Go-将base64字符串保存到文件

Question

so.. I have a base64 encoded string which I need to decode, check it's width and height and then save to file. However.. I've consistently been getting a corrupt image file saved.

package server

import (
    "encoding/base64"
    "errors"
    "io"
    "os"
    "strings"

    "image"
    _ "image/gif"
    _ "image/jpeg"
    _ "image/png"
)

var (
    ErrBucket       = errors.New("Invalid bucket!")
    ErrSize         = errors.New("Invalid size!")
    ErrInvalidImage = errors.New("Invalid image!")
)

func saveImageToDisk(fileNameBase, data, bucket string) (string, error) {
    idx := strings.Index(data, ";base64,")
    if idx < 0 {
        return "", ErrInvalidImage
    }

    reader := base64.NewDecoder(base64.StdEncoding, strings.NewReader(data[idx+8:]))
    imgCfg, fmt, err := image.DecodeConfig(reader)
    if err != nil {
        return "", err
    }

    if imgCfg.Width != 750 || imgCfg.Height != 685 {
        return "", ErrSize
    }

    fileName := fileNameBase + "." + fmt
    f, err := os.Create(fileName)
    if err != nil {
        return "", err
    }

    _, err = io.Copy(f, reader)
    f.Close()

    return fileName, err
}

It does save A file.. and the base64 I'm testing with is valid as per online base64 to image converters. Any help?

http://pastebin.com/u18eRv7d Here's the base64 string i'm using (data in the func)

Answer 1

The main problem of those code is, when you read the io.Reader, the cursor won't back to the first. And you already read the io.Reader on the on decoding image config. So when you write it to file, it will read next until EOF (which maybe not your data)

I don't know exactly how to reset the io.Reader back to first but I write a work around way to make it work :

import (
    "encoding/base64"
    "errors"
    "strings"

    "image"
    _ "image/gif"
    _ "image/jpeg"
    _ "image/png"
    "io/ioutil"
    "bytes"
)

var (
    ErrBucket       = errors.New("Invalid bucket!")
    ErrSize         = errors.New("Invalid size!")
    ErrInvalidImage = errors.New("Invalid image!")
)

func saveImageToDisk(fileNameBase, data string) (string, error) {
    idx := strings.Index(data, ";base64,")
    if idx < 0 {
        return "", ErrInvalidImage
    }
    reader := base64.NewDecoder(base64.StdEncoding, strings.NewReader(data[idx+8:]))
    buff := bytes.Buffer{}
    _, err := buff.ReadFrom(reader)
    if err != nil {
        return "", err
    }
    imgCfg, fm, err := image.DecodeConfig(bytes.NewReader(buff.Bytes()))
    if err != nil {
        return "", err
    }

    if imgCfg.Width != 750 || imgCfg.Height != 685 {
        return "", ErrSize
    }

    fileName := fileNameBase + "." + fm
    ioutil.WriteFile(fileName, buff.Bytes(), 0644)

    return fileName, err
}