archive/zip: replace Writer.Comment field with SetComment method

A method is more in keeping with the rest of the Writer API and
incidentally allows the comment error to be reported earlier.

Fixes #22737.

Change-Id: I1eee2103a0720c76d0c394ccd6541e6219996dc0
Reviewed-on: https://go-review.googlesource.com/79415
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

src/archive/zip/writer.go

@@ -26,13 +26,11 @@ type Writer struct {
 	last        *fileWriter
 	closed      bool
 	compressors map[uint16]Compressor
+	comment     string
 
 	// testHookCloseSizeOffset if non-nil is called with the size
 	// of offset of the central directory at Close.
 	testHookCloseSizeOffset func(size, offset uint64)
-
-	// Comment is the central directory comment and must be set before Close is called.
-	Comment string
 }
 
 type header struct {
@@ -62,13 +60,19 @@ func (w *Writer) Flush() error {
 	return w.cw.w.(*bufio.Writer).Flush()
 }
 
-// Close finishes writing the zip file by writing the central directory.
-// It does not (and cannot) close the underlying writer.
-func (w *Writer) Close() error {
-	if len(w.Comment) > uint16max {
+// SetComment sets the end-of-central-directory comment field.
+// It can only be called before Close.
+func (w *Writer) SetComment(comment string) error {
+	if len(comment) > uint16max {
 		return errors.New("zip: Writer.Comment too long")
 	}
+	w.comment = comment
+	return nil
+}
 
+// Close finishes writing the zip file by writing the central directory.
+// It does not (and cannot) close the underlying writer.
+func (w *Writer) Close() error {
 	if w.last != nil && !w.last.closed {
 		if err := w.last.close(); err != nil {
 			return err
@@ -189,11 +193,11 @@ func (w *Writer) Close() error {
 	b.uint16(uint16(records))        // number of entries total
 	b.uint32(uint32(size))           // size of directory
 	b.uint32(uint32(offset))         // start of directory
-	b.uint16(uint16(len(w.Comment))) // byte size of EOCD comment
+	b.uint16(uint16(len(w.comment))) // byte size of EOCD comment
 	if _, err := w.cw.Write(buf[:]); err != nil {
 		return err
 	}
-	if _, err := io.WriteString(w.cw, w.Comment); err != nil {
+	if _, err := io.WriteString(w.cw, w.comment); err != nil {
 		return err
 	}
 

src/archive/zip/writer_test.go

@@ -106,7 +106,16 @@ func TestWriterComment(t *testing.T) {
 		// write a zip file
 		buf := new(bytes.Buffer)
 		w := NewWriter(buf)
-		w.Comment = test.comment
+		if err := w.SetComment(test.comment); err != nil {
+			if test.ok {
+				t.Fatalf("SetComment: unexpected error %v", err)
+			}
+			continue
+		} else {
+			if !test.ok {
+				t.Fatalf("SetComment: unexpected success, want error")
+			}
+		}
 
 		if err := w.Close(); test.ok == (err != nil) {
 			t.Fatal(err)