package rootstate

import (
	"os"
	"path/filepath"
	"strings"

	"golang.org/x/sys/unix"
)

func ReadTrustedLink(path string) (string, error) {
	if !filepath.IsAbs(path) || filepath.Clean(path) != path {
		return "", os.ErrPermission
	}
	s, err := Open(filepath.Dir(path), false, true)
	if err != nil {
		return "", err
	}
	defer s.Close()
	return s.(*directory).readLink(filepath.Base(path))
}

func (d *directory) readLink(name string) (string, error) {
	if !component(strings.ReplaceAll(name, "@", "_")) {
		return "", os.ErrPermission
	}
	if err := d.check(); err != nil {
		return "", err
	}
	fd, err := unix.Openat(int(d.file.Fd()), name, unix.O_PATH|unix.O_NOFOLLOW|unix.O_CLOEXEC, 0)
	if err != nil {
		return "", err
	}
	defer unix.Close(fd)
	var st unix.Stat_t
	if err := unix.Fstat(fd, &st); err != nil {
		return "", err
	}
	if st.Uid != d.owner || st.Mode&unix.S_IFMT != unix.S_IFLNK || st.Nlink != 1 {
		return "", os.ErrPermission
	}
	var buf [4096]byte
	n, err := unix.Readlinkat(fd, "", buf[:])
	if err != nil {
		return "", err
	}
	if n == len(buf) {
		return "", os.ErrPermission
	}
	return string(buf[:n]), nil
}