if err != nil {
			return err
		}
		defer gz.Close()
		r = gz
	case formatS2:
		r = s2.NewReader(bf)
	case formatZstd:
		// Limit to 16 MiB per stream.
		dec, err := zstd.NewReader(bf, zstd.WithDecoderMaxWindow(16<<20))
		if err != nil {
			return err
		}
		defer dec.Close()
		r = dec
	case formatBZ2:
		ctx, cancel := context.WithCancel(ctx)
		defer cancel()