undo CL 95760043 / b2131d729e52

Breaks Camlistore by introducing a datarace. See comments on
https://golang.org/cl/95760043/ for details.

I'll add a new test to lock-in the current behavior in a
subsequent CL.

I don't think Camlistore is particularly unique here: it's doing
the obvious thing to stream a multipart body to a server
using a goroutine feeding the multipart writer.

««« original CL description
mime/multipart: delay reading random source

If a user sets his/her own boundary string with SetBoundary,
we don't need to call randomBoundary at all.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/95760043
»»»

LGTM=ruiu
R=ruiu
CC=golang-codereviews, mathieu.lonjaret
https://golang.org/cl/117600043

src/pkg/mime/multipart/writer.go

@@ -25,15 +25,13 @@ type Writer struct {
 // writing to w.
 func NewWriter(w io.Writer) *Writer {
 	return &Writer{
-		w: w,
+		w:        w,
+		boundary: randomBoundary(),
 	}
 }
 
 // Boundary returns the Writer's boundary.
 func (w *Writer) Boundary() string {
-	if w.boundary == "" {
-		w.boundary = randomBoundary()
-	}
 	return w.boundary
 }
 
@@ -67,7 +65,7 @@ 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()
+	return "multipart/form-data; boundary=" + w.boundary
 }
 
 func randomBoundary() string {
@@ -91,9 +89,9 @@ func (w *Writer) CreatePart(header textproto.MIMEHeader) (io.Writer, error) {
 	}
 	var b bytes.Buffer
 	if w.lastpart != nil {
-		fmt.Fprintf(&b, "\r\n--%s\r\n", w.Boundary())
+		fmt.Fprintf(&b, "\r\n--%s\r\n", w.boundary)
 	} else {
-		fmt.Fprintf(&b, "--%s\r\n", w.Boundary())
+		fmt.Fprintf(&b, "--%s\r\n", w.boundary)
 	}
 	// TODO(bradfitz): move this to textproto.MimeHeader.Write(w), have it sort
 	// and clean, like http.Header.Write(w) does.
@@ -159,7 +157,7 @@ func (w *Writer) Close() error {
 		}
 		w.lastpart = nil
 	}
-	_, err := fmt.Fprintf(w.w, "\r\n--%s--\r\n", w.Boundary())
+	_, err := fmt.Fprintf(w.w, "\r\n--%s--\r\n", w.boundary)
 	return err
 }