mime/multipart: add Size to FileHeader

This change makes it possible to retrieve the size of a file part
without having to Seek to determine file-size.

Resolves #19501

Change-Id: I7b9994c4cf41c9b06a046eb7046f8952ae1f15e9
Reviewed-on: https://go-review.googlesource.com/39223Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/mime/multipart/formdata.go

@@ -79,7 +79,7 @@ func (r *Reader) readForm(maxMemory int64) (_ *Form, err error) {
 			if err != nil {
 				return nil, err
 			}
-			_, err = io.Copy(file, io.MultiReader(&b, p))
+			size, err := io.Copy(file, io.MultiReader(&b, p))
 			if cerr := file.Close(); err == nil {
 				err = cerr
 			}
@@ -88,8 +88,10 @@ func (r *Reader) readForm(maxMemory int64) (_ *Form, err error) {
 				return nil, err
 			}
 			fh.tmpfile = file.Name()
+			fh.Size = size
 		} else {
 			fh.content = b.Bytes()
+			fh.Size = int64(len(fh.content))
 			maxMemory -= n
 		}
 		form.File[name] = append(form.File[name], fh)
@@ -128,6 +130,7 @@ func (f *Form) RemoveAll() error {
 type FileHeader struct {
 	Filename string
 	Header   textproto.MIMEHeader
+	Size     int64
 
 	content []byte
 	tmpfile string

src/mime/multipart/formdata_test.go

@@ -44,6 +44,9 @@ func testFile(t *testing.T, fh *FileHeader, efn, econtent string) File {
 	if fh.Filename != efn {
 		t.Errorf("filename = %q, want %q", fh.Filename, efn)
 	}
+	if fh.Size != int64(len(econtent)) {
+		t.Errorf("size = %d, want %d", fh.Size, len(econtent))
+	}
 	f, err := fh.Open()
 	if err != nil {
 		t.Fatal("opening file:", err)