HTTP Post containing binary data in golang

Question

I hope I can explain this right. I'm trying to make an HTTP post request that contains binary data (a file). This is for DeepStack image processing. In Python I have the following working:

image_data = open(file,"rb").read()
        try:
            response = requests.post("http://deepstack.local:82/v1/vision/detection",files={"image":image_data},timeout=15).json()

In Go, I started with the basic example from here: https://golangtutorial.dev/tips/http-post-json-go/

Modifying this a bit for my use, the relevant lines are:

    data, err :=  ioutil.ReadFile(tempPath + file.Name())
    if err != nil {
        log.Print(err)
    }

    httpposturl := "http://deepstack.local:82/v1/vision/custom/combined"
    fmt.Println("HTTP JSON POST URL:", httpposturl)

    var jsonData = []byte(`{"image": ` + data + `}`)

    request, error := http.NewRequest("POST", httpposturl, bytes.NewBuffer(jsonData))
    request.Header.Set("Content-Type", "application/json; charset=UTF-8")

This results in an error:

invalid operation: `{"image": ` + data (mismatched types untyped string and []byte)`

the "data" variable at this point is []uint8 ([]byte). I realize, at a high level, what is wrong. I'm trying to join two data types that are not the same. That's about it though. I've tried a bunch of stuff that I'm pretty sure anyone familiar with Go would immediately realize was wrong (declaring jsonData as a byte, converting data to a string, using os.Open instead of ioutil.ReadFile, etc.). I'm just kind of stumbling around blind though. I can't find an example that doesn't use a plain string as the JSON data.

I would appreciate any thoughts.

--- ANSWER ---

I'm marking Dietrich Epp's answer as accepted, because he gave me what I asked for. However, RedBlue in the comments gave me what I actually needed. Thank you both. The code below is modified just a bit from this answer: https://stackoverflow.com/a/56696333/2707357

Change the url variable to your DeepStack server, and the file name to one that actually exists, and the response body should return the necessary information.

package main

import (
    "bytes"
    "fmt"
    "io"
    "io/ioutil"
    "mime/multipart"
    "net/http"
    "os"
)

func createMultipartFormData(fieldName, fileName string) (bytes.Buffer, *multipart.Writer) {
    var b bytes.Buffer
    var err error
    w := multipart.NewWriter(&b)
    var fw io.Writer
    file := mustOpen(fileName)
    if fw, err = w.CreateFormFile(fieldName, file.Name()); err != nil {
        fmt.Println("Error: ", err)
    }
    if _, err = io.Copy(fw, file); err != nil {
        fmt.Println("Error: ", err)
    }
    w.Close()
    return b, w
}

func mustOpen(f string) *os.File {
    r, err := os.Open(f)
    if err != nil {
        pwd, _ := os.Getwd()
        fmt.Println("PWD: ", pwd)
        panic(err)
    }
    return r
}

func main() {
    url := "http://deepstack.local:82/v1/vision/custom/combined"
    b, w := createMultipartFormData("image", "C:\\go_sort\\temp\\person.jpg")

    req, err := http.NewRequest("POST", url, &b)
    if err != nil {
        return
    }
    // Don't forget to set the content type, this will contain the boundary.
    req.Header.Set("Content-Type", w.FormDataContentType())

    client := &http.Client{}
    response, error := client.Do(req)
    if err != nil {
        panic(error)
    }
    defer response.Body.Close()

    fmt.Println("response Status:", response.Status)
    fmt.Println("response Headers:", response.Header)
    body, _ := ioutil.ReadAll(response.Body)
    fmt.Println("response Body:", string(body))
}

Is your goal to post a multipart form as in the Python code or JSON body as you are attempting in the Go code? — , Jun 23 '22 at 19:45
The python code works, so yes, I believe it should be a multipart form. — BeanBagKing, Jun 23 '22 at 20:23

score -1 · Accepted Answer · answered Jun 23 '22 at 20:26

It's really such a small error. This is all your question boils down to, as far as I can tell:

var data []byte // with some value
jsonData := []byte(`{"image": ` + data + `}`)

All you have to do is change this to use append() or something similar:

jsonData := append(
    append([]byte(`{"image": `), data...),
    '}')

The reason is that you can't use + to concatenate []byte in Go. You can use append(), though.

HTTP Post containing binary data in golang

1 Answers1