Function flushPartiallyCompletedSegment

packages/react-server/src/ReactFizzServer.js:5948–5987

(
  request: Request,
  destination: Destination,
  boundary: SuspenseBoundary,
  segment: Segment,
)

}

function flushPartiallyCompletedSegment(
  request: Request,
  destination: Destination,
  boundary: SuspenseBoundary,
  segment: Segment,
): boolean {
  if (segment.status === FLUSHED) {
    // We've already flushed this inline.
    return true;
  }

  const hoistableState = boundary.contentState;

  const segmentID = segment.id;
  if (segmentID === -1) {
    // This segment wasn't previously referred to. This happens at the root of
    // a boundary. We make kind of a leap here and assume this is the root.
    const rootSegmentID = (segment.id = boundary.rootSegmentID);

    if (rootSegmentID === -1) {
      throw new Error(
        'A root segment ID must have been assigned by now. This is a bug in React.',
      );
    }

    return flushSegmentContainer(request, destination, segment, hoistableState);
  } else if (segmentID === boundary.rootSegmentID) {
    // When we emit postponed boundaries, we might have assigned the ID already
    // but it's still the root segment so we can't inject it into the parent yet.
    return flushSegmentContainer(request, destination, segment, hoistableState);
  } else {
    flushSegmentContainer(request, destination, segment, hoistableState);
    return writeCompletedSegmentInstruction(
      destination,
      request.resumableState,
      request.renderState,
      segmentID,
    );
  }
}

let flushingPartialBoundaries = false;