Using MultipartRequest to upload file

Question

I'm using Goa v3 to design an endpoint that allows me to upload files (more precisely, images) with a multipart/form-data POST request. I have declared the following Service:

var _ = Service("images", func() {
    HTTP(func() {
        Path("/images")
    })

    Method("upload", func() {  
        HTTP(func() {
            POST("/")
            MultipartRequest()
        })

        Payload(func() {
            Description("Multipart request Payload")
            Attribute("File", Bytes, "File")
        })

        Result(ImageList)
    })
})

I run the goa gen and the goa example commands to generate the boilerplate code. Apart from the cmd directory, the example code generates the images.go main file and a multipart.go file to declare the encoder and decoder logic, e.g.:

func ImagesUploadDecoderFunc(mr *multipart.Reader, p **images.UploadPayload) error {
    // Add multipart request decoder logic here
    return nil
}

I can use the mr.NextPart() and obtain a reference to the image file apparently, but I'm still not sure how should I map this to the Bytes field in the images.UploadPayload type (or maybe I should declare another type of field to handle Files??).

I can't find any example in the Goa documentation.

Gerardo Roza · Accepted Answer · 2019-10-13 00:42:52Z

Ok, I finally understood how the multipart.Reader works, and I came up with a solution.

First let's clarify that differently from how Goa usually works (mapping 'automatically' the requests params with the Payload fields), with MultipartRequest(), I have to make the mapping on my own, so the Payload can actually have any structure.

In my case, I re-defined my Payload structure as follows:

// ImageUpload single image upload element
var ImageUpload = Type("ImageUpload", func() {
    Description("A single Image Upload type")
    Attribute("type", String)
    Attribute("bytes", Bytes)
    Attribute("name", String)
})

// ImageUploadPayload is a list of files
var ImageUploadPayload = Type("ImageUploadPayload", func() {
    Description("Image Upload Payload")

    Attribute("Files", ArrayOf(ImageUpload), "Collection of uploaded files")
})

In a nutshell, I want to support uploading several files, each with its mime-type, filename and data.

To achieve this, I implemented the multipart.go decoder function like this:

func ImagesUploadDecoderFunc(mr *multipart.Reader, p **images.ImageUploadPayload) error {
    res := images.ImageUploadPayload{}

    for {
        p, err := mr.NextPart()
        if err == io.EOF {
            break
        }

        if err != nil {
            fmt.Fprintln(os.Stderr, err)
            return err
        }

        _, params, err := mime.ParseMediaType(p.Header.Get("Content-Disposition"))
        if err != nil {
            // can't process this entry, it probably isn't an image
            continue
        }

        disposition, _, err := mime.ParseMediaType(p.Header.Get("Content-Type"))
        // the disposition can be, for example 'image/jpeg' or 'video/mp4'
        // I want to support only image files!
        if err != nil || !strings.HasPrefix(disposition, "image/") {
            // can't process this entry, it probably isn't an image
            continue
        }

        if params["name"] == "file" {
            bytes, err := ioutil.ReadAll(p)
            if err != nil {
                // can't process this entry, for some reason
                fmt.Fprintln(os.Stderr, err)
                continue
            }
            filename := params["filename"]
            imageUpload := images.ImageUpload{
                Type:  &disposition,
                Bytes: bytes,
                Name:  &filename,
            }
            res.Files = append(res.Files, &imageUpload)
        }
    }
    *p = &res
    return nil
}

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