feature #2530 Throw exception on circular reference detection (yceruto)

This PR was merged into the 1.x branch.

Discussion
----------

Throw exception on circular reference detection

Out-the-box this isn't common... but into Symfony framework this kind of issue can happen frequently when we're overriding a template from third-party bundle (see main issue https://github.com/symfony/symfony/issues/17557)

```twig
{# app/Resources/AcmeBlogBundle/views/Blog/index.html.twig #}
{% extends '@AcmeBlog/Blog/index.html.twig' %}

{% block title 'New Title' %}
```

This exception avoids inifite loop or maximum execution time for this case, by detecting a recursive loading when a template extends from itself, thinking to override the parent template which has the same namespace + template name.

Commits
-------

074e1fbe Throw exception on circular reference detection

lib/Twig/Environment.php

@@ -58,6 +58,7 @@ class Twig_Environment
     private $runtimeLoaders = array();
     private $runtimes = array();
     private $optionsHash;
+    private $loading = array();
 
     /**
      * Constructor.
@@ -472,7 +473,22 @@ class Twig_Environment
             $this->initRuntime();
         }
 
-        return $this->loadedTemplates[$cls] = new $cls($this);
+        if (isset($this->loading[$cls])) {
+            throw new Twig_Error_Runtime(sprintf('Circular reference detected for Twig template "%s", path: %s.', $name, implode(' -> ', array_merge($this->loading, array($name)))));
+        }
+
+        $this->loading[$cls] = $name;
+
+        try {
+            $this->loadedTemplates[$cls] = new $cls($this);
+            unset($this->loading[$cls]);
+        } catch (\Exception $e) {
+            unset($this->loading[$cls]);
+
+            throw $e;
+        }
+
+        return $this->loadedTemplates[$cls];
     }
 
     /**

test/Twig/Tests/EnvironmentTest.php

@@ -480,6 +480,33 @@ EOF
         $this->assertEquals('foo', $twig->render('func_string_named_args'));
     }
 
+    /**
+     * @expectedException Twig_Error_Runtime
+     * @expectedExceptionMessage Circular reference detected for Twig template "base.html.twig", path: base.html.twig -> base.html.twig in "base.html.twig" at line 1
+     */
+    public function testFailLoadTemplateOnCircularReference()
+    {
+        $twig = new Twig_Environment(new Twig_Loader_Array(array(
+            'base.html.twig' => '{% extends "base.html.twig" %}',
+        )));
+
+        $twig->loadTemplate('base.html.twig');
+    }
+
+    /**
+     * @expectedException Twig_Error_Runtime
+     * @expectedExceptionMessage Circular reference detected for Twig template "base1.html.twig", path: base1.html.twig -> base2.html.twig -> base1.html.twig in "base1.html.twig" at line 1
+     */
+    public function testFailLoadTemplateOnComplexCircularReference()
+    {
+        $twig = new Twig_Environment(new Twig_Loader_Array(array(
+            'base1.html.twig' => '{% extends "base2.html.twig" %}',
+            'base2.html.twig' => '{% extends "base1.html.twig" %}',
+        )));
+
+        $twig->loadTemplate('base1.html.twig');
+    }
+
     protected function getMockLoader($templateName, $templateContent)
     {
         // to be removed in 2.0