net/http: support multipart/mixed in Request.MultipartReader

Fixes #23959

GitHub-Last-Rev: 08ce026f52f9fd65b49d99745dffed46a3951585
GitHub-Pull-Request: golang/go#24012
Change-Id: I7e71c41330346dbc4dad6ba813cabfa8a54e2f66
Reviewed-on: https://go-review.googlesource.com/95975
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/request.go

@@ -411,7 +411,7 @@ var multipartByReader = &multipart.Form{
 }
 
 // MultipartReader returns a MIME multipart reader if this is a
-// multipart/form-data POST request, else returns nil and an error.
+// multipart/form-data or a multipart/mixed POST request, else returns nil and an error.
 // Use this function instead of ParseMultipartForm to
 // process the request body as a stream.
 func (r *Request) MultipartReader() (*multipart.Reader, error) {
@@ -422,16 +422,16 @@ func (r *Request) MultipartReader() (*multipart.Reader, error) {
 		return nil, errors.New("http: multipart handled by ParseMultipartForm")
 	}
 	r.MultipartForm = multipartByReader
-	return r.multipartReader()
+	return r.multipartReader(true)
 }
 
-func (r *Request) multipartReader() (*multipart.Reader, error) {
+func (r *Request) multipartReader(allowMixed bool) (*multipart.Reader, error) {
 	v := r.Header.Get("Content-Type")
 	if v == "" {
 		return nil, ErrNotMultipart
 	}
 	d, params, err := mime.ParseMediaType(v)
-	if err != nil || d != "multipart/form-data" {
+	if err != nil || !(d == "multipart/form-data" || allowMixed && d == "multipart/mixed") {
 		return nil, ErrNotMultipart
 	}
 	boundary, ok := params["boundary"]
@@ -1207,7 +1207,7 @@ func (r *Request) ParseMultipartForm(maxMemory int64) error {
 		return nil
 	}
 
-	mr, err := r.multipartReader()
+	mr, err := r.multipartReader(false)
 	if err != nil {
 		return err
 	}

src/net/http/request_test.go

@@ -143,6 +143,16 @@ func TestMultipartReader(t *testing.T) {
 		t.Errorf("expected multipart; error: %v", err)
 	}
 
+	req = &Request{
+		Method: "POST",
+		Header: Header{"Content-Type": {`multipart/mixed; boundary="foo123"`}},
+		Body:   ioutil.NopCloser(new(bytes.Buffer)),
+	}
+	multipart, err = req.MultipartReader()
+	if multipart == nil {
+		t.Errorf("expected multipart; error: %v", err)
+	}
+
 	req.Header = Header{"Content-Type": {"text/plain"}}
 	multipart, err = req.MultipartReader()
 	if multipart != nil {