How can I receive an uploaded file using a Golang net/http server?

Question

I'm playing around with Mux and net/http. Lately, I'm trying to get a simple server with one endpoint to accept a file upload.

Here's the code I've got so far:

server.go

package main

import (
    "fmt"
    "github.com/gorilla/mux"
    "log"
    "net/http"
)

func main() {
    router := mux.NewRouter()
    router.
        Path("/upload").
        Methods("POST").
        HandlerFunc(UploadCsv)
    fmt.Println("Starting")
    log.Fatal(http.ListenAndServe(":8080", router))
}

endpoint.go

package main

import (
    "fmt"
    "net/http"
)

func UploadFile(w http.ResponseWriter, r *http.Request) {
    err := r.ParseMultipartForm(5 * 1024 * 1024)
    if err != nil {
        panic(err)
    }

    fmt.Println(r.FormValue("fileupload"))
}

I think I've narrowed the issue down to actually retrieving the body from the request inside UploadFile. When I run this cURL command:

curl http://localhost:8080/upload -F "[email protected]" -vvv

I get an empty response (as expected; I'm not printing to the ResponseWriter), but I just get a new (empty) line printed at the prompt where I'm running the server, instead of the request body.

I'm sending the file as multipart (AFAIK, implied by using -F rather than -d in cURL), and cURL's verbose output is showing 502 bytes sent:

$ curl http://localhost:8080/upload -F "[email protected]" -vvv
*   Trying ::1...
* TCP_NODELAY set
* Connected to localhost (::1) port 8080 (#0)
> POST /upload HTTP/1.1
> Host: localhost:8080
> User-Agent: curl/7.51.0
> Accept: */*
> Content-Length: 520
> Expect: 100-continue
> Content-Type: multipart/form-data; boundary=------------------------b578878d86779dc5
> 
< HTTP/1.1 100 Continue
< HTTP/1.1 200 OK
< Date: Fri, 18 Nov 2016 19:01:50 GMT
< Content-Length: 0
< Content-Type: text/plain; charset=utf-8
< 
* Curl_http_done: called premature == 0
* Connection #0 to host localhost left intact

What's the proper way to receive files uploaded as multipart form data using a net/http server in Go?

Answer 1

Here's a quick example

func ReceiveFile(w http.ResponseWriter, r *http.Request) {
    r.ParseMultipartForm(32 << 20) // limit your max input length!
    var buf bytes.Buffer
    // in your case file would be fileupload
    file, header, err := r.FormFile("file")
    if err != nil {
        panic(err)
    }
    defer file.Close()
    name := strings.Split(header.Filename, ".")
    fmt.Printf("File name %s\n", name[0])
    // Copy the file data to my buffer
    io.Copy(&buf, file)
    // do something with the contents...
    // I normally have a struct defined and unmarshal into a struct, but this will
    // work as an example
    contents := buf.String()
    fmt.Println(contents)
    // I reset the buffer in case I want to use it again
    // reduces memory allocations in more intense projects
    buf.Reset()
    // do something else
    // etc write header
    return
}