Fix xml deserialization when xsi:nil="true" is set

Add xml deserialization null check based on xsi:nil
Expand visitor interface with a null data detect function
Add default implementation for null detection in abstract visitor
Add round-trip test for serializing ad de-serializing null object

src/AbstractVisitor.php

@@ -63,4 +63,13 @@ protected function getElementType($typeArray)
         }
     }
 
+    /**
+     * @param mixed $data
+     *
+     * @return bool
+     */
+    public function isNullData($data)
+    {
+        return $data === null;
+    }
 }

src/GraphNavigator.php

@@ -119,9 +119,10 @@ public function accept($data, array $type = null, Context $context)
 
             $type = array('name' => $typeName, 'params' => array());
         }
+
         // If the data is null, we have to force the type to null regardless of the input in order to
         // guarantee correct handling of null values, and not have any internal auto-casting behavior.
-        else if ($context instanceof SerializationContext && null === $data) {
+        if ($visitor instanceof VisitorInterface && true === $visitor->isNullData($data)) {
             $type = array('name' => 'NULL', 'params' => array());
         }
 

src/VisitorInterface.php

@@ -138,4 +138,14 @@ public function getNavigator();
      * @return object|array|scalar
      */
     public function getResult();
+
+    /**
+     * Determine if the value evaluates to null.
+     * Used by the navigator to determine the correct data type.
+     *
+     * @param mixed $data
+     *
+     * @return bool
+     */
+    public function isNullData($data);
 }

src/XmlDeserializationVisitor.php

@@ -387,4 +387,26 @@ private function getDomDocumentTypeEntitySubset($data)
 
         return $internalSubset;
     }
+
+    /**
+     * Consider xml element value null if the xsi:nil attribute is set and therefore can't have a value
+     * @see https://www.w3.org/TR/xmlschema-1/#xsi_nil
+     *
+     * @param $data
+     *
+     * @return bool
+     */
+    public function isNullData($data)
+    {
+        if ($data instanceof \SimpleXMLElement) {
+            $xsiAttributes = $data->attributes('http://www.w3.org/2001/XMLSchema-instance');
+
+            //We have to keep the isset quiet, some tests give error: `Node no longer exists`; even though it evaluates to false
+            if (@isset($xsiAttributes['nil']) && (string) $xsiAttributes['nil'] === 'true') {
+                return true;
+            }
+        }
+
+        return $data === null;
+    }
 }

tests/Fixtures/ObjectWithNullProperty.php

@@ -18,7 +18,21 @@
 
 namespace JMS\Serializer\Tests\Fixtures;
 
+use JMS\Serializer\Annotation\Type;
+
 class ObjectWithNullProperty extends SimpleObject
 {
+    /**
+     * @var null
+     * @Type("string")
+     */
     private $nullProperty = null;
+
+    /**
+     * @return null
+     */
+    public function getNullProperty()
+    {
+        return $this->nullProperty;
+    }
 }

tests/Serializer/BaseSerializationTest.php

@@ -166,6 +166,25 @@ public function testSerializeNullObject()
         );
     }
 
+    public function testDeserializeNullObject()
+    {
+        if (!$this->hasDeserializer()) {
+            $this->markTestSkipped(sprintf('No deserializer available for format `%s`', $this->getFormat()));
+        }
+
+        $obj = new ObjectWithNullProperty('foo', 'bar');
+
+        /** @var ObjectWithNullProperty $dObj */
+        $dObj = $this->serializer->deserialize(
+            $this->getContent('simple_object_nullable'),
+            ObjectWithNullProperty::class,
+            $this->getFormat()
+        );
+
+        $this->assertEquals($obj, $dObj);
+        $this->assertNull($dObj->getNullProperty());
+    }
+
     /**
      * @dataProvider getTypes
      */
@@ -1345,6 +1364,19 @@ public function testObjectWithNullableArrays()
         $this->assertEquals($this->getContent('nullable_arrays'), $this->serializer->serialize($object, $this->getFormat()));
     }
 
+    public function testIsNullDataWithNull()
+    {
+        /** @var VisitorInterface $visitor */
+        foreach ($this->serializationVisitors as $visitor) {
+            $this->assertTrue($visitor->isNullData(null));
+        }
+
+        /** @var VisitorInterface $visitor */
+        foreach ($this->deserializationVisitors as $visitor) {
+            $this->assertTrue($visitor->isNullData(null));
+        }
+    }
+
     abstract protected function getContent($key);
 
     abstract protected function getFormat();

tests/Serializer/XmlSerializationTest.php

@@ -26,6 +26,7 @@
 use JMS\Serializer\Handler\HandlerRegistry;
 use JMS\Serializer\Metadata\StaticPropertyMetadata;
 use JMS\Serializer\Naming\CamelCaseNamingStrategy;
+use JMS\Serializer\Naming\PropertyNamingStrategyInterface;
 use JMS\Serializer\Naming\SerializedNameAnnotationStrategy;
 use JMS\Serializer\SerializationContext;
 use JMS\Serializer\Serializer;
@@ -52,6 +53,7 @@
 use JMS\Serializer\Tests\Fixtures\SimpleClassObject;
 use JMS\Serializer\Tests\Fixtures\SimpleObject;
 use JMS\Serializer\Tests\Fixtures\SimpleSubClassObject;
+use JMS\Serializer\XmlDeserializationVisitor;
 use JMS\Serializer\XmlSerializationVisitor;
 use PhpCollection\Map;
 
@@ -488,6 +490,15 @@ public function testDeserializeEmptyString()
         $this->deserialize('', 'stdClass');
     }
 
+    public function testIsNullDataWithXSINilLabeledElement()
+    {
+        $namingStrategy = $this->getMockBuilder(PropertyNamingStrategyInterface::class)->getMock();
+        $visitor = new XmlDeserializationVisitor($namingStrategy);
+        $element = simplexml_load_string('<empty xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:nil="true"/>');
+
+        $this->assertTrue($visitor->isNullData($element));
+    }
+
     private function xpathFirstToString(\SimpleXMLElement $xml, $xpath)
     {
         $nodes = $xml->xpath($xpath);