BigPipeTest.php

<?php

declare (strict_types=1);
namespace Drupal\Tests\big_pipe\Functional;

use Behat\Mink\Element\NodeElement;
use Drupal\big_pipe\Render\Placeholder\BigPipeStrategy;
use Drupal\big_pipe\Render\BigPipe;
use Drupal\big_pipe_test\BigPipePlaceholderTestCases;
use Drupal\Component\Serialization\Json;
use Drupal\Component\Utility\Html;
use Drupal\Component\Utility\UrlHelper;
use Drupal\Core\Database\Database;
use Drupal\Core\Logger\RfcLogLevel;
use Drupal\Core\Url;
use Drupal\Tests\BrowserTestBase;

/**
 * Tests BigPipe's no-JS detection & response delivery (with and without JS).
 *
 * Covers:
 * - big_pipe_page_attachments()
 * - \Drupal\big_pipe\Controller\BigPipeController
 * - \Drupal\big_pipe\EventSubscriber\HtmlResponseBigPipeSubscriber
 * - \Drupal\big_pipe\Render\BigPipe
 *
 * @group big_pipe
 */
class BigPipeTest extends BrowserTestBase {
    
    /**
     * Modules to enable.
     *
     * @var array
     */
    protected static $modules = [
        'big_pipe',
        'big_pipe_test',
        'dblog',
    ];
    
    /**
     * {@inheritdoc}
     */
    protected $defaultTheme = 'stark';
    
    /**
     * {@inheritdoc}
     */
    protected function setUp() : void {
        parent::setUp();
        // Ignore the <meta> refresh that big_pipe.module sets. It causes a redirect
        // to a page that sets another cookie, which causes BrowserTestBase to lose
        // the session cookie. To avoid this problem, tests should first call
        // drupalGet() and then call checkForMetaRefresh() manually, and then reset
        // $this->maximumMetaRefreshCount and $this->metaRefreshCount.
        // @see doMetaRefresh()
        $this->maximumMetaRefreshCount = 0;
    }
    
    /**
     * Performs a single <meta> refresh explicitly.
     *
     * This test disables the automatic <meta> refresh checking, each time it is
     * desired that this runs, a test case must explicitly call this.
     *
     * @see setUp()
     */
    protected function performMetaRefresh() {
        $this->maximumMetaRefreshCount = 1;
        $this->checkForMetaRefresh();
        $this->maximumMetaRefreshCount = 0;
        $this->metaRefreshCount = 0;
    }
    
    /**
     * Tests BigPipe's no-JS detection.
     *
     * Covers:
     * - big_pipe_page_attachments()
     * - \Drupal\big_pipe\Controller\BigPipeController
     */
    public function testNoJsDetection() : void {
        $no_js_to_js_markup = '<script>document.cookie = "' . BigPipeStrategy::NOJS_COOKIE . '=1; path=/; expires=Thu, 01 Jan 1970 00:00:00 GMT"</script>';
        // 1. No session (anonymous).
        $this->drupalGet(Url::fromRoute('<front>'));
        $this->assertSessionCookieExists('0');
        $this->assertBigPipeNoJsCookieExists('0');
        $this->assertSession()
            ->responseNotContains('<noscript><meta http-equiv="Refresh" content="0; URL=');
        $this->assertSession()
            ->responseNotContains($no_js_to_js_markup);
        // 2. Session (authenticated).
        $this->drupalLogin($this->rootUser);
        $this->assertSessionCookieExists('1');
        $this->assertBigPipeNoJsCookieExists('0');
        $this->assertSession()
            ->responseContains('<noscript><meta http-equiv="Refresh" content="0; URL=' . base_path() . 'big_pipe/no-js?destination=' . UrlHelper::encodePath(base_path() . 'user/1?check_logged_in=1') . '" />' . "\n" . '</noscript>');
        $this->assertSession()
            ->responseNotContains($no_js_to_js_markup);
        $this->assertBigPipeNoJsMetaRefreshRedirect();
        $this->assertBigPipeNoJsCookieExists('1');
        $this->assertSession()
            ->responseNotContains('<noscript><meta http-equiv="Refresh" content="0; URL=');
        $this->assertSession()
            ->responseContains($no_js_to_js_markup);
        $this->drupalLogout();
        // Close the prior connection and remove the collected state.
        $this->getSession()
            ->reset();
        // 3. Session (anonymous).
        $this->drupalGet(Url::fromRoute('user.login', [], [
            'query' => [
                'trigger_session' => 1,
            ],
        ]));
        $this->drupalGet(Url::fromRoute('user.login'));
        $this->assertSessionCookieExists('1');
        $this->assertBigPipeNoJsCookieExists('0');
        $this->assertSession()
            ->responseContains('<noscript><meta http-equiv="Refresh" content="0; URL=' . base_path() . 'big_pipe/no-js?destination=' . base_path() . 'user/login" />' . "\n" . '</noscript>');
        $this->assertSession()
            ->responseNotContains($no_js_to_js_markup);
        $this->assertBigPipeNoJsMetaRefreshRedirect();
        $this->assertBigPipeNoJsCookieExists('1');
        $this->assertSession()
            ->responseNotContains('<noscript><meta http-equiv="Refresh" content="0; URL=');
        $this->assertSession()
            ->responseContains($no_js_to_js_markup);
        // Close the prior connection and remove the collected state.
        $this->getSession()
            ->reset();
        // Edge case: route with '_no_big_pipe' option.
        $this->drupalGet(Url::fromRoute('no_big_pipe'));
        $this->assertSessionCookieExists('0');
        $this->assertBigPipeNoJsCookieExists('0');
        $this->assertSession()
            ->responseNotContains('<noscript><meta http-equiv="Refresh" content="0; URL=');
        $this->assertSession()
            ->responseNotContains($no_js_to_js_markup);
        $this->drupalLogin($this->rootUser);
        $this->drupalGet(Url::fromRoute('no_big_pipe'));
        $this->assertSessionCookieExists('1');
        $this->assertBigPipeNoJsCookieExists('0');
        $this->assertSession()
            ->responseNotContains('<noscript><meta http-equiv="Refresh" content="0; URL=');
        $this->assertSession()
            ->responseNotContains($no_js_to_js_markup);
    }
    
    /**
     * Tests BigPipe-delivered HTML responses when JavaScript is enabled.
     *
     * Covers:
     * - \Drupal\big_pipe\EventSubscriber\HtmlResponseBigPipeSubscriber
     * - \Drupal\big_pipe\Render\BigPipe
     * - \Drupal\big_pipe\Render\BigPipe::sendPlaceholders()
     *
     * @see \Drupal\big_pipe_test\BigPipePlaceholderTestCases
     */
    public function testBigPipe() : void {
        // Simulate production.
        $this->config('system.logging')
            ->set('error_level', ERROR_REPORTING_HIDE)
            ->save();
        $this->drupalLogin($this->rootUser);
        $this->assertSessionCookieExists('1');
        $this->assertBigPipeNoJsCookieExists('0');
        $connection = Database::getConnection();
        $log_count = $connection->select('watchdog')
            ->countQuery()
            ->execute()
            ->fetchField();
        // By not calling performMetaRefresh() here, we simulate JavaScript being
        // enabled, because as far as the BigPipe module is concerned, JavaScript is
        // enabled in the browser as long as the BigPipe no-JS cookie is *not* set.
        // @see setUp()
        // @see performMetaRefresh()
        $this->drupalGet(Url::fromRoute('big_pipe_test'));
        $this->assertBigPipeResponseHeadersPresent();
        $this->assertSession()
            ->responseHeaderNotContains('X-Drupal-Cache-Tags', 'cache_tag_set_in_lazy_builder');
        $this->setCsrfTokenSeedInTestEnvironment();
        $cases = $this->getTestCases();
        $this->assertBigPipeNoJsPlaceholders([
            $cases['edge_case__invalid_html']->bigPipeNoJsPlaceholder => $cases['edge_case__invalid_html']->embeddedHtmlResponse,
            $cases['html_attribute_value']->bigPipeNoJsPlaceholder => $cases['html_attribute_value']->embeddedHtmlResponse,
            $cases['html_attribute_value_subset']->bigPipeNoJsPlaceholder => $cases['html_attribute_value_subset']->embeddedHtmlResponse,
        ]);
        $this->assertBigPipePlaceholders([
            $cases['html']->bigPipePlaceholderId => Json::encode($cases['html']->embeddedAjaxResponseCommands),
            $cases['edge_case__html_non_lazy_builder_suspend']->bigPipePlaceholderId => Json::encode($cases['edge_case__html_non_lazy_builder_suspend']->embeddedAjaxResponseCommands),
            $cases['edge_case__html_non_lazy_builder']->bigPipePlaceholderId => Json::encode($cases['edge_case__html_non_lazy_builder']->embeddedAjaxResponseCommands),
            $cases['exception__lazy_builder']->bigPipePlaceholderId => NULL,
            $cases['exception__embedded_response']->bigPipePlaceholderId => NULL,
        ], [
            0 => $cases['html']->bigPipePlaceholderId,
            1 => $cases['edge_case__html_non_lazy_builder']->bigPipePlaceholderId,
            // The suspended placeholder is replaced after the non-suspended
            // placeholder even though it appears first in the page.
            // @see Drupal\big_pipe\Render\BigPipe\Render::sendPlaceholders()
2 => $cases['edge_case__html_non_lazy_builder_suspend']->bigPipePlaceholderId,
        ]);
        $this->assertSession()
            ->responseContains('</body>');
        // Verifying BigPipe assets are present.
        $this->assertNotEmpty($this->getDrupalSettings());
        $this->assertContains('big_pipe/big_pipe', explode(',', $this->getDrupalSettings()['ajaxPageState']['libraries']), 'BigPipe asset library is present.');
        // Verify that the two expected exceptions are logged as errors.
        $this->assertEquals($log_count + 2, (int) $connection->select('watchdog')
            ->countQuery()
            ->execute()
            ->fetchField(), 'Two new watchdog entries.');
        // Using dynamic select queries with the method range() allows contrib
        // database drivers the ability to insert their own limit and offset
        // functionality.
        $records = $connection->select('watchdog', 'w')
            ->fields('w')
            ->orderBy('wid', 'DESC')
            ->range(0, 2)
            ->execute()
            ->fetchAll();
        $this->assertEquals(RfcLogLevel::ERROR, $records[0]->severity);
        $this->assertStringContainsString('Oh noes!', (string) unserialize($records[0]->variables)['@message']);
        $this->assertEquals(RfcLogLevel::ERROR, $records[1]->severity);
        $this->assertStringContainsString('You are not allowed to say llamas are not cool!', (string) unserialize($records[1]->variables)['@message']);
        // Verify that 4xx responses work fine. (4xx responses are handled by
        // subrequests to a route pointing to a controller with the desired output.)
        $this->drupalGet(Url::fromUri('base:non-existing-path'));
        // Simulate development.
        // Verifying BigPipe provides useful error output when an error occurs
        // while rendering a placeholder if verbose error logging is enabled.
        $this->config('system.logging')
            ->set('error_level', ERROR_REPORTING_DISPLAY_VERBOSE)
            ->save();
        $this->drupalGet(Url::fromRoute('big_pipe_test'));
        // The 'edge_case__html_exception' case throws an exception.
        $this->assertSession()
            ->pageTextContains('The website encountered an unexpected error. Try again later');
        $this->assertSession()
            ->pageTextContains('You are not allowed to say llamas are not cool!');
        // Check that stop signal and closing body tag are absent.
        $this->assertSession()
            ->responseNotContains(BigPipe::STOP_SIGNAL);
        $this->assertSession()
            ->responseNotContains('</body>');
        // The exception is expected. Do not interpret it as a test failure.
        unlink($this->root . '/' . $this->siteDirectory . '/error.log');
    }
    
    /**
     * Tests BigPipe-delivered HTML responses when JavaScript is disabled.
     *
     * Covers:
     * - \Drupal\big_pipe\EventSubscriber\HtmlResponseBigPipeSubscriber
     * - \Drupal\big_pipe\Render\BigPipe
     * - \Drupal\big_pipe\Render\BigPipe::sendNoJsPlaceholders()
     *
     * @see \Drupal\big_pipe_test\BigPipePlaceholderTestCases
     */
    public function testBigPipeNoJs() : void {
        // Simulate production.
        $this->config('system.logging')
            ->set('error_level', ERROR_REPORTING_HIDE)
            ->save();
        $this->drupalLogin($this->rootUser);
        $this->assertSessionCookieExists('1');
        $this->assertBigPipeNoJsCookieExists('0');
        // By calling performMetaRefresh() here, we simulate JavaScript being
        // disabled, because as far as the BigPipe module is concerned, it is
        // enabled in the browser when the BigPipe no-JS cookie is set.
        // @see setUp()
        // @see performMetaRefresh()
        $this->performMetaRefresh();
        $this->assertBigPipeNoJsCookieExists('1');
        $this->drupalGet(Url::fromRoute('big_pipe_test'));
        $this->assertBigPipeResponseHeadersPresent();
        $this->assertSession()
            ->responseHeaderNotContains('X-Drupal-Cache-Tags', 'cache_tag_set_in_lazy_builder');
        $this->setCsrfTokenSeedInTestEnvironment();
        $cases = $this->getTestCases();
        $this->assertBigPipeNoJsPlaceholders([
            $cases['edge_case__invalid_html']->bigPipeNoJsPlaceholder => $cases['edge_case__invalid_html']->embeddedHtmlResponse,
            $cases['html_attribute_value']->bigPipeNoJsPlaceholder => $cases['html_attribute_value']->embeddedHtmlResponse,
            $cases['html_attribute_value_subset']->bigPipeNoJsPlaceholder => $cases['html_attribute_value_subset']->embeddedHtmlResponse,
            $cases['html']->bigPipeNoJsPlaceholder => $cases['html']->embeddedHtmlResponse,
            $cases['edge_case__html_non_lazy_builder']->bigPipeNoJsPlaceholder => $cases['edge_case__html_non_lazy_builder']->embeddedHtmlResponse,
            $cases['exception__lazy_builder']->bigPipePlaceholderId => NULL,
            $cases['exception__embedded_response']->bigPipePlaceholderId => NULL,
        ]);
        // Verifying there are no BigPipe placeholders & replacements.
        $this->assertSession()
            ->responseHeaderEquals('BigPipe-Test-Placeholders', '<none>');
        // Verifying BigPipe start/stop signals are absent.
        $this->assertSession()
            ->responseNotContains(BigPipe::START_SIGNAL);
        $this->assertSession()
            ->responseNotContains(BigPipe::STOP_SIGNAL);
        // Verifying BigPipe assets are absent.
        $this->assertArrayNotHasKey('bigPipePlaceholderIds', $this->getDrupalSettings());
        $this->assertArrayNotHasKey('ajaxPageState', $this->getDrupalSettings());
        $this->assertSession()
            ->responseContains('</body>');
        // Verify that 4xx responses work fine. (4xx responses are handled by
        // subrequests to a route pointing to a controller with the desired output.)
        $this->drupalGet(Url::fromUri('base:non-existing-path'));
        // Simulate development.
        // Verifying BigPipe provides useful error output when an error occurs
        // while rendering a placeholder if verbose error logging is enabled.
        $this->config('system.logging')
            ->set('error_level', ERROR_REPORTING_DISPLAY_VERBOSE)
            ->save();
        $this->drupalGet(Url::fromRoute('big_pipe_test'));
        // The 'edge_case__html_exception' case throws an exception.
        $this->assertSession()
            ->pageTextContains('The website encountered an unexpected error. Try again later');
        $this->assertSession()
            ->pageTextContains('You are not allowed to say llamas are not cool!');
        $this->assertSession()
            ->responseNotContains('</body>');
        // The exception is expected. Do not interpret it as a test failure.
        unlink($this->root . '/' . $this->siteDirectory . '/error.log');
    }
    
    /**
     * Tests BigPipe with a multi-occurrence placeholder.
     */
    public function testBigPipeMultiOccurrencePlaceholders() : void {
        $this->drupalLogin($this->rootUser);
        $this->assertSessionCookieExists('1');
        $this->assertBigPipeNoJsCookieExists('0');
        // By not calling performMetaRefresh() here, we simulate JavaScript being
        // enabled, because as far as the BigPipe module is concerned, JavaScript is
        // enabled in the browser as long as the BigPipe no-JS cookie is *not* set.
        // @see setUp()
        // @see performMetaRefresh()
        $this->drupalGet(Url::fromRoute('big_pipe_test_multi_occurrence'));
        // cspell:disable-next-line
        $big_pipe_placeholder_id = 'callback=Drupal%5CCore%5CRender%5CElement%5CStatusMessages%3A%3ArenderMessages&amp;args%5B0%5D&amp;token=_HAdUpwWmet0TOTe2PSiJuMntExoshbm1kh2wQzzzAA';
        $expected_placeholder_replacement = '<script type="application/vnd.drupal-ajax" data-big-pipe-replacement-for-placeholder-with-id="' . $big_pipe_placeholder_id . '">';
        $this->assertSession()
            ->pageTextContains('The count is 1.');
        $this->assertSession()
            ->responseNotContains('The count is 2.');
        $this->assertSession()
            ->responseNotContains('The count is 3.');
        $raw_content = $this->getSession()
            ->getPage()
            ->getContent();
        $this->assertSame(1, substr_count($raw_content, $expected_placeholder_replacement), 'Only one placeholder replacement was found for the duplicate #lazy_builder arrays.');
        // By calling performMetaRefresh() here, we simulate JavaScript being
        // disabled, because as far as the BigPipe module is concerned, it is
        // enabled in the browser when the BigPipe no-JS cookie is set.
        // @see setUp()
        // @see performMetaRefresh()
        $this->performMetaRefresh();
        $this->assertBigPipeNoJsCookieExists('1');
        $this->drupalGet(Url::fromRoute('big_pipe_test_multi_occurrence'));
        $this->assertSession()
            ->pageTextContains('The count is 1.');
        $this->assertSession()
            ->responseNotContains('The count is 2.');
        $this->assertSession()
            ->responseNotContains('The count is 3.');
    }
    
    /**
     * @internal
     */
    protected function assertBigPipeResponseHeadersPresent() : void {
        // Check that Cache-Control header set to "private".
        $this->assertSession()
            ->responseHeaderContains('Cache-Control', 'private');
        $this->assertSession()
            ->responseHeaderEquals('Surrogate-Control', 'no-store, content="BigPipe/1.0"');
        $this->assertSession()
            ->responseHeaderEquals('X-Accel-Buffering', 'no');
    }
    
    /**
     * Asserts expected BigPipe no-JS placeholders are present and replaced.
     *
     * @param array $expected_big_pipe_nojs_placeholders
     *   Keys: BigPipe no-JS placeholder markup. Values: expected replacement
     *   markup.
     *
     * @internal
     */
    protected function assertBigPipeNoJsPlaceholders(array $expected_big_pipe_nojs_placeholders) : void {
        $this->assertSetsEqual(array_keys($expected_big_pipe_nojs_placeholders), array_map('rawurldecode', explode(' ', $this->getSession()
            ->getResponseHeader('BigPipe-Test-No-Js-Placeholders'))));
        foreach ($expected_big_pipe_nojs_placeholders as $big_pipe_nojs_placeholder => $expected_replacement) {
            // Checking whether the replacement for the BigPipe no-JS placeholder
            // $big_pipe_nojs_placeholder is present.
            $this->assertSession()
                ->responseNotContains($big_pipe_nojs_placeholder);
            if ($expected_replacement !== NULL) {
                $this->assertSession()
                    ->responseContains($expected_replacement);
            }
        }
    }
    
    /**
     * Asserts expected BigPipe placeholders are present and replaced.
     *
     * @param array $expected_big_pipe_placeholders
     *   Keys: BigPipe placeholder IDs. Values: expected AJAX response.
     * @param array $expected_big_pipe_placeholder_stream_order
     *   Keys: BigPipe placeholder IDs. Values: expected AJAX response. Keys are
     *   defined in the order that they are expected to be rendered & streamed.
     *
     * @internal
     */
    protected function assertBigPipePlaceholders(array $expected_big_pipe_placeholders, array $expected_big_pipe_placeholder_stream_order) : void {
        $this->assertSetsEqual(array_keys($expected_big_pipe_placeholders), explode(' ', $this->getSession()
            ->getResponseHeader('BigPipe-Test-Placeholders')));
        $placeholder_positions = [];
        $placeholder_replacement_positions = [];
        foreach ($expected_big_pipe_placeholders as $big_pipe_placeholder_id => $expected_ajax_response) {
            // Verify expected placeholder.
            $expected_placeholder_html = '<span data-big-pipe-placeholder-id="' . $big_pipe_placeholder_id . '">';
            $this->assertSession()
                ->responseContains($expected_placeholder_html);
            $pos = strpos($this->getSession()
                ->getPage()
                ->getContent(), $expected_placeholder_html);
            $placeholder_positions[$pos] = $big_pipe_placeholder_id;
            // Verify expected placeholder replacement.
            $expected_placeholder_replacement = '<script type="application/vnd.drupal-ajax" data-big-pipe-replacement-for-placeholder-with-id="' . $big_pipe_placeholder_id . '">';
            $xpath = '//script[@data-big-pipe-replacement-for-placeholder-with-id="' . Html::decodeEntities($big_pipe_placeholder_id) . '"]';
            if ($expected_ajax_response === NULL) {
                $this->assertSession()
                    ->elementNotExists('xpath', $xpath);
                $this->assertSession()
                    ->responseNotContains($expected_placeholder_replacement);
                continue;
            }
            $this->assertSession()
                ->elementTextContains('xpath', $xpath, $expected_ajax_response);
            $this->assertSession()
                ->responseContains($expected_placeholder_replacement);
            $pos = strpos($this->getSession()
                ->getPage()
                ->getContent(), $expected_placeholder_replacement);
            $placeholder_replacement_positions[$pos] = $big_pipe_placeholder_id;
        }
        ksort($placeholder_positions, SORT_NUMERIC);
        $this->assertEquals(array_keys($expected_big_pipe_placeholders), array_values($placeholder_positions));
        $placeholders = array_map(function (NodeElement $element) {
            return $element->getAttribute('data-big-pipe-placeholder-id');
        }, $this->cssSelect('[data-big-pipe-placeholder-id]'));
        $this->assertSameSize($expected_big_pipe_placeholders, array_unique($placeholders));
        $expected_big_pipe_placeholders_with_replacements = [];
        foreach ($expected_big_pipe_placeholder_stream_order as $big_pipe_placeholder_id) {
            $expected_big_pipe_placeholders_with_replacements[$big_pipe_placeholder_id] = $expected_big_pipe_placeholders[$big_pipe_placeholder_id];
        }
        $this->assertEquals($expected_big_pipe_placeholders_with_replacements, array_filter($expected_big_pipe_placeholders));
        $this->assertSetsEqual(array_keys($expected_big_pipe_placeholders_with_replacements), array_values($placeholder_replacement_positions));
        $this->assertSame(count($expected_big_pipe_placeholders_with_replacements), preg_match_all('/' . preg_quote('<script type="application/vnd.drupal-ajax" data-big-pipe-replacement-for-placeholder-with-id="', '/') . '/', $this->getSession()
            ->getPage()
            ->getContent()));
        // Verifying BigPipe start/stop signals.
        $this->assertSession()
            ->responseContains(BigPipe::START_SIGNAL);
        $this->assertSession()
            ->responseContains(BigPipe::STOP_SIGNAL);
        $start_signal_position = strpos($this->getSession()
            ->getPage()
            ->getContent(), BigPipe::START_SIGNAL);
        $stop_signal_position = strpos($this->getSession()
            ->getPage()
            ->getContent(), BigPipe::STOP_SIGNAL);
        $this->assertTrue($start_signal_position < $stop_signal_position, 'BigPipe start signal appears before stop signal.');
        // Verifying BigPipe placeholder replacements and start/stop signals were
        // streamed in the correct order.
        $expected_stream_order = array_keys($expected_big_pipe_placeholders_with_replacements);
        array_unshift($expected_stream_order, BigPipe::START_SIGNAL);
        array_push($expected_stream_order, BigPipe::STOP_SIGNAL);
        $actual_stream_order = $placeholder_replacement_positions + [
            $start_signal_position => BigPipe::START_SIGNAL,
            $stop_signal_position => BigPipe::STOP_SIGNAL,
        ];
        ksort($actual_stream_order, SORT_NUMERIC);
        $this->assertEquals($expected_stream_order, array_values($actual_stream_order));
    }
    
    /**
     * Ensures CSRF tokens can be generated for the current user's session.
     */
    protected function setCsrfTokenSeedInTestEnvironment() {
        // Retrieve the CSRF token from the child site from its serialized session
        // record in the database.
        $session_data = $this->container
            ->get('session_handler.write_safe')
            ->read($this->getSession()
            ->getCookie($this->getSessionName()));
        $csrf_token_seed = unserialize(explode('_sf2_meta|', $session_data)[1])['s'];
        // Ensure that the session is started before accessing a session bag.
        // Otherwise the value stored in the bag is lost when subsequent session
        // access triggers a session start automatically.
        
        /** @var \Symfony\Component\HttpFoundation\RequestStack $request_stack */
        $request_stack = $this->container
            ->get('request_stack');
        $session = $request_stack->getSession();
        if (!$session->isStarted()) {
            $session->start();
        }
        // Store the CSRF token in the test runners session metadata bag.
        $this->container
            ->get('session_manager.metadata_bag')
            ->setCsrfTokenSeed($csrf_token_seed);
    }
    
    /**
     * @return \Drupal\big_pipe_test\BigPipePlaceholderTestCase[]
     */
    protected function getTestCases($has_session = TRUE) {
        return BigPipePlaceholderTestCases::cases($this->container, $this->rootUser);
    }
    
    /**
     * Asserts whether arrays A and B are equal, when treated as sets.
     *
     * @todo This method is broken. Fix it in
     *   https://www.drupal.org/project/drupal/issues/3144926
     *
     * @internal
     */
    protected function assertSetsEqual(array $a, array $b) : void {
        $result = count($a) == count($b) && !array_diff_assoc($a, $b);
    }
    
    /**
     * Asserts whether a BigPipe no-JS cookie exists or not.
     *
     * @internal
     */
    protected function assertBigPipeNoJsCookieExists(string $expected) : void {
        $this->assertCookieExists('big_pipe_nojs', $expected, 'BigPipe no-JS');
    }
    
    /**
     * Asserts whether a session cookie exists or not.
     *
     * @internal
     */
    protected function assertSessionCookieExists(string $expected) : void {
        $this->assertCookieExists($this->getSessionName(), $expected, 'Session');
    }
    
    /**
     * Asserts whether a cookie exists on the client or not.
     *
     * @internal
     */
    protected function assertCookieExists(string $cookie_name, string $expected, string $cookie_label) : void {
        $this->assertEquals($expected, !empty($this->getSession()
            ->getCookie($cookie_name)), $expected ? "{$cookie_label} cookie exists." : "{$cookie_label} cookie does not exist.");
    }
    
    /**
     * Calls ::performMetaRefresh() and asserts the responses.
     *
     * @internal
     */
    protected function assertBigPipeNoJsMetaRefreshRedirect() : void {
        $original_url = $this->getSession()
            ->getCurrentUrl();
        // Disable automatic following of redirects by the HTTP client, so that this
        // test can analyze the response headers of each redirect response.
        $this->getSession()
            ->getDriver()
            ->getClient()
            ->followRedirects(FALSE);
        $this->performMetaRefresh();
        $headers[0] = $this->getSession()
            ->getResponseHeaders();
        $statuses[0] = $this->getSession()
            ->getStatusCode();
        $this->performMetaRefresh();
        $headers[1] = $this->getSession()
            ->getResponseHeaders();
        $statuses[1] = $this->getSession()
            ->getStatusCode();
        $this->getSession()
            ->getDriver()
            ->getClient()
            ->followRedirects(TRUE);
        $this->assertEquals($original_url, $this->getSession()
            ->getCurrentUrl(), 'Redirected back to the original location.');
        // First response: redirect.
        $this->assertEquals(302, $statuses[0], 'The first response was a 302 (redirect).');
        $this->assertStringStartsWith('big_pipe_nojs=1', $headers[0]['Set-Cookie'][0], 'The first response sets the big_pipe_nojs cookie.');
        $this->assertEquals($original_url, $headers[0]['Location'][0], 'The first response redirected back to the original page.');
        $this->assertEmpty(array_diff([
            'cookies:big_pipe_nojs',
            'session.exists',
        ], explode(' ', $headers[0]['X-Drupal-Cache-Contexts'][0])), 'The first response varies by the "cookies:big_pipe_nojs" and "session.exists" cache contexts.');
        $this->assertFalse(isset($headers[0]['Surrogate-Control']), 'The first response has no "Surrogate-Control" header.');
        // Second response: redirect followed.
        $this->assertEquals(200, $statuses[1], 'The second response was a 200.');
        $this->assertEmpty(array_diff([
            'cookies:big_pipe_nojs',
            'session.exists',
        ], explode(' ', $headers[0]['X-Drupal-Cache-Contexts'][0])), 'The second response varies by the "cookies:big_pipe_nojs" and "session.exists" cache contexts.');
        $this->assertEquals('no-store, content="BigPipe/1.0"', $headers[1]['Surrogate-Control'][0], 'The second response has a "Surrogate-Control" header.');
        // Check that the <meta> refresh is absent, only one redirect ever happens.
        $this->assertSession()
            ->responseNotContains('<noscript><meta http-equiv="Refresh" content="0; URL=');
    }

}