edlp_d8/core/modules/big_pipe/tests/modules/big_pipe_test/big_pipe_test.module

<?php

/**
 * @file
 * Support module for BigPipe testing.
 */

/**
 * Implements hook_page_top().
 */
function big_pipe_test_page_top(array &$page_top) {
  // Ensure this hook is invoked on every page load.
  $page_top['#cache']['max-age'] = 0;

  if (\Drupal::request()->query->get('trigger_session')) {
    $_SESSION['big_pipe_test'] = TRUE;
  }
}