http - 使用 mime/multipart 上传会损坏文件

Question

我写了一个服务器，有一个上传图片的路由。这是一个接收一些参数的表单:标题、描述、可见性和图片。该页面还使用 Authentication header 。

func UploadPictureRoute(prv *services.Provider) http.HandlerFunc {
    return func(w http.ResponseWriter, r *http.Request) {
        user, err := auth.ValidateRequest(prv, w, r)
        if auth.RespondError(w, err) {
            return
        }

        r.ParseMultipartForm(10 << 20) // 10 meg max

        title := r.FormValue("title")
        desc := r.FormValue("description")
        visib := r.FormValue("visibility")
        visibInt, err := strconv.Atoi(visib)
        visibility := int8(visibInt) // Visibility can be either 0, 1, 2

        if err != nil {
            w.WriteHeader(http.StatusBadRequest)
        }

        file, _, err := r.FormFile("picture")
        if err != nil {
            w.WriteHeader(http.StatusBadRequest)
            return
        }
        defer file.Close()

        mimeType, _, err := mimetype.DetectReader(file) // Package gabriel-vasile/mimetype
        if err != nil {
            w.WriteHeader(http.StatusBadRequest)
            return
        }

        if !utils.IsValidMimetype(mimeType) { // Basically just comparing to image/png, image/jpg. Crashes here
            w.WriteHeader(http.StatusBadRequest)
            return
        }

        parentFolder := prv.PicturePath + "/" + strconv.FormatInt(*user.ID, 10) + "/"

        _, err = os.Stat(parentFolder)
        if os.IsNotExist(err) {
            err = os.MkdirAll(parentFolder, os.ModePerm)
            if err != nil {
                w.WriteHeader(http.StatusInternalServerError)
                return
            }
        }

        pict := model.Picture{
            Title:       title,
            Description: desc,
            Creator:     &user,
            Visibility:  visibility,
            Ext:         utils.GetExtForMimetype(mimeType),
        }

        pict, err = dal.CreatePicture(prv, pict)
        if err != nil {
            w.WriteHeader(http.StatusInternalServerError)
            return
        }

        outputFile, err := os.Create(parentFolder + strconv.FormatInt(*pict.ID, 10) + "." + pict.Ext)
        if err != nil {
            w.WriteHeader(http.StatusInternalServerError)
            dal.DeletePicture(prv, pict)
        }
        defer outputFile.Close()

        _, err = io.Copy(outputFile, file)
        if err != nil {
            w.WriteHeader(http.StatusInternalServerError)
            return
        }
// Responding 
    }
}

使用 Postman 效果很好，它可以正确上传文件，一切都按预期进行。

但是，我未能编写一个 go http 客户端来上传文件:

type uploadResponse struct {
    URLID string
}

func main() {
    filename := "filename.png"

    file, err := os.Open(filename)
    if err != nil {
        fmt.Println(err)
    }
    defer file.Close()

    // Just to be sure we're really at the start of the file
    _, err = file.Seek(0, io.SeekStart)
    if err != nil {
        fmt.Println("Can't read the file")
        return
    }

    mime, _, err := mimetype.DetectReader(file)
    if err != nil {
        fmt.Println("Can't read the file")
        return
    }

    if !api.IsMimetypeAllowed(mime) { // Just check if it's a jpg/png/gif. This works on the client
        fmt.Println("This filetype can't be uploaded to a Scinna server.")
        fmt.Println("Please convert this file to jpeg, png or gif.")

        return
    }


    body := &bytes.Buffer{}
    writer := multipart.NewWriter(body)

    title, _ := writer.CreateFormField("title")
    title.Write([]byte("My picture title"))

    desc, _ := writer.CreateFormField("description")
    desc.Write([]byte("My picture description"))

    visib, _ := writer.CreateFormField("visibility")
    visib.Write([]byte("0"))

    pict, _ := writer.CreateFormFile("picture", file.Name())
    _, err = io.Copy(pict, file)

    writer.Close()

    r, err := http.NewRequest("POST", "https://myapi.local/pictures", body)
    if err != nil {
        panic(err)
    }
    r.Header.Add("Content-Type", writer.FormDataContentType())
    r.Header.Add("Authorization", "Bearer "+config.Token)

    client := &http.Client{}
    client.Do(r)
}

此代码会导致服务器崩溃。图片的 mimetype 变为 application/octet-stream 并且图像标题已损坏(它仍然在某些编辑器中打开，但 EyesOfGnome 基本上表示该图片不是 JPG/PNG 文件，因为它可以'找不到开头的神奇数字)

如何修复HTTP go客户端才能成功上传图片？

Answer 1

对 mimetype.DetectReader(file) 的调用读取文件的一部分。对 _, err = io.Copy(pict, file) 的调用会读取文件的其余部分。要读取整个文件，请返回到调用 io.Copy 之前的偏移量 0。

文件在偏移量 0 处打开。调用 Open 后无需立即查找偏移量 0。

通过交换调用顺序来修复问题:

...

mime, _, err := mimetype.DetectReader(file)
if err != nil {
    fmt.Println("Can't read the file")
    return
}

// Rewind to the start of the file
_, err = file.Seek(0, io.SeekStart)
if err != nil {
    fmt.Println("Can't read the file")
    return
}

...

服务器也有类似的问题。检测类型后回退:

mimeType, _, err := mimetype.DetectReader(file) // Package gabriel-vasile/mimetype
if err != nil {
    w.WriteHeader(http.StatusBadRequest)
    return
}

// Rewind to the start of the file
_, err = file.Seek(0, io.SeekStart)
if err != nil {
    w.WriteHeader(http.StatusInternalServerError)
    return
}