tests(assertions): Implement leaked metadata detection for QueryResultAssertionTrait (drupal-graphql#1207)

This adds detection of leaked cacheability metadata to assertResults and assertErrors
in the QueryResultAssertionTrait.

The context is serialized in the error message because this is always possible and while
serialized content isn't the most readable it provides the most robust method of easily 
showing what metadata was leaked.

Fixes drupal-graphql#1206

Author: Kingdutch

tests/src/Traits/QueryResultAssertionTrait.php

@@ -3,6 +3,8 @@
 namespace Drupal\Tests\graphql\Traits;
 
 use Drupal\Core\Cache\CacheableMetadata;
+use Drupal\Core\Render\RenderContext;
+use Drupal\Core\Render\RendererInterface;
 use Drupal\graphql\GraphQL\Execution\ExecutionResult;
 use GraphQL\Server\OperationParams;
 
@@ -16,6 +18,11 @@ trait QueryResultAssertionTrait {
    */
   protected $server;
 
+  /**
+   * @var \Drupal\Core\Render\RendererInterface
+   */
+  protected $renderer;
+
   /**
    * Return the default cache max age for this test case.
    *
@@ -81,16 +88,23 @@ protected function defaultMutationCacheMetaData() {
    *   The expected cache metadata object.
    */
   protected function assertResults($query, array $variables, array $expected, CacheableMetadata $metadata = NULL): void {
-    $result = $this->server->executeOperation(
-      OperationParams::create([
-        'query' => $query,
-        'variables' => $variables,
-      ])
+    $context = new RenderContext();
+    $result = $this->getRenderer()->executeInRenderContext(
+      $context,
+      function () use ($query, $variables) {
+        return $this->server->executeOperation(
+          OperationParams::create([
+            'query' => $query,
+            'variables' => $variables,
+          ])
+        );
+      }
     );
 
     $this->assertResultErrors($result, []);
     $this->assertResultData($result, $expected);
     $this->assertResultMetadata($result, $metadata ?: $this->defaultCacheMetaData());
+    self::assertTrue($context->isEmpty(), "Metadata was leaked during operation execution: {$context->serialize()}");
   }
 
   /**
@@ -106,15 +120,22 @@ protected function assertResults($query, array $variables, array $expected, Cach
    *   The expected cache metadata object.
    */
   protected function assertErrors($query, array $variables, $expected, CacheableMetadata $metadata): void {
-    $result = $this->server->executeOperation(
-      OperationParams::create([
-        'query' => $query,
-        'variables' => $variables,
-      ])
+    $context = new RenderContext();
+    $result = $this->getRenderer()->executeInRenderContext(
+      $context,
+      function () use ($query, $variables) {
+        return $this->server->executeOperation(
+          OperationParams::create([
+            'query' => $query,
+            'variables' => $variables,
+          ])
+        );
+      }
     );
 
     $this->assertResultErrors($result, $expected);
     $this->assertResultMetadata($result, $metadata);
+    self::assertTrue($context->isEmpty(), "Metadata was leaked during operation execution: {$context->serialize()}");
   }
 
   /**
@@ -210,4 +231,20 @@ private function assertResultMetadata(ExecutionResult $result, CacheableMetadata
     $this->assertEmpty($unexpectedTags, 'Unexpected cache tags: ' . implode(', ', $unexpectedTags));
   }
 
+  /**
+   * Get the Drupal renderer.
+   *
+   * Uses either the renderer available in the test class or fetches the Drupal
+   * renderer service.
+   *
+   * @return \Drupal\Core\Render\RendererInterface
+   *   The renderer service for the test.
+   */
+  private function getRenderer() : RendererInterface {
+    if (!isset($this->renderer)) {
+      $this->renderer = \Drupal::service('renderer');
+    }
+    return $this->renderer;
+  }
+
 }