You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: backend.go
    path of file: ./repos/goldfish/vendor/github.com/hashicorp/vault/builtin/audit/file
    
    the code of the file until where you have to start completion: package file

import (
	"fmt"
	"io/ioutil"
	"os"
	"path/filepath"
	"strconv"
	"strings"
	"sync"

	"github.com/hashicorp/vault/audit"
	"github.com/hashicorp/vault/helper/salt"
	"github.com/hashicorp/vault/logical"
)

func Factory(conf *audit.BackendConfig) (audit.Backend, error) {
	if conf.SaltConfig == nil {
		return nil, fmt.Errorf("nil salt config")
	}
	if conf.SaltView == nil {
		return nil, fmt.Errorf("nil salt view")
	}

	path, ok := conf.Config["file_path"]
	if 
    
func (b *Backend) open() error {
	if b.f != nil {
		return nil
	}
	if err := os.MkdirAll(filepath.Dir(b.path), b.mode); err != nil {
		return err
	}

	var err error
	b.f, err = os.OpenFile(b.path, os.O_APPEND|os.O_WRONLY|os.O_CREATE, b.mode)
	if err != nil {
		return err
	}

	// Change the file mode in case the log file already existed. We special
	// case /dev/null since we can't chmod it
	switch b.path {
	case "/dev/null":
	default:
		err = os.Chmod(b.path, b.mode)
		if err != nil {
			return err
		}
	}

	return nil
}