// Package safeio reads and writes local files without following symlinked path components.
package safeio

import (
	"errors"
	"io"
	"os"
	"path/filepath"
	"strings"
	"unicode/utf8"
)

var ErrUnsafeFile = errors.New("unsafe file")

// ValidateCanonicalPath rejects relative or lexically non-canonical paths before they are opened.
func ValidateCanonicalPath(path string) error {
	if !filepath.IsAbs(path) || filepath.Clean(path) != path || strings.Contains(path, string(filepath.Separator)+".."+string(filepath.Separator)) {
		return ErrUnsafeFile
	}
	return nil
}

func readBoundedRegularFile(path string, file *os.File, maximum int64) ([]byte, error) {
	if maximum < 0 || maximum == int64(^uint64(0)>>1) {
		return nil, ErrUnsafeFile
	}
	info, err := file.Stat()
	if err != nil || !info.Mode().IsRegular() || !hasSingleLink(info) {
		return nil, ErrUnsafeFile
	}
	contents, err := io.ReadAll(io.LimitReader(file, maximum+1))
	if err != nil || int64(len(contents)) > maximum {
		return nil, ErrUnsafeFile
	}
	after, err := file.Stat()
	if err != nil || !after.Mode().IsRegular() || !hasSingleLink(after) || !os.SameFile(info, after) {
		return nil, ErrUnsafeFile
	}
	current, err := os.Stat(path)
	if err != nil || !os.SameFile(info, current) {
		return nil, ErrUnsafeFile
	}
	return contents, nil
}

func ReadCanonicalUTF8(path string, maximum int64) (string, error) {
	contents, err := ReadCanonicalRegular(path, maximum)
	if err != nil {
		return "", err
	}
	if !utf8.Valid(contents) {
		return "", ErrUnsafeFile
	}
	return string(contents), nil
}

func WriteCanonicalNewFile(path string, contents []byte, mode os.FileMode) error {
	if err := ValidateCanonicalPath(path); err != nil {
		return err
	}
	parent := filepath.Dir(path)
	if err := requireCanonicalDirectory(parent); err != nil {
		return err
	}
	if info, err := os.Lstat(path); err == nil {
		if !info.Mode().IsRegular() || info.Mode()&os.ModeSymlink != 0 || info.Mode()&os.ModeType != 0 {
			return ErrUnsafeFile
		}
		return ErrUnsafeFile
	} else if !errors.Is(err, os.ErrNotExist) {
		return ErrUnsafeFile
	}
	file, err := os.OpenFile(path, os.O_WRONLY|os.O_CREATE|os.O_EXCL, mode)
	if err != nil {
		return ErrUnsafeFile
	}
	defer file.Close()
	if _, err := file.Write(contents); err != nil {
		_ = os.Remove(path)
		return ErrUnsafeFile
	}
	if err := file.Sync(); err != nil {
		_ = os.Remove(path)
		return ErrUnsafeFile
	}
	return nil
}

func requireCanonicalDirectory(path string) error {
	if err := ValidateCanonicalPath(path); err != nil {
		return err
	}
	resolved, err := filepath.EvalSymlinks(path)
	if err != nil || resolved != path {
		return ErrUnsafeFile
	}
	info, err := os.Stat(path)
	if err != nil || !info.IsDir() {
		return ErrUnsafeFile
	}
	return nil
}