Allow backwards compatible null property behavior.

Per the Mustache spec, keys of a value higher in the context stack shadow similarly named keys lower in the stack. For example, in the template `{{# foo }}{{ bar }}{{/ foo }}` if the value for `foo` has a method, property, or key named `bar`, it will prevent looking lower in the context stack for a another value named `bar`.

In Mustache.php, setting the value of an array key to null prevents lookups higher in the context stack. The behavior should have been identical for object properties (and ArrayAccess) as well, but a bug in the context lookup logic meant that a property which exists but is set to null would not prevent further context lookup.

This change adds a `buggy_property_shadowing` configuration option to preserve the behavior from Mustache.php <= 2.14.2, to ease the transition to the correct behavior.

See #410

src/Mustache/Context.php

@@ -17,16 +17,21 @@ class Mustache_Context
     private $stack      = array();
     private $blockStack = array();
 
+    private $buggyPropertyShadowing = false;
+
     /**
      * Mustache rendering Context constructor.
      *
-     * @param mixed $context Default rendering context (default: null)
+     * @param mixed $context                Default rendering context (default: null)
+     * @param bool  $buggyPropertyShadowing See Mustache_Engine::useBuggyPropertyShadowing (default: false)
      */
-    public function __construct($context = null)
+    public function __construct($context = null, $buggyPropertyShadowing = false)
     {
         if ($context !== null) {
             $this->stack = array($context);
         }
+
+        $this->buggyPropertyShadowing = $buggyPropertyShadowing;
     }
 
     /**
@@ -223,15 +228,24 @@ private function findVariableInStack($id, array $stack)
                             return $frame->$id;
                         }
 
-                        if (property_exists($frame, $id)) {
-                            $rp = new \ReflectionProperty($frame, $id);
-                            if ($rp->isPublic()) {
-                                return $frame->$id;
+                        // Preserve backwards compatibility with a property shadowing bug in
+                        // Mustache.php <= 2.14.2
+                        // See https://github.com/bobthecow/mustache.php/pull/410
+                        if ($this->buggyPropertyShadowing) {
+                            if ($frame instanceof ArrayAccess && isset($frame[$id])) {
+                                return $frame[$id];
+                            }
+                        } else {
+                            if (property_exists($frame, $id)) {
+                                $rp = new \ReflectionProperty($frame, $id);
+                                if ($rp->isPublic()) {
+                                    return $frame->$id;
+                                }
                             }
-                        }
 
-                        if ($frame instanceof ArrayAccess && isset($frame[$id])) {
-                            return $frame[$id];
+                            if ($frame instanceof ArrayAccess && $frame->offsetExists($id)) {
+                                return $frame[$id];
+                            }
                         }
                     }
                     break;

src/Mustache/Engine.php

@@ -57,6 +57,7 @@ class Mustache_Engine
     private $strictCallables = false;
     private $pragmas = array();
     private $delimiters;
+    private $buggyPropertyShadowing = false;
 
     // Services
     private $tokenizer;
@@ -218,6 +219,10 @@ public function __construct(array $options = array())
                 $this->pragmas[$pragma] = true;
             }
         }
+
+        if (isset($options['buggy_property_shadowing'])) {
+            $this->buggyPropertyShadowing = (bool) $options['buggy_property_shadowing'];
+        }
     }
 
     /**
@@ -268,6 +273,16 @@ public function getCharset()
         return $this->charset;
     }
 
+    /**
+     * Check whether to use buggy property shadowing.
+     *
+     * See https://github.com/bobthecow/mustache.php/pull/410
+     */
+    public function useBuggyPropertyShadowing()
+    {
+        return $this->buggyPropertyShadowing;
+    }
+
     /**
      * Get the current globally enabled pragmas.
      *

src/Mustache/Template.php

@@ -143,7 +143,7 @@ protected function isIterable($value)
      */
     protected function prepareContextStack($context = null)
     {
-        $stack = new Mustache_Context();
+        $stack = new Mustache_Context(null, $this->mustache->useBuggyPropertyShadowing());
 
         $helpers = $this->mustache->getHelpers();
         if (!$helpers->isEmpty()) {

test/Mustache/Test/EngineTest.php

@@ -376,6 +376,15 @@ public function testCustomDelimiters()
         $tpl = $mustache->loadTemplate('[[> one ]]');
         $this->assertEquals('b', $tpl->render(array('a' => 'b')));
     }
+
+    public function testBuggyPropertyShadowing()
+    {
+        $mustache = new Mustache_Engine();
+        $this->assertFalse($mustache->useBuggyPropertyShadowing());
+
+        $mustache = new Mustache_Engine(array('buggy_property_shadowing' => true));
+        $this->assertTrue($mustache->useBuggyPropertyShadowing());
+    }
 }
 
 class MustacheStub extends Mustache_Engine