package safeio

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

// AtomicWriteFile writes data to path atomically by writing to a temp file in
// the same directory and renaming it into place. On any error before rename,
// the temp file is removed. The parent directory is created if needed.
func AtomicWriteFile(path string, data []byte, perm fs.FileMode) error {
	dir := filepath.Dir(path)
	if err := os.MkdirAll(dir, 0o755); err != nil {
		return fmt.Errorf("create parent directory: %w", err)
	}

	tmp, err := os.CreateTemp(dir, ".atomic-*")
	if err != nil {
		return fmt.Errorf("create temp file: %w", err)
	}
	tmpPath := tmp.Name()
	cleanup := true
	defer func() {
		if cleanup {
			os.Remove(tmpPath)
		}
	}()

	if _, err := tmp.Write(data); err != nil {
		tmp.Close()
		return fmt.Errorf("write temp file: %w", err)
	}
	if err := tmp.Sync(); err != nil {
		tmp.Close()
		return fmt.Errorf("sync temp file: %w", err)
	}
	if err := tmp.Close(); err != nil {
		return fmt.Errorf("close temp file: %w", err)
	}
	if err := os.Chmod(tmpPath, perm); err != nil {
		return fmt.Errorf("chmod temp file: %w", err)
	}
	if err := os.Rename(tmpPath, path); err != nil {
		return fmt.Errorf("rename temp file: %w", err)
	}
	cleanup = false

	// Best-effort sync of the parent directory.
	if f, err := os.Open(dir); err == nil {
		_ = f.Sync()
		f.Close()
	}
	return nil
}