Issue #1975442 by thedavidmeister, dcam: Fixed drupal_render() should always return a string.

CHANGELOG.txt

@@ -1,6 +1,8 @@
 
 Drupal 7.23, xxxx-xx-xx (development version)
 -----------------------
+- Fixed drupal_render() to always return an empty string when there is no
+  output, rather than sometimes returning NULL (minor API change).
 - Added protection to cache_clear_all() to ensure that non-cache tables cannot
   be truncated (API addition: a new isValidBin() method has been added to the
   default database cache implementation).

includes/common.inc

@@ -5795,23 +5795,23 @@ function drupal_render_page($page) {
  * array to be rendered independently and prevents them from being rendered
  * more than once on subsequent calls to drupal_render() (e.g., as part of a
  * larger array). If the same array or array element is passed more than once
- * to drupal_render(), it simply returns a NULL value.
+ * to drupal_render(), it simply returns an empty string.
  *
- * @param $elements
+ * @param array $elements
  *   The structured array describing the data to be rendered.
  *
- * @return
+ * @return string
  *   The rendered HTML.
  */
 function drupal_render(&$elements) {
   // Early-return nothing if user does not have access.
   if (empty($elements) || (isset($elements['#access']) && !$elements['#access'])) {
-    return;
+    return '';
   }
 
   // Do not print elements twice.
   if (!empty($elements['#printed'])) {
-    return;
+    return '';
   }
 
   // Try to fetch the element's markup from cache and return.
@@ -5847,7 +5847,7 @@ function drupal_render(&$elements) {
 
   // Allow #pre_render to abort rendering.
   if (!empty($elements['#printed'])) {
-    return;
+    return '';
   }
 
   // Get the children of the element, sorted by weight.

modules/simpletest/tests/common.test

@@ -1564,6 +1564,60 @@ class DrupalRenderTestCase extends DrupalWebTestCase {
     parent::setUp('common_test');
   }
 
+  /**
+   * Tests the output drupal_render() for some elementary input values.
+   */
+  function testDrupalRenderBasics() {
+    $types = array(
+      array(
+        'name' => 'null',
+        'value' => NULL,
+        'expected' => '',
+      ),
+      array(
+        'name' => 'no value',
+        'expected' => '',
+      ),
+      array(
+        'name' => 'empty string',
+        'value' => '',
+        'expected' => '',
+      ),
+      array(
+        'name' => 'no access',
+        'value' => array(
+          '#markup' => 'foo',
+          '#access' => FALSE,
+        ),
+        'expected' => '',
+      ),
+      array(
+        'name' => 'previously printed',
+        'value' => array(
+          '#markup' => 'foo',
+          '#printed' => TRUE,
+        ),
+        'expected' => '',
+      ),
+      array(
+        'name' => 'printed in prerender',
+        'value' => array(
+          '#markup' => 'foo',
+          '#pre_render' => array('common_test_drupal_render_printing_pre_render'),
+        ),
+        'expected' => '',
+      ),
+      array(
+        'name' => 'basic renderable array',
+        'value' => array('#markup' => 'foo'),
+        'expected' => 'foo',
+      ),
+    );
+    foreach($types as $type) {
+      $this->assertIdentical(drupal_render($type['value']), $type['expected'], '"' . $type['name'] . '" input rendered correctly by drupal_render().');
+    }
+  }
+
   /**
    * Test sorting by weight.
    */

modules/simpletest/tests/common_test.module

@@ -100,6 +100,14 @@ function common_test_destination() {
   print "The destination: " . check_plain($destination['destination']);
 }
 
+/**
+ * Applies #printed to an element to help test #pre_render.
+ */
+function common_test_drupal_render_printing_pre_render($elements) {
+  $elements['#printed'] = TRUE;
+  return $elements;
+}
+
 /**
  * Implements hook_TYPE_alter().
  */