//go:build !windows

package safeio

import (
	"errors"
	"io"
	"os"

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

func lockCanonicalPrivateClaimOperation() func() {
	return func() {}
}

func claimCanonicalPrivateRegular(source, claim string) (bool, error) {
	directory, sourceName, err := openCanonicalParentDirectory(source)
	if err != nil {
		return false, ErrUnsafeFile
	}
	defer unix.Close(directory)
	claimName := claim[len(claim)-len(claimBaseName(claim)):]
	if err := requirePrivateRegularAtLinks(directory, sourceName, 1); err != nil {
		if requireSamePrivatePairAt(directory, sourceName, claimName) == nil {
			return false, nil
		}
		if isPrivateClaimAbsentOrOrphanAt(directory, sourceName, claimName) {
			return false, nil
		}
		return false, ErrUnsafeFile
	}
	if err := unix.Linkat(directory, sourceName, directory, claimName, 0); err != nil {
		if errors.Is(err, unix.EEXIST) && privateRegularAtAllowedLinks(directory, claimName, 1, 2) == nil {
			return false, nil
		}
		return false, ErrUnsafeFile
	}
	if err := requireSamePrivatePairAt(directory, sourceName, claimName); err != nil {
		return false, ErrUnsafeFile
	}
	if err := unix.Fsync(directory); err != nil {
		return false, ErrUnsafeFile
	}
	return true, nil
}

func readCanonicalPrivateClaim(source, claim string, maximum int64) ([]byte, bool, error) {
	directory, sourceName, err := openCanonicalParentDirectory(source)
	if err != nil {
		return nil, false, ErrUnsafeFile
	}
	defer unix.Close(directory)
	claimName := claim[len(claim)-len(claimBaseName(claim)):]
	if err := requireSamePrivatePairAt(directory, sourceName, claimName); err != nil {
		if isPrivateClaimAbsentOrOrphanAt(directory, sourceName, claimName) {
			return nil, false, nil
		}
		return nil, false, ErrUnsafeFile
	}
	descriptor, err := unix.Openat(directory, sourceName, unix.O_RDONLY|unix.O_CLOEXEC|unix.O_NOFOLLOW, 0)
	if err != nil {
		return nil, false, ErrUnsafeFile
	}
	file := os.NewFile(uintptr(descriptor), "tht-safeio-oidc-claim")
	if file == nil {
		unix.Close(descriptor)
		return nil, false, ErrUnsafeFile
	}
	defer file.Close()
	var before unix.Stat_t
	if err := unix.Fstat(descriptor, &before); err != nil || !privateRegularStatWithLinks(before, 2) {
		return nil, false, ErrUnsafeFile
	}
	contents, err := io.ReadAll(io.LimitReader(file, maximum+1))
	if err != nil || int64(len(contents)) > maximum {
		return nil, false, ErrUnsafeFile
	}
	var after unix.Stat_t
	if err := unix.Fstat(descriptor, &after); err != nil || !sameUnixPrivateFile(before, after) || !privateRegularStatWithLinks(after, 2) {
		return nil, false, ErrUnsafeFile
	}
	if err := requireSamePrivatePairAt(directory, sourceName, claimName); err != nil {
		return nil, false, ErrUnsafeFile
	}
	return contents, true, nil
}

func removeCanonicalPrivateClaim(source, claim string) (bool, error) {
	directory, sourceName, err := openCanonicalParentDirectory(source)
	if err != nil {
		return false, ErrUnsafeFile
	}
	defer unix.Close(directory)
	claimName := claim[len(claim)-len(claimBaseName(claim)):]
	if err := requireSamePrivatePairAt(directory, sourceName, claimName); err != nil {
		if isPrivateClaimAbsentOrOrphanAt(directory, sourceName, claimName) {
			return false, nil
		}
		return false, ErrUnsafeFile
	}
	if err := unix.Unlinkat(directory, sourceName, 0); err != nil {
		return false, ErrUnsafeFile
	}
	if err := requirePrivateRegularAtLinks(directory, claimName, 1); err != nil {
		return false, ErrUnsafeFile
	}
	if err := unix.Unlinkat(directory, claimName, 0); err != nil {
		return false, ErrUnsafeFile
	}
	if err := unix.Fsync(directory); err != nil {
		return false, ErrUnsafeFile
	}
	return true, nil
}

func claimBaseName(path string) string {
	for index := len(path) - 1; index >= 0; index-- {
		if path[index] == byte(os.PathSeparator) {
			return path[index+1:]
		}
	}
	return path
}

func privateRegularAtAllowedLinks(directory int, name string, allowed ...uint64) error {
	var stat unix.Stat_t
	if err := unix.Fstatat(directory, name, &stat, unix.AT_SYMLINK_NOFOLLOW); err != nil {
		return ErrUnsafeFile
	}
	for _, links := range allowed {
		if privateRegularStatWithLinks(stat, links) {
			return nil
		}
	}
	return ErrUnsafeFile
}

func requirePrivateRegularAtLinks(directory int, name string, links uint64) error {
	return privateRegularAtAllowedLinks(directory, name, links)
}

func requireSamePrivatePairAt(directory int, source, claim string) error {
	var left, right unix.Stat_t
	if err := unix.Fstatat(directory, source, &left, unix.AT_SYMLINK_NOFOLLOW); err != nil || !privateRegularStatWithLinks(left, 2) {
		return ErrUnsafeFile
	}
	if err := unix.Fstatat(directory, claim, &right, unix.AT_SYMLINK_NOFOLLOW); err != nil || !privateRegularStatWithLinks(right, 2) {
		return ErrUnsafeFile
	}
	if !sameUnixPrivateFile(left, right) {
		return ErrUnsafeFile
	}
	return nil
}

func isPrivateClaimAbsentOrOrphanAt(directory int, source, claim string) bool {
	var sourceStat, claimStat unix.Stat_t
	sourceErr := unix.Fstatat(directory, source, &sourceStat, unix.AT_SYMLINK_NOFOLLOW)
	claimErr := unix.Fstatat(directory, claim, &claimStat, unix.AT_SYMLINK_NOFOLLOW)
	if errors.Is(sourceErr, unix.ENOENT) && errors.Is(claimErr, unix.ENOENT) {
		return true
	}
	return errors.Is(sourceErr, unix.ENOENT) && claimErr == nil && privateRegularStatWithLinks(claimStat, 1)
}

func privateRegularStatWithLinks(stat unix.Stat_t, links uint64) bool {
	return stat.Mode&unix.S_IFMT == unix.S_IFREG && uint64(stat.Nlink) == links && stat.Mode&0o7777 == 0o600
}

func sameUnixPrivateFile(left, right unix.Stat_t) bool {
	return left.Dev == right.Dev && left.Ino == right.Ino && left.Size == right.Size && left.Mtim == right.Mtim
}