Prepend content to file by golang

record.go

package main

import (
	"bufio"
	"fmt"
	"log"
	"os"
)

func main() {
	for i := 0; i < 10; i++ {
		err := NewRecord("test.txt").Prepend(fmt.Sprintf("%02d", i))
		if err != nil {
			log.Fatalf("failed to prepend: %+v", err)
		}
	}
}

type Record struct {
	Filename string
	Contents []string
}

func NewRecord(filename string) *Record {
	return &Record{
		Filename: filename,
		Contents: make([]string, 0),
	}
}

func (r *Record) readLines() error {
	if _, err := os.Stat(r.Filename); err != nil {
		return nil
	}

	f, err := os.OpenFile(r.Filename, os.O_RDONLY, 0600)
	if err != nil {
		return err
	}
	defer f.Close()

	scanner := bufio.NewScanner(f)
	for scanner.Scan() {
		if tmp := scanner.Text(); len(tmp) != 0 {
			r.Contents = append(r.Contents, tmp)
		}
	}

	return nil
}

func (r *Record) Prepend(content string) error {
	err := r.readLines()
	if err != nil {
		return err
	}

	f, err := os.OpenFile(r.Filename, os.O_CREATE|os.O_WRONLY, 0600)
	if err != nil {
		return err
	}
	defer f.Close()

	writer := bufio.NewWriter(f)
	writer.WriteString(fmt.Sprintf("%s\n", content))
	for _, line := range r.Contents {
		_, err := writer.WriteString(fmt.Sprintf("%s\n", line))
		if err != nil {
			return err
		}
	}

	if err := writer.Flush(); err != nil {
		return err
	}

	return nil
}