void HTTPSession::sendHeaders()

in proxygen/lib/http/session/HTTPSession.cpp [1536:1607]
61 lines of code
25 McCabe index (conditional complexity)

void HTTPSession::sendHeaders(HTTPTransaction* txn,
                              const HTTPMessage& headers,
                              HTTPHeaderSize* size,
                              bool includeEOM) noexcept {
  CHECK(started_);
  unique_ptr<IOBuf> goawayBuf;
  if (draining_ && isUpstream() && codec_->isReusable() &&
      allTransactionsStarted()) {
    // For HTTP/1.1, add Connection: close
    // For SPDY/H2, save the goaway for AFTER the request
    auto writeBuf = writeBuf_.move();
    drainImpl();
    goawayBuf = writeBuf_.move();
    writeBuf_.append(std::move(writeBuf));
  }
  if (isUpstream() || (txn->isPushed() && headers.isRequest())) {
    // upstream picks priority
    if (getHTTP2PrioritiesEnabled()) {
      auto pri = getMessagePriority(&headers);
      if (pri.streamDependency == txn->getID()) {
        LOG(ERROR) << "Attempted to create circular dependency txn=" << *this;
      } else {
        txn->onPriorityUpdate(pri);
      }
    }
  }

  const bool wasReusable = codec_->isReusable();
  const uint64_t oldOffset = sessionByteOffset();
  auto exAttributes = txn->getExAttributes();
  auto assocStream = txn->getAssocTxnId();
  if (exAttributes) {
    codec_->generateExHeader(
        writeBuf_, txn->getID(), headers, *exAttributes, includeEOM, size);
  } else if (headers.isRequest() && assocStream) {
    // Only PUSH_PROMISE (not push response) has an associated stream
    codec_->generatePushPromise(
        writeBuf_, txn->getID(), headers, *assocStream, includeEOM, size);
  } else {
    codec_->generateHeader(writeBuf_, txn->getID(), headers, includeEOM, size);
  }
  const uint64_t newOffset = sessionByteOffset();

  // for push response count towards the MAX_CONCURRENT_STREAMS limit
  if (isDownstream() && headers.isResponse() && txn->isPushed()) {
    incrementOutgoingStreams(txn);
  }

  // For all upstream headers, addFirstHeaderByteEvent should be added
  // For all downstream, only response headers need addFirstHeaderByteEvent
  bool shouldAddFirstHeaderByteEvent =
      isUpstream() || (isDownstream() && headers.isResponse());
  if (shouldAddFirstHeaderByteEvent && newOffset > oldOffset &&
      !txn->testAndSetFirstHeaderByteSent() && byteEventTracker_) {
    byteEventTracker_->addFirstHeaderByteEvent(newOffset, txn);
  }

  if (size) {
    VLOG(4) << *this << " sending headers, size=" << size->compressed
            << ", uncompressedSize=" << size->uncompressed;
  }
  if (goawayBuf) {
    VLOG(4) << *this << " moved GOAWAY to end of writeBuf";
    writeBuf_.append(std::move(goawayBuf));
  }
  if (includeEOM) {
    CHECK_GE(newOffset, oldOffset);
    commonEom(txn, newOffset - oldOffset, true);
  }
  scheduleWrite();
  onHeadersSent(headers, wasReusable);
}