code: func writeFileMapRolling(ctx context.Context, bs blobserver.StatReceiver, file *Builder, r io.Reader) (blob.Ref, error) {
  	n, spans, err := writeFileChunks(ctx, bs, file, r)
  	if err != nil {
  		return blob.Ref{}, err
  	}
  	// The top-level content parts
  	return uploadBytes(ctx, bs, file, n, spans).Get()
  }
docs: // writeFileMap uploads chunks of r to bs while populating fileMap and
  // finally uploading fileMap. The returned blobref is of fileMap's
  // JSON blob. It uses rolling checksum for the chunks sizes.