err
	}

	defer func() { _ = root.Close() }()

	err = root.MkdirAll(rel, dirPerms)
	if err != nil {
		return err
	}

	// WalkDir does not follow a symbolic link, and Lchown gives the
	// link itself to owner, not what it points to.
	err = fs.WalkDir(root.FS(), ".",
		func(name string, _ fs.DirEntry, err error) error {
			if err != nil {
				return err
			}

			return root.Lchown(name, uid, gid)
		})
	if err != nil {
		return err
	}

	err = root.Chmod(".", dirPerms)
	if err != nil {
		return err
	}

	return root.Chmod(rel, dirPerms)
}