archive/zip: clarify that CreateHeader takes ownership of FileHeader

Fixes #11144

Change-Id: I1da0b72ef00a84c9b5751be0e72ad07d664bc98b
Reviewed-on: https://go-review.googlesource.com/10883Reviewed-by: Andrew Gerrand <adg@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

src/archive/zip/writer.go

@@ -195,14 +195,20 @@ func (w *Writer) Create(name string) (io.Writer, error) {
 // CreateHeader adds a file to the zip file using the provided FileHeader
 // for the file metadata.
 // It returns a Writer to which the file contents should be written.
+//
 // The file's contents must be written to the io.Writer before the next
-// call to Create, CreateHeader, or Close.
+// call to Create, CreateHeader, or Close. The provided FileHeader fh
+// must not be modified after a call to CreateHeader.
 func (w *Writer) CreateHeader(fh *FileHeader) (io.Writer, error) {
 	if w.last != nil && !w.last.closed {
 		if err := w.last.close(); err != nil {
 			return nil, err
 		}
 	}
+	if len(w.dir) > 0 && w.dir[len(w.dir)-1].FileHeader == fh {
+		// See https://golang.org/issue/11144 confusion.
+		return nil, errors.New("archive/zip: invalid duplicate FileHeader")
+	}
 
 	fh.Flags |= 0x8 // we will write a data descriptor