return err
		}
		defer dec.Close()
		r = dec
	case formatBZ2:
		ctx, cancel := context.WithCancel(ctx)
		defer cancel()
		r = pbzip2.NewReader(ctx, bf, pbzip2.DecompressionOptions(
			pbzip2.BZConcurrency((runtime.GOMAXPROCS(0)+1)/2),
			pbzip2.BZConcurrencyPool(bz2Limiter)))
	case formatLZ4:
		r = lz4.NewReader(bf)
	case formatUnknown:
		r = bf
	default:
		return fmt.Errorf("Unsupported format %s", f)