pkg/io/opener.go
func (o opener) Reader(ctx context.Context, path string) (io.ReadCloser, error) {
	g, err := o.openGCS(path)
	if err != nil {
		return nil, fmt.Errorf("bad gcs path: %v", err)
	}
	if g == nil {
		return os.Open(path)
	}
	return g.NewReader(ctx)
}
func (o opener) Reader(ctx context.Context, path string) (io.ReadCloser, error) {
	g, err := o.openGCS(path)
	if err != nil {
		return nil, fmt.Errorf("bad gcs path: %v", err)
	}
	if g == nil {
		return os.Open(path)
	}
	return g.NewReader(ctx)
}
// Reader will open the path for reading, returning an IsNotExist() error when missing