//go:build linux

package reportbuf

import (
	"log/slog"
	"os"
	"syscall"

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

// chownFile gives name, a regular file or symbolic link inside root, to
// uid and gid, unless it has a second name (hard link): all the names
// of a file share its owner, and the other could be outside root. It
// opens name through root with O_PATH, which reads nothing from the
// file; os.Root adds O_NOFOLLOW, and the two together open a symbolic
// link itself, never what it points to. It reads the number of names
// from the open file and gives that file to them, so a hard link
// swapped in at name is caught too. It logs a file it leaves.
//
// This file is built only on Linux, because O_PATH and AT_EMPTY_PATH
// exist only there; prepare-data-dir runs only in the Linux image. On
// any other system chownFile returns an error (see datadir_other.go).
func chownFile(root *os.Root, name string, uid, gid int) error {
	file, err := root.OpenFile(name, unix.O_PATH, 0)
	if err != nil {
		return err
	}

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

	info, err := file.Stat()
	if err != nil {
		return err
	}

	// File.Chown would fail: fchown refuses a file opened with O_PATH.
	// Fchownat with an empty name and AT_EMPTY_PATH acts on the open
	// file instead.
	stat, ok := info.Sys().(*syscall.Stat_t)
	if ok && stat.Nlink == 1 {
		return unix.Fchownat(int(file.Fd()), "", uid, gid, unix.AT_EMPTY_PATH)
	}

	slog.Warn("owner not changed: file has a second name (hard link)",
		"path", file.Name())

	return nil
}