Function flushCompletedQueues

packages/react-server/src/ReactFizzServer.js:5991–6170

(
  request: Request,
  destination: Destination,
)

let flushingPartialBoundaries = false;

function flushCompletedQueues(
  request: Request,
  destination: Destination,
): void {
  beginWriting(destination);
  try {
    // The structure of this is to go through each queue one by one and write
    // until the sink tells us to stop. When we should stop, we still finish writing
    // that item fully and then yield. At that point we remove the already completed
    // items up until the point we completed them.

    if (request.pendingRootTasks > 0) {
      // When there are pending root tasks we don't want to flush anything
      return;
    }

    let i;
    const completedRootSegment = request.completedRootSegment;
    if (completedRootSegment !== null) {
      if (completedRootSegment.status === POSTPONED) {
        return;
      }

      const completedPreambleSegments = request.completedPreambleSegments;
      if (completedPreambleSegments === null) {
        // The preamble isn't ready yet even though the root is so we omit flushing
        return;
      }

      flushedByteSize = request.byteSize; // Start counting bytes
      // TODO: Count the size of the preamble chunks too.
      let skipBlockingShell = false;
      if (enableFizzBlockingRender) {
        const blockingRenderMaxSize = getBlockingRenderMaxSize(request);
        if (flushedByteSize > blockingRenderMaxSize) {
          skipBlockingShell = true;
          const maxSizeKb = Math.round(blockingRenderMaxSize / 1000);
          const error = new Error(
            'This rendered a large document (>' +
              maxSizeKb +
              ' kB) without any Suspense ' +
              'boundaries around most of it. That can delay initial paint longer than ' +
              'necessary. To improve load performance, add a <Suspense> or <SuspenseList> ' +
              'around the content you expect to be below the header or below the fold. ' +
              'In the meantime, the content will deopt to paint arbitrary incomplete ' +
              'pieces of HTML.',
          );
          const errorInfo: ThrownInfo = {};
          logRecoverableError(request, error, errorInfo, null);
        }
      }
      flushPreamble(
        request,
        destination,
        completedRootSegment,
        completedPreambleSegments,
        skipBlockingShell,
      );