code: func EnsureFile(ctx context.Context, fs FileSystem, path string, content io.Reader) error {
  	return fs.EnsureFile(ctx, path, func(f *os.File) error {
  		_, err := io.Copy(f, content)
  		return err
  	})
  }
docs: // EnsureFile creates a file with the given content.
  // It will create full directory path to the file if necessary.