void appendDataToReadBufferCommon()

in quic/state/QuicStreamFunctions.cpp [86:238]
121 lines of code
41 McCabe index (conditional complexity)

void appendDataToReadBufferCommon(
    QuicStreamLike& stream,
    StreamBuffer buffer,
    folly::Function<void(uint64_t, uint64_t)>&& connFlowControlVisitor) {
  auto& readBuffer = stream.readBuffer;
  auto it = readBuffer.begin();

  auto bufferEndOffset = buffer.offset + buffer.data.chainLength();

  folly::Optional<uint64_t> bufferEofOffset;
  if (buffer.eof) {
    bufferEofOffset = bufferEndOffset;
  } else if (buffer.data.chainLength() == 0) {
    VLOG(10) << "Empty stream without EOF";
    return;
  }

  if (stream.finalReadOffset && bufferEofOffset &&
      *stream.finalReadOffset != *bufferEofOffset) {
    throw QuicTransportException(
        "Invalid EOF", TransportErrorCode::FINAL_SIZE_ERROR);
  } else if (bufferEofOffset) {
    // Do some consistency checks on the stream.
    if (stream.maxOffsetObserved > *bufferEofOffset) {
      throw QuicTransportException(
          "EOF in middle of stream", TransportErrorCode::FINAL_SIZE_ERROR);
    }
    stream.finalReadOffset = bufferEofOffset;
  } else if (stream.finalReadOffset) {
    // We did not receive a segment with an EOF set.
    if (buffer.offset + buffer.data.chainLength() > *stream.finalReadOffset) {
      throw QuicTransportException(
          "Invalid data after EOF", TransportErrorCode::FINAL_SIZE_ERROR);
    }
  }
  // Update the flow control information before changing max offset observed on
  // the stream.
  connFlowControlVisitor(stream.maxOffsetObserved, bufferEndOffset);
  stream.maxOffsetObserved =
      std::max(stream.maxOffsetObserved, bufferEndOffset);

  if (buffer.data.chainLength() == 0) {
    // Nothing more to do since we already processed the EOF
    // case.
    return;
  }

  if (buffer.offset < stream.currentReadOffset) {
    // trim the buffer to start from stream read offset.
    buffer.data.trimStartAtMost(stream.currentReadOffset - buffer.offset);
    buffer.offset = stream.currentReadOffset;
    if (buffer.data.chainLength() == 0) {
      return;
    }
  }

  // Nothing in the buffer, just append it.
  if (it == readBuffer.end()) {
    readBuffer.emplace_back(std::move(buffer));
    return;
  }

  // Start overlap will point to the first buffer that overlaps with the
  // current buffer and End overlap will point to the last buffer that overlaps.
  // They must always be set together.
  folly::Optional<std::deque<StreamBuffer>::iterator> startOverlap;
  folly::Optional<std::deque<StreamBuffer>::iterator> endOverlap;

  StreamBuffer* current = &buffer;
  bool currentAlreadyInserted = false;
  bool done = false;
  it = std::lower_bound(
      it,
      readBuffer.end(),
      current->offset,
      [](const StreamBuffer& listValue, uint64_t offset) {
        // First element where the end offset is > start offset of the buffer.
        return (listValue.offset + listValue.data.chainLength()) < offset;
      });

  // The invariant we're trying to maintain here is that individual
  // elements of the readBuffer are assuredly non contiguous sections
  // of the stream.
  for (; it != readBuffer.end() && !done; ++it) {
    auto currentEnd = current->offset + current->data.chainLength();
    auto itEnd = it->offset + it->data.chainLength();
    if (current->offset == it->offset && currentEnd == itEnd) {
      // Exact overlap. Done.
      done = true;
    } else if (current->offset >= it->offset && currentEnd <= itEnd) {
      // Subset overlap
      done = true;
    } else if (
        current->offset <= it->offset && currentEnd >= it->offset &&
        currentEnd <= itEnd) {
      // Left overlap. Done.
      it->data.trimStartAtMost(currentEnd - it->offset);
      if (it->data.chainLength() > 0) {
        current->data.append(it->data.move());
      }
      if (!startOverlap) {
        startOverlap = it;
      }
      endOverlap = it + 1;
      done = true;
    } else if (current->offset < it->offset && currentEnd < it->offset) {
      // Left, no overlap. Done.
      if (!startOverlap) {
        startOverlap = it;
        endOverlap = it;
      }
      done = true;
    } else if (current->offset <= it->offset && currentEnd > it->offset) {
      // Complete overlap. Need to move on.
      if (!startOverlap) {
        startOverlap = it;
      }
      endOverlap = it + 1;
    } else if (
        current->offset >= it->offset && current->offset <= itEnd &&
        currentEnd > itEnd) {
      // Right overlap. Not done.
      current->data.trimStartAtMost(itEnd - current->offset);
      it->data.append(current->data.move());
      current = &(*it);
      currentAlreadyInserted = true;
      DCHECK(!startOverlap);
      startOverlap = it + 1;
      endOverlap = it + 1;
    }
  }

  // Could have also been completely to the right of the last element.
  if (startOverlap && !currentAlreadyInserted) {
    DCHECK(endOverlap);
    DCHECK(
        *startOverlap != readBuffer.end() || *endOverlap == readBuffer.end());
    auto insertIt = readBuffer.erase(*startOverlap, *endOverlap);
    readBuffer.emplace(insertIt, std::move(*current));
    return;
  } else if (currentAlreadyInserted) {
    DCHECK(startOverlap);
    DCHECK(endOverlap);
    DCHECK(
        *startOverlap != readBuffer.end() || *endOverlap == readBuffer.end());
    readBuffer.erase(*startOverlap, *endOverlap);
    return;
  }
  auto last = readBuffer.end() - 1;
  if (current->offset > last->offset + last->data.chainLength()) {
    readBuffer.emplace_back(std::move(*current));
  }
}