//go:build !windows

package backup

import (
	"crypto/rand"
	"encoding/hex"
	"errors"
	"io"
	"os"
	"path/filepath"
	"strings"

	"github.com/aritmolab/thothii/tools/tht/internal/safeio"
	"golang.org/x/sys/unix"
)

type restoreTargetIdentity struct {
	exists bool
	device uint64
	inode  uint64
}

func replaceRestoreFile(target string, contents []byte, mode os.FileMode) error {
	if safeio.ValidateCanonicalPath(target) != nil || mode&os.ModeType != 0 || mode.Perm() == 0 {
		return safeio.ErrUnsafeFile
	}
	components := strings.Split(strings.TrimPrefix(target, string(os.PathSeparator)), string(os.PathSeparator))
	if len(components) < 2 || components[0] == "" || components[len(components)-1] == "" {
		return safeio.ErrUnsafeFile
	}
	directory, err := unix.Open(string(os.PathSeparator), unix.O_RDONLY|unix.O_CLOEXEC|unix.O_DIRECTORY, 0)
	if err != nil {
		return safeio.ErrUnsafeFile
	}
	defer unix.Close(directory)
	for _, component := range components[:len(components)-1] {
		next, openErr := unix.Openat(directory, component, unix.O_RDONLY|unix.O_CLOEXEC|unix.O_DIRECTORY|unix.O_NOFOLLOW, 0)
		if openErr != nil {
			return safeio.ErrUnsafeFile
		}
		unix.Close(directory)
		directory = next
	}
	name := components[len(components)-1]
	identity, err := inspectRestoreTargetAt(directory, name)
	if err != nil {
		return safeio.ErrUnsafeFile
	}
	temporary, err := writeRestoreTemporaryAt(directory, contents, mode.Perm())
	if err != nil {
		return safeio.ErrUnsafeFile
	}
	defer func() { _ = unix.Unlinkat(directory, temporary, 0) }()
	current, err := inspectRestoreTargetAt(directory, name)
	if err != nil || current != identity {
		return safeio.ErrUnsafeFile
	}
	if err := unix.Renameat(directory, temporary, directory, name); err != nil {
		return safeio.ErrUnsafeFile
	}
	temporary = ""
	if err := unix.Fsync(directory); err != nil {
		return safeio.ErrUnsafeFile
	}
	return nil
}

func inspectRestoreTargetAt(directory int, name string) (restoreTargetIdentity, error) {
	var status unix.Stat_t
	err := unix.Fstatat(directory, name, &status, unix.AT_SYMLINK_NOFOLLOW)
	if errors.Is(err, unix.ENOENT) {
		return restoreTargetIdentity{}, nil
	}
	if err != nil || status.Mode&unix.S_IFMT != unix.S_IFREG || status.Nlink != 1 {
		return restoreTargetIdentity{}, safeio.ErrUnsafeFile
	}
	return restoreTargetIdentity{exists: true, device: uint64(status.Dev), inode: status.Ino}, nil
}

func writeRestoreTemporaryAt(directory int, contents []byte, mode os.FileMode) (string, error) {
	for attempt := 0; attempt < 16; attempt++ {
		random := make([]byte, 8)
		if _, err := rand.Read(random); err != nil {
			return "", err
		}
		name := ".tht-restore-" + hex.EncodeToString(random) + ".tmp"
		descriptor, err := unix.Openat(directory, name, unix.O_WRONLY|unix.O_CREAT|unix.O_EXCL|unix.O_CLOEXEC|unix.O_NOFOLLOW, uint32(mode))
		if errors.Is(err, unix.EEXIST) {
			continue
		}
		if err != nil {
			return "", err
		}
		file := os.NewFile(uintptr(descriptor), filepath.Base(name))
		if file == nil {
			unix.Close(descriptor)
			return "", safeio.ErrUnsafeFile
		}
		if err := file.Chmod(mode); err == nil {
			var written int
			written, err = file.Write(contents)
			if err == nil && written != len(contents) {
				err = io.ErrShortWrite
			}
		}
		if err == nil {
			err = file.Sync()
		}
		closeErr := file.Close()
		if err == nil {
			err = closeErr
		}
		if err != nil {
			_ = unix.Unlinkat(directory, name, 0)
			return "", err
		}
		return name, nil
	}
	return "", safeio.ErrUnsafeFile
}