http: fix ParseMultipartForm after MultipartReader error

Addresses part of issue 2093
Thanks to espians....@

R=golang-dev, nigeltao
CC=golang-dev
https://golang.org/cl/4808087

src/pkg/http/request.go

@@ -729,6 +729,9 @@ func (r *Request) ParseForm() (err os.Error) {
 // ParseMultipartForm calls ParseForm if necessary.
 // After one call to ParseMultipartForm, subsequent calls have no effect.
 func (r *Request) ParseMultipartForm(maxMemory int64) os.Error {
+	if r.MultipartForm == multipartByReader {
+		return os.NewError("http: multipart handled by MultipartReader")
+	}
 	if r.Form == nil {
 		err := r.ParseForm()
 		if err != nil {
@@ -738,9 +741,6 @@ func (r *Request) ParseMultipartForm(maxMemory int64) os.Error {
 	if r.MultipartForm != nil {
 		return nil
 	}
-	if r.MultipartForm == multipartByReader {
-		return os.NewError("http: multipart handled by MultipartReader")
-	}
 
 	mr, err := r.multipartReader()
 	if err == ErrNotMultipart {

src/pkg/http/request_test.go

@@ -219,6 +219,18 @@ func TestEmptyMultipartRequest(t *testing.T) {
 	testMissingFile(t, req)
 }
 
+func TestRequestMultipartCallOrder(t *testing.T) {
+	req := newTestMultipartRequest(t)
+	_, err := req.MultipartReader()
+	if err != nil {
+		t.Fatalf("MultipartReader: %v", err)
+	}
+	err = req.ParseMultipartForm(1024)
+	if err == nil {
+		t.Errorf("expected an error from ParseMultipartForm after call to MultipartReader")
+	}
+}
+
 func testMissingFile(t *testing.T, req *Request) {
 	f, fh, err := req.FormFile("missing")
 	if f != nil {