Issue #2197359 by xjm, alexpott: Random failure in Drupal\system\Tests\Common\RenderTest.

core/modules/system/lib/Drupal/system/Tests/Common/RenderTest.php

@@ -453,7 +453,7 @@ function testDrupalRenderCache() {
    * Tests post-render cache callbacks functionality.
    */
   function testDrupalRenderPostRenderCache() {
-    $context = array('foo' => $this->randomString());
+    $context = array('foo' => $this->randomContextValue());
     $test_element = array();
     $test_element['#markup'] = '';
     $test_element['#attached']['js'][] = array('type' => 'setting', 'data' => array('foo' => 'bar'));
@@ -553,9 +553,9 @@ function testDrupalRenderChildrenPostRenderCache() {
     // Create an element with a child and subchild. Each element has the same
     // #post_render_cache callback, but with different contexts.
     drupal_static_reset('_drupal_add_js');
-    $context_1 = array('foo' => $this->randomString());
-    $context_2 = array('bar' => $this->randomString());
-    $context_3 = array('baz' => $this->randomString());
+    $context_1 = array('foo' => $this->randomContextValue());
+    $context_2 = array('bar' => $this->randomContextValue());
+    $context_3 = array('baz' => $this->randomContextValue());
     $test_element = array(
       '#type' => 'details',
       '#cache' => array(
@@ -770,7 +770,7 @@ function testDrupalRenderChildrenPostRenderCache() {
    * Tests post-render cache-integrated 'render_cache_placeholder' element.
    */
   function testDrupalRenderRenderCachePlaceholder() {
-    $context = array('bar' => $this->randomString());
+    $context = array('bar' => $this->randomContextValue());
     $test_element = array(
       '#type' => 'render_cache_placeholder',
       '#context' => $context,
@@ -846,4 +846,25 @@ protected function parseDrupalSettings($html) {
     return $parsed_settings;
   }
 
+  /**
+   * Generates a random context value for the post-render cache tests.
+   *
+   * The #context array used by the post-render cache callback will generally
+   * be used to provide metadata like entity IDs, field machine names, paths,
+   * etc. for JavaScript replacement of content or assets. In this test, the
+   * callbacks common_test_post_render_cache() and
+   * common_test_post_render_cache_placeholder() render the context inside test
+   * HTML, so using any random string would sometimes cause random test
+   * failures because the test output would be unparseable. Instead, we provide
+   * random tokens for replacement.
+   *
+   * @see common_test_post_render_cache()
+   * @see common_test_post_render_cache_placeholder()
+   * @see https://drupal.org/node/2151609
+   */
+  protected function randomContextValue() {
+    $tokens = array('llama', 'alpaca', 'camel', 'moose', 'elk');
+    return $tokens[mt_rand(0, 4)];
+  }
+
 }