golang POST data using the Content-Type multipart/

Question

I'm trying to upload images from my computer to a website using go. Usually, I use a bash script that sends a file and a key to the server:

curl -F "image"=@"IMAGEFILE" -F "key"="KEY" URL

it works fine, but I'm trying to convert this request into my golang program.

http://matt.aimonetti.net/posts/2013/07/01/golang-multipart-file-upload-example/

I tried this link and many others, but, for each code that I try, the response from the server is "no image sent", and I've no idea why. If someone knows what's happening with the example above.

Thanks

Answer 1

I have found this tutorial very helpful to clarify my confusions about file uploading in Go.

Basically you upload the file via ajax using form-data on a client and use the following small snippet of Go code on the server:

file, handler, err := r.FormFile("img") // img is the key of the form-data
if err != nil {
    fmt.Println(err)
    return
}
defer file.Close()

fmt.Println("File is good")
fmt.Println(handler.Filename)
fmt.Println()
fmt.Println(handler.Header)


f, err := os.OpenFile(handler.Filename, os.O_WRONLY|os.O_CREATE, 0666)
if err != nil {
    fmt.Println(err)
    return
}
defer f.Close()
io.Copy(f, file)

Here r is *http.Request. P.S. this just stores the file in the same folder and does not perform any security checks.

Answer 2

Here's some sample code.

In short, you'll need to use the mime/multipart package to build the form.

package main

import (
    "bytes"
    "fmt"
    "io"
    "mime/multipart"
    "net/http"
    "net/http/httptest"
    "net/http/httputil"
    "os"
    "strings"
)

func main() {

    var client *http.Client
    var remoteURL string
    {
        //setup a mocked http client.
        ts := httptest.NewTLSServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
            b, err := httputil.DumpRequest(r, true)
            if err != nil {
                panic(err)
            }
            fmt.Printf("%s", b)
        }))
        defer ts.Close()
        client = ts.Client()
        remoteURL = ts.URL
    }

    //prepare the reader instances to encode
    values := map[string]io.Reader{
        "file":  mustOpen("main.go"), // lets assume its this file
        "other": strings.NewReader("hello world!"),
    }
    err := Upload(client, remoteURL, values)
    if err != nil {
        panic(err)
    }
}

func Upload(client *http.Client, url string, values map[string]io.Reader) (err error) {
    // Prepare a form that you will submit to that URL.
    var b bytes.Buffer
    w := multipart.NewWriter(&b)
    for key, r := range values {
        var fw io.Writer
        if x, ok := r.(io.Closer); ok {
            defer x.Close()
        }
        // Add an image file
        if x, ok := r.(*os.File); ok {
            if fw, err = w.CreateFormFile(key, x.Name()); err != nil {
                return
            }
        } else {
            // Add other fields
            if fw, err = w.CreateFormField(key); err != nil {
                return
            }
        }
        if _, err = io.Copy(fw, r); err != nil {
            return err
        }

    }
    // Don't forget to close the multipart writer.
    // If you don't close it, your request will be missing the terminating boundary.
    w.Close()

    // Now that you have a form, you can submit it to your handler.
    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())

    // Submit the request
    res, err := client.Do(req)
    if err != nil {
        return
    }

    // Check the response
    if res.StatusCode != http.StatusOK {
        err = fmt.Errorf("bad status: %s", res.Status)
    }
    return
}

func mustOpen(f string) *os.File {
    r, err := os.Open(f)
    if err != nil {
        panic(err)
    }
    return r
}

Answer 3

Regarding attila-o's post, the request header doesn't have the boundary since the Writer is closed already.

// after the close, the bounday will be nil.
w.Close()
...
req.Header.Set("Content-Type", w.FormDataContentType())

So, it should close after the set, I think.

req.Header.Set("Content-Type", w.FormDataContentType())
w.Close()