package main

import (
	"errors"
	"fmt"
	"io"
	"os"
	"path/filepath"
)

// checkNew fails if path already exists.
func checkNew(path string) error {
	if _, err := os.Lstat(path); err == nil {
		return fmt.Errorf("%s already exists; outputs are never overwritten", path)
	} else if !errors.Is(err, os.ErrNotExist) {
		return err
	}
	return nil
}

// writeAtomic stages the output in a private temporary file next to path and
// publishes it only after fn succeeded (spec §56). Publication creates path
// without replacing an existing file: a hard link where the file system
// supports it, otherwise an exclusive create and copy. On any failure no
// partial output remains.
func writeAtomic(path string, fn func(io.Writer) error) (err error) {
	if err := checkNew(path); err != nil {
		return err
	}
	tmp, err := os.CreateTemp(filepath.Dir(path), ".datekeys-*")
	if err != nil {
		return err
	}
	name := tmp.Name()
	defer func() {
		tmp.Close()
		os.Remove(name)
	}()
	if err := fn(tmp); err != nil {
		return err
	}
	if err := tmp.Sync(); err != nil {
		return err
	}
	if err := tmp.Close(); err != nil {
		return err
	}
	if err := os.Link(name, path); err == nil {
		return nil
	} else if errors.Is(err, os.ErrExist) {
		return fmt.Errorf("%s already exists; outputs are never overwritten", path)
	}
	return copyExclusive(name, path)
}

func copyExclusive(from, to string) error {
	src, err := os.Open(from)
	if err != nil {
		return err
	}
	defer src.Close()
	dst, err := os.OpenFile(to, os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0o600)
	if err != nil {
		return err
	}
	_, copyErr := io.Copy(dst, src)
	syncErr := dst.Sync()
	closeErr := dst.Close()
	if err := errors.Join(copyErr, syncErr, closeErr); err != nil {
		os.Remove(to)
		return err
	}
	return nil
}