Function flushSegment

packages/react-server/src/ReactFizzServer.js:5672–5820

(
  request: Request,
  destination: Destination,
  segment: Segment,
  hoistableState: null | HoistableState,
)

let flushedByteSize = 0;

function flushSegment(
  request: Request,
  destination: Destination,
  segment: Segment,
  hoistableState: null | HoistableState,
): boolean {
  const boundary = segment.boundary;
  if (boundary === null) {
    // Not a suspense boundary.
    return flushSubtree(request, destination, segment, hoistableState);
  }

  boundary.parentFlushed = true;
  // This segment is a Suspense boundary. We need to decide whether to
  // emit the content or the fallback now.
  if (boundary.status === CLIENT_RENDERED) {
    // Emit a client rendered suspense boundary wrapper.
    // We never queue the inner boundary so we'll never emit its content or partial segments.

    const row = boundary.row;
    if (row !== null) {
      // Since this boundary end up client rendered, we can unblock future suspense list rows.
      // This means that they may appear out of order if the future rows succeed but this is
      // a client rendered row.
      if (--row.pendingTasks === 0) {
        finishSuspenseListRow(request, row);
      }
    }

    if (__DEV__) {
      writeStartClientRenderedSuspenseBoundary(
        destination,
        request.renderState,
        boundary.errorDigest,
        boundary.errorMessage,
        boundary.errorStack,
        boundary.errorComponentStack,
      );
    } else {
      writeStartClientRenderedSuspenseBoundary(
        destination,
        request.renderState,
        boundary.errorDigest,
        null,
        null,
        null,
      );
    }
    // Flush the fallback.
    flushSubtree(request, destination, segment, hoistableState);

    return writeEndClientRenderedSuspenseBoundary(
      destination,
      request.renderState,
    );
  } else if (boundary.status !== COMPLETED) {
    if (boundary.status === PENDING) {
      // For pending boundaries we lazily assign an ID to the boundary