//go:build !windows

package safeio

import (
	"io/fs"
	"os"
	"strings"

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

// ReadCanonicalRegular opens an absolute canonical path component by component from the root
// descriptor. O_NOFOLLOW rejects symlinks at every component, and the open directory descriptors
// prevent later parent replacement from redirecting the final open.
func ReadCanonicalRegular(path string, maximum int64) ([]byte, error) {
	if err := ValidateCanonicalPath(path); err != nil {
		return nil, err
	}
	components := strings.Split(strings.TrimPrefix(path, string(os.PathSeparator)), string(os.PathSeparator))
	if len(components) == 0 || components[0] == "" {
		return nil, ErrUnsafeFile
	}

	directory, err := unix.Open(string(os.PathSeparator), unix.O_RDONLY|unix.O_CLOEXEC|unix.O_DIRECTORY, 0)
	if err != nil {
		return nil, ErrUnsafeFile
	}
	directories := []int{directory}
	defer func() { closeUnixDescriptors(directories) }()

	for _, component := range components[:len(components)-1] {
		nextDirectory, err := unix.Openat(directory, component, unix.O_RDONLY|unix.O_CLOEXEC|unix.O_DIRECTORY|unix.O_NOFOLLOW, 0)
		if err != nil {
			return nil, ErrUnsafeFile
		}
		directory = nextDirectory
		directories = append(directories, directory)
	}

	descriptor, err := unix.Openat(directory, components[len(components)-1], unix.O_RDONLY|unix.O_CLOEXEC|unix.O_NOFOLLOW|unix.O_NONBLOCK, 0)
	if err != nil {
		return nil, ErrUnsafeFile
	}
	file := os.NewFile(uintptr(descriptor), "thothctl-safeio")
	if file == nil {
		unix.Close(descriptor)
		return nil, ErrUnsafeFile
	}
	defer file.Close()
	var before, after unix.Stat_t
	if err := unix.Fstat(int(file.Fd()), &before); err != nil || before.Nlink > 1 || before.Mode&unix.S_IFMT != unix.S_IFREG {
		return nil, ErrUnsafeFile
	}
	contents, err := readBoundedRegularFile(file, maximum)
	if err != nil {
		return nil, ErrUnsafeFile
	}
	// Checking the retained descriptor alone misses a pathname replacement while the
	// read is in progress.  The parent descriptor and final name must still resolve to
	// the exact open file after reading.
	if err := unix.Fstat(int(file.Fd()), &after); err != nil || after.Nlink > 1 || after.Mode != before.Mode || after.Ino != before.Ino || after.Dev != before.Dev || after.Size != before.Size {
		return nil, ErrUnsafeFile
	}
	var named unix.Stat_t
	if err := unix.Fstatat(directory, components[len(components)-1], &named, unix.AT_SYMLINK_NOFOLLOW); err != nil || named.Nlink > 1 || named.Mode != before.Mode || named.Ino != before.Ino || named.Dev != before.Dev {
		return nil, ErrUnsafeFile
	}
	if !recheckUnixParents(components, directories) {
		return nil, ErrUnsafeFile
	}
	return contents, nil
}

func closeUnixDescriptors(descriptors []int) {
	for _, descriptor := range descriptors {
		unix.Close(descriptor)
	}
}

func writeCanonicalExclusive(path string, contents []byte, mode fs.FileMode) error {
	if err := ValidateCanonicalPath(path); err != nil || len(contents) > 16<<20 || mode.Perm() != 0o600 {
		return ErrUnsafeFile
	}
	components := strings.Split(strings.TrimPrefix(path, string(os.PathSeparator)), string(os.PathSeparator))
	if len(components) == 0 || components[0] == "" {
		return ErrUnsafeFile
	}
	dir, err := unix.Open("/", unix.O_RDONLY|unix.O_DIRECTORY|unix.O_CLOEXEC, 0)
	if err != nil {
		return ErrUnsafeFile
	}
	defer func() { unix.Close(dir) }()
	for _, component := range components[:len(components)-1] {
		next, err := unix.Openat(dir, component, unix.O_RDONLY|unix.O_DIRECTORY|unix.O_CLOEXEC|unix.O_NOFOLLOW, 0)
		if err != nil {
			return ErrUnsafeFile
		}
		unix.Close(dir)
		dir = next
	}
	var parentBefore unix.Stat_t
	if err := unix.Fstat(dir, &parentBefore); err != nil {
		return ErrUnsafeFile
	}
	fd, err := unix.Openat(dir, components[len(components)-1], unix.O_WRONLY|unix.O_CREAT|unix.O_EXCL|unix.O_CLOEXEC|unix.O_NOFOLLOW, uint32(mode.Perm()))
	if err != nil {
		return ErrUnsafeFile
	}
	f := os.NewFile(uintptr(fd), "thothctl-safeio-output")
	if f == nil {
		unix.Close(fd)
		return ErrUnsafeFile
	}
	defer f.Close()
	if err := f.Chmod(mode); err != nil {
		return ErrUnsafeFile
	}
	if _, err := f.Write(contents); err != nil {
		return ErrUnsafeFile
	}
	if err := f.Sync(); err != nil {
		return ErrUnsafeFile
	}
	var opened, named, parentAfter unix.Stat_t
	if err := unix.Fstat(fd, &opened); err != nil || opened.Nlink != 1 {
		return ErrUnsafeFile
	}
	if err := unix.Fstatat(dir, components[len(components)-1], &named, unix.AT_SYMLINK_NOFOLLOW); err != nil || named.Nlink != 1 || named.Ino != opened.Ino || named.Dev != opened.Dev {
		return ErrUnsafeFile
	}
	if err := unix.Fstat(dir, &parentAfter); err != nil || parentAfter.Ino != parentBefore.Ino || parentAfter.Dev != parentBefore.Dev || !recheckUnixParentPath(components[:len(components)-1], dir) {
		return ErrUnsafeFile
	}
	return nil
}

func validateCanonicalOutputPath(path string) error {
	if err := ValidateCanonicalPath(path); err != nil {
		return err
	}
	components := strings.Split(strings.TrimPrefix(path, string(os.PathSeparator)), string(os.PathSeparator))
	if len(components) < 2 || components[0] == "" {
		return ErrUnsafeFile
	}
	dir, err := unix.Open("/", unix.O_RDONLY|unix.O_DIRECTORY|unix.O_CLOEXEC, 0)
	if err != nil {
		return ErrUnsafeFile
	}
	defer func() { unix.Close(dir) }()
	for _, component := range components[:len(components)-1] {
		next, err := unix.Openat(dir, component, unix.O_RDONLY|unix.O_DIRECTORY|unix.O_CLOEXEC|unix.O_NOFOLLOW, 0)
		if err != nil {
			return ErrUnsafeFile
		}
		unix.Close(dir)
		dir = next
	}
	var st unix.Stat_t
	if err := unix.Fstatat(dir, components[len(components)-1], &st, unix.AT_SYMLINK_NOFOLLOW); err == nil {
		return ErrUnsafeFile
	} else if err != unix.ENOENT {
		return ErrUnsafeFile
	}
	return nil
}

func recheckUnixParents(components []string, retained []int) bool {
	if len(retained) != len(components) {
		return false
	}
	dir, err := unix.Open("/", unix.O_RDONLY|unix.O_DIRECTORY|unix.O_CLOEXEC, 0)
	if err != nil {
		return false
	}
	defer func() { unix.Close(dir) }()
	for i, component := range components[:len(components)-1] {
		next, e := unix.Openat(dir, component, unix.O_RDONLY|unix.O_DIRECTORY|unix.O_CLOEXEC|unix.O_NOFOLLOW, 0)
		if e != nil {
			return false
		}
		var got, want unix.Stat_t
		if unix.Fstat(next, &got) != nil || unix.Fstat(retained[i+1], &want) != nil || got.Ino != want.Ino || got.Dev != want.Dev {
			unix.Close(next)
			return false
		}
		unix.Close(dir)
		dir = next
	}
	return true
}

func recheckUnixParentPath(components []string, retained int) bool {
	dir, err := unix.Open("/", unix.O_RDONLY|unix.O_DIRECTORY|unix.O_CLOEXEC, 0)
	if err != nil {
		return false
	}
	defer func() { unix.Close(dir) }()
	for _, component := range components {
		next, e := unix.Openat(dir, component, unix.O_RDONLY|unix.O_DIRECTORY|unix.O_CLOEXEC|unix.O_NOFOLLOW, 0)
		if e != nil {
			return false
		}
		unix.Close(dir)
		dir = next
	}
	var got, want unix.Stat_t
	return unix.Fstat(dir, &got) == nil && unix.Fstat(retained, &want) == nil && got.Ino == want.Ino && got.Dev == want.Dev
}