mime/multipart: quote boundary in Content-Type if necessary

Fixes #26532

Change-Id: Ic086c90503c7b24982f947c828c7ccf016ddbf69
Reviewed-on: https://go-review.googlesource.com/c/154120
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/mime/multipart/writer.go

@@ -72,7 +72,13 @@ func (w *Writer) SetBoundary(boundary string) error {
 // FormDataContentType returns the Content-Type for an HTTP
 // multipart/form-data with this Writer's Boundary.
 func (w *Writer) FormDataContentType() string {
-	return "multipart/form-data; boundary=" + w.boundary
+	b := w.boundary
+	// We must quote the boundary if it contains any of the
+	// tspecials characters defined by RFC 2045, or space.
+	if strings.ContainsAny(b, `()<>@,;:\"/[]?= `) {
+		b = `"` + b + `"`
+	}
+	return "multipart/form-data; boundary=" + b
 }
 
 func randomBoundary() string {

src/mime/multipart/writer_test.go

@@ -7,6 +7,7 @@ package multipart
 import (
 	"bytes"
 	"io/ioutil"
+	"mime"
 	"net/textproto"
 	"strings"
 	"testing"
@@ -94,6 +95,7 @@ func TestWriterSetBoundary(t *testing.T) {
 		{"my-separator", true},
 		{"with space", true},
 		{"badspace ", false},
+		{"(boundary)", true},
 	}
 	for i, tt := range tests {
 		var b bytes.Buffer
@@ -107,6 +109,17 @@ func TestWriterSetBoundary(t *testing.T) {
 			if got != tt.b {
 				t.Errorf("boundary = %q; want %q", got, tt.b)
 			}
+
+			ct := w.FormDataContentType()
+			mt, params, err := mime.ParseMediaType(ct)
+			if err != nil {
+				t.Errorf("could not parse Content-Type %q: %v", ct, err)
+			} else if mt != "multipart/form-data" {
+				t.Errorf("unexpected media type %q; want %q", mt, "multipart/form-data")
+			} else if b := params["boundary"]; b != tt.b {
+				t.Errorf("unexpected boundary parameter %q; want %q", b, tt.b)
+			}
+
 			w.Close()
 			wantSub := "\r\n--" + tt.b + "--\r\n"
 			if got := b.String(); !strings.Contains(got, wantSub) {