mime/multipart: convert Reader from interface to struct

It was always a weird interface but I didn't know what I
was doing at the time.  rsc questioned me about it then
but didn't press on it during review.  Then adg bugged me
about it too recently.

So clean it up. It parallels the Writer struct too.

R=golang-dev, r, rsc
CC=golang-dev
https://golang.org/cl/4602063

src/pkg/http/request.go

@@ -188,7 +188,7 @@ var multipartByReader = &multipart.Form{
 // multipart/form-data 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, os.Error) {
+func (r *Request) MultipartReader() (*multipart.Reader, os.Error) {
 	if r.MultipartForm == multipartByReader {
 		return nil, os.NewError("http: MultipartReader called twice")
 	}
@@ -199,7 +199,7 @@ func (r *Request) MultipartReader() (multipart.Reader, os.Error) {
 	return r.multipartReader()
 }
 
-func (r *Request) multipartReader() (multipart.Reader, os.Error) {
+func (r *Request) multipartReader() (*multipart.Reader, os.Error) {
 	v := r.Header.Get("Content-Type")
 	if v == "" {
 		return nil, ErrNotMultipart

src/pkg/mime/multipart/formdata.go

@@ -19,7 +19,7 @@ import (
 // a Content-Disposition of "form-data".
 // It stores up to maxMemory bytes of the file parts in memory
 // and the remainder on disk in temporary files.
-func (r *multiReader) ReadForm(maxMemory int64) (f *Form, err os.Error) {
+func (r *Reader) ReadForm(maxMemory int64) (f *Form, err os.Error) {
 	form := &Form{make(map[string][]string), make(map[string][]*FileHeader)}
 	defer func() {
 		if err != nil {

src/pkg/mime/multipart/multipart.go

@@ -28,21 +28,6 @@ var headerRegexp *regexp.Regexp = regexp.MustCompile("^([a-zA-Z0-9\\-]+): *([^\r
 
 var emptyParams = make(map[string]string)
 
-// Reader is an iterator over parts in a MIME multipart body.
-// Reader's underlying parser consumes its input as needed.  Seeking
-// isn't supported.
-type Reader interface {
-	// NextPart returns the next part in the multipart or an error.
-	// When there are no more parts, the error os.EOF is returned.
-	NextPart() (*Part, os.Error)
-
-	// ReadForm parses an entire multipart message whose parts have
-	// a Content-Disposition of "form-data".
-	// It stores up to maxMemory bytes of the file parts in memory
-	// and the remainder on disk in temporary files.
-	ReadForm(maxMemory int64) (*Form, os.Error)
-}
-
 // A Part represents a single part in a multipart body.
 type Part struct {
 	// The headers of the body, if any, with the keys canonicalized
@@ -51,7 +36,7 @@ type Part struct {
 	Header textproto.MIMEHeader
 
 	buffer *bytes.Buffer
-	mr     *multiReader
+	mr     *Reader
 
 	disposition       string
 	dispositionParams map[string]string
@@ -91,9 +76,9 @@ func (p *Part) parseContentDisposition() {
 
 // NewReader creates a new multipart Reader reading from r using the
 // given MIME boundary.
-func NewReader(reader io.Reader, boundary string) Reader {
+func NewReader(reader io.Reader, boundary string) *Reader {
 	b := []byte("\r\n--" + boundary + "--")
-	return &multiReader{
+	return &Reader{
 		bufReader: bufio.NewReader(reader),
 
 		nlDashBoundary:   b[:len(b)-2],
@@ -102,9 +87,7 @@ func NewReader(reader io.Reader, boundary string) Reader {
 	}
 }
 
-// Implementation ....
-
-func newPart(mr *multiReader) (*Part, os.Error) {
+func newPart(mr *Reader) (*Part, os.Error) {
 	bp := &Part{
 		Header: make(map[string][]string),
 		mr:     mr,
@@ -188,7 +171,10 @@ func (bp *Part) Close() os.Error {
 	return nil
 }
 
-type multiReader struct {
+// Reader is an iterator over parts in a MIME multipart body.
+// Reader's underlying parser consumes its input as needed.  Seeking
+// isn't supported.
+type Reader struct {
 	bufReader *bufio.Reader
 
 	currentPart *Part
@@ -197,7 +183,9 @@ type multiReader struct {
 	nlDashBoundary, dashBoundaryDash, dashBoundary []byte
 }
 
-func (mr *multiReader) NextPart() (*Part, os.Error) {
+// NextPart returns the next part in the multipart or an error.
+// When there are no more parts, the error os.EOF is returned.
+func (mr *Reader) NextPart() (*Part, os.Error) {
 	if mr.currentPart != nil {
 		mr.currentPart.Close()
 	}
@@ -247,7 +235,7 @@ func (mr *multiReader) NextPart() (*Part, os.Error) {
 	panic("unreachable")
 }
 
-func (mr *multiReader) isBoundaryDelimiterLine(line []byte) bool {
+func (mr *Reader) isBoundaryDelimiterLine(line []byte) bool {
 	// http://tools.ietf.org/html/rfc2046#section-5.1
 	//   The boundary delimiter line is then defined as a line
 	//   consisting entirely of two hyphen characters ("-",

src/pkg/mime/multipart/multipart_test.go

@@ -25,7 +25,7 @@ func TestHorizontalWhitespace(t *testing.T) {
 }
 
 func TestBoundaryLine(t *testing.T) {
-	mr := NewReader(strings.NewReader(""), "myBoundary").(*multiReader)
+	mr := NewReader(strings.NewReader(""), "myBoundary")
 	if !mr.isBoundaryDelimiterLine([]byte("--myBoundary\r\n")) {
 		t.Error("expected")
 	}