mime/multipart: allow setting the Writer boundary

Fixes #4490

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6924044

src/pkg/mime/multipart/writer.go

@@ -30,11 +30,38 @@ func NewWriter(w io.Writer) *Writer {
 	}
 }
 
-// Boundary returns the Writer's randomly selected boundary string.
+// Boundary returns the Writer's boundary.
 func (w *Writer) Boundary() string {
 	return w.boundary
 }
 
+// SetBoundary overrides the Writer's default randomly-generated
+// boundary separator with an explicit value.
+//
+// SetBoundary must be called before any parts are created, may only
+// contain certain ASCII characters, and must be 1-69 bytes long.
+func (w *Writer) SetBoundary(boundary string) error {
+	if w.lastpart != nil {
+		return errors.New("mime: SetBoundary called after write")
+	}
+	// rfc2046#section-5.1.1
+	if len(boundary) < 1 || len(boundary) > 69 {
+		return errors.New("mime: invalid boundary length")
+	}
+	for _, b := range boundary {
+		if 'A' <= b && b <= 'Z' || 'a' <= b && b <= 'z' || '0' <= b && b <= '9' {
+			continue
+		}
+		switch b {
+		case '\'', '(', ')', '+', '_', ',', '-', '.', '/', ':', '=', '?':
+			continue
+		}
+		return errors.New("mime: invalid boundary character")
+	}
+	w.boundary = boundary
+	return nil
+}
+
 // FormDataContentType returns the Content-Type for an HTTP
 // multipart/form-data with this Writer's Boundary.
 func (w *Writer) FormDataContentType() string {

src/pkg/mime/multipart/writer_test.go

@@ -7,6 +7,7 @@ package multipart
 import (
 	"bytes"
 	"io/ioutil"
+	"strings"
 	"testing"
 )
 
@@ -76,3 +77,37 @@ func TestWriter(t *testing.T) {
 		t.Fatalf("expected end of parts; got %v, %v", part, err)
 	}
 }
+
+func TestWriterSetBoundary(t *testing.T) {
+	var b bytes.Buffer
+	w := NewWriter(&b)
+	tests := []struct {
+		b  string
+		ok bool
+	}{
+		{"abc", true},
+		{"", false},
+		{"ungültig", false},
+		{"!", false},
+		{strings.Repeat("x", 69), true},
+		{strings.Repeat("x", 70), false},
+		{"bad!ascii!", false},
+		{"my-separator", true},
+	}
+	for i, tt := range tests {
+		err := w.SetBoundary(tt.b)
+		got := err == nil
+		if got != tt.ok {
+			t.Errorf("%d. boundary %q = %v (%v); want %v", i, tt.b, got, err, tt.ok)
+		} else if tt.ok {
+			got := w.Boundary()
+			if got != tt.b {
+				t.Errorf("boundary = %q; want %q", got, tt.b)
+			}
+		}
+	}
+	w.Close()
+	if got := b.String(); !strings.Contains(got, "\r\n--my-separator--\r\n") {
+		t.Errorf("expected my-separator in output. got: %q", got)
+	}
+}