net/http: `r.FormValue()` ignores the return value of `ParseMultipartForm()`

[saleemjaffer]

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

go version go1.8.1 darwin/amd64

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/saleem/proj"
GORACE=""
GOROOT="/usr/local/Cellar/go/1.8.1/libexec"
GOTOOLDIR="/usr/local/Cellar/go/1.8.1/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/tx/9dlgzqg11c30qh8p6l3czn000000gn/T/go-build451820530=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"
PKG_CONFIG="pkg-config"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"

What did you do?

I have a Go Http endpoint that handles image uploads. From the client I make a POST request, with a content type multipart/form-data. I send some string key, value pairs and an image file.

On the server when I try to read the string values sent using request.FormValue("key_name") I get an empty string. This is the source code of request.FormValue from net/http:

func (r *Request) FormValue(key string) string {
	if r.Form == nil {
		r.ParseMultipartForm(defaultMaxMemory)
	}
	if vs := r.Form[key]; len(vs) > 0 {
		return vs[0]
	}
	return ""
}

The return value of ParseMultipartForm is ignored. It is the ParseMultipartForm function which is failing. Why is the return value ignored?

What did you expect to see?

The error returned from ParseMultipartForm should have been handled in request.FormValue of the net/http package

What did you see instead?

The error returned from ParseMultipartForm is not handled.

[bradfitz]

Request.FormValue is a convenience, best-effort method. If it's not convenient, don't use it.

We can't change its signature (https://golang.org/doc/go1compat) even if we wanted to.

Typical caller code will check FormValue for acceptable values and return an HTTP 400 Bad Request or similar if the request is bogus or missing form values. If you really need to know specifically how the request is malformed, call ParseMultipartForm yourself.