Issue #3143096 by jedihe, jyotimishra123, shetpooja04, alexpott, mradcliffe: [DX]: throw an exception if #lazy_builder callback does not return a (renderable) array

core/lib/Drupal/Core/Render/Renderer.php

@@ -353,6 +353,23 @@ class Renderer implements RendererInterface {
     // Build the element if it is still empty.
     if (isset($elements['#lazy_builder'])) {
       $new_elements = $this->doCallback('#lazy_builder', $elements['#lazy_builder'][0], $elements['#lazy_builder'][1]);
+      // Throw an exception if #lazy_builder callback does not return an array;
+      // provide helpful details for troubleshooting.
+      if (!is_array($new_elements)) {
+        $callable = $elements['#lazy_builder'][0];
+        $callable_name = '[unknown]';
+        if ($callable instanceof \Closure) {
+          $callable_name = '[closure]';
+        }
+        elseif (is_array($callable)) {
+          $callable_name = implode('::', $callable);
+        }
+        elseif (is_string($callable)) {
+          $callable_name = $callable;
+        }
+        $wrong_type = gettype($new_elements);
+        throw new \LogicException("#lazy_builder callbacks must return a valid renderable array, got $wrong_type from $callable_name");
+      }
       // Retain the original cacheability metadata, plus cache keys.
       CacheableMetadata::createFromRenderArray($elements)
         ->merge(CacheableMetadata::createFromRenderArray($new_elements))

core/tests/Drupal/Tests/Core/Render/RendererPlaceholdersTest.php

@@ -962,6 +962,54 @@ class RendererPlaceholdersTest extends RendererTestBase {
     $this->renderer->renderRoot($element);
   }
 
+  /**
+   * Tests lazy builders (string callable) that do not return a renderable.
+   *
+   * @covers ::render
+   * @covers ::doRender
+   */
+  public function testNonArrayReturnFromLazyBuilderStringCallable() {
+    $element = [];
+    $element['#lazy_builder'] = ['Drupal\Tests\Core\Render\PlaceholdersTest::callbackNonArrayReturn', []];
+
+    $this->expectException(\LogicException::class);
+    $this->expectExceptionMessage('#lazy_builder callbacks must return a valid renderable array, got boolean from Drupal\Tests\Core\Render\PlaceholdersTest::callbackNonArrayReturn');
+    $this->renderer->renderRoot($element);
+  }
+
+  /**
+   * Tests lazy builders (array callable) that do not return a renderable.
+   *
+   * @covers ::render
+   * @covers ::doRender
+   */
+  public function testNonArrayReturnFromLazyBuilderArrayCallable() {
+    $element = [];
+    $element['#lazy_builder'] = [['Drupal\Tests\Core\Render\PlaceholdersTest', 'callbackNonArrayReturn'], []];
+
+    $this->expectException(\LogicException::class);
+    $this->expectExceptionMessage('#lazy_builder callbacks must return a valid renderable array, got boolean from Drupal\Tests\Core\Render\PlaceholdersTest::callbackNonArrayReturn');
+    $this->renderer->renderRoot($element);
+  }
+
+  /**
+   * Tests lazy builders (closure) that do not return a renderable.
+   *
+   * @covers ::render
+   * @covers ::doRender
+   */
+  public function testNonArrayReturnFromLazyBuilderClosure() {
+    $element = [];
+    $closure = function () {
+      return NULL;
+    };
+    $element['#lazy_builder'] = [$closure, []];
+
+    $this->expectException(\LogicException::class);
+    $this->expectExceptionMessage('#lazy_builder callbacks must return a valid renderable array, got NULL from [closure]');
+    $this->renderer->renderRoot($element);
+  }
+
   /**
    * Create an element with a child and subchild. Each element has the same
    * #lazy_builder callback, but with different contexts. They don't modify

core/tests/Drupal/Tests/Core/Render/RendererTestBase.php

@@ -317,11 +317,21 @@ class PlaceholdersTest implements TrustedCallbackInterface {
     return $build;
   }
 
+  /**
+   * Returns invalid renderable to #lazy_builder callback.
+   *
+   * @return bool
+   *   TRUE.
+   */
+  public static function callbackNonArrayReturn() {
+    return TRUE;
+  }
+
   /**
    * {@inheritdoc}
    */
   public static function trustedCallbacks() {
-    return ['callbackTagCurrentTemperature', 'callbackPerUser', 'callback'];
+    return ['callbackTagCurrentTemperature', 'callbackPerUser', 'callback', 'callbackNonArrayReturn'];
   }
 
 }