folly::Expected parseLongHeaderVariants()

in quic/codec/Decode.cpp [988:1065]
67 lines of code
16 McCabe index (conditional complexity)

folly::Expected<ParsedLongHeader, TransportErrorCode> parseLongHeaderVariants(
    LongHeader::Types type,
    ParsedLongHeaderInvariant parsedLongHeaderInvariant,
    folly::io::Cursor& cursor,
    QuicNodeType nodeType) {
  if (type == LongHeader::Types::Retry) {
    // The integrity tag is kRetryIntegrityTagLen bytes in length, and the
    // token must be at least one byte, so the remaining length must
    // be > kRetryIntegrityTagLen.
    if (cursor.totalLength() <= kRetryIntegrityTagLen) {
      VLOG(5) << "Not enough bytes for retry token";
      return folly::makeUnexpected(TransportErrorCode::FRAME_ENCODING_ERROR);
    }

    Buf token;
    cursor.clone(token, cursor.totalLength() - kRetryIntegrityTagLen);

    return ParsedLongHeader(
        LongHeader(
            type,
            std::move(parsedLongHeaderInvariant.invariant),
            token ? token->moveToFbString().toStdString() : std::string()),
        PacketLength(0, 0));
  }

  // TODO Checking kMinInitialDestinationConnIdLength isn't necessary
  // if this packet is in response to a retry.
  if (type == LongHeader::Types::Initial && nodeType == QuicNodeType::Server &&
      parsedLongHeaderInvariant.invariant.dstConnId.size() <
          kMinInitialDestinationConnIdLength) {
    VLOG(5)
        << "Dest Conn-Id length in client initial packet must be >= 8 bytes.";
    return folly::makeUnexpected(TransportErrorCode::FRAME_ENCODING_ERROR);
  }

  Buf token;
  if (type == LongHeader::Types::Initial) {
    auto tokenLen = decodeQuicInteger(cursor);
    if (!tokenLen) {
      VLOG(5) << "Token len not found in Long header";
      return folly::makeUnexpected(TransportErrorCode::FRAME_ENCODING_ERROR);
    }
    if (!cursor.canAdvance(tokenLen->first)) {
      VLOG(5) << "Not enough input bytes to read input token";
      return folly::makeUnexpected(TransportErrorCode::FRAME_ENCODING_ERROR);
    }

    if (tokenLen->first > 0) {
      Buf tokenBuf;
      // If tokenLen > token's actual length then the cursor will throw.
      cursor.clone(tokenBuf, tokenLen->first);
      token = std::move(tokenBuf);
    }
  }
  auto pktLen = decodeQuicInteger(cursor);
  if (!pktLen) {
    VLOG(5) << "Packet len not found in Long header";
    return folly::makeUnexpected(TransportErrorCode::FRAME_ENCODING_ERROR);
  }
  if (!cursor.canAdvance(pktLen->first)) {
    VLOG(5) << "Not enough input bytes to read packet number";
    return folly::makeUnexpected(TransportErrorCode::FRAME_ENCODING_ERROR);
  }
  size_t packetNumLen =
      parsePacketNumberLength(parsedLongHeaderInvariant.initialByte);
  if (!cursor.canAdvance(packetNumLen)) {
    return folly::makeUnexpected(TransportErrorCode::FRAME_ENCODING_ERROR);
  }
  if (packetNumLen > kMaxPacketNumEncodingSize) {
    return folly::makeUnexpected(TransportErrorCode::FRAME_ENCODING_ERROR);
  }
  return ParsedLongHeader(
      LongHeader(
          type,
          std::move(parsedLongHeaderInvariant.invariant),
          token ? token->moveToFbString().toStdString() : std::string()),
      PacketLength(pktLen->first, pktLen->second));
}