Make sure serialzation context is immutable

src/Context.php

@@ -130,7 +130,7 @@ public function setAttribute(string $key, $value): self
         return $this;
     }
 
-    private function assertMutable(): void
+    final protected function assertMutable(): void
     {
         if (!$this->initialized) {
             return;

src/SerializationContext.php

@@ -43,6 +43,8 @@ public function initialize(string $format, VisitorInterface $visitor, GraphNavig
      */
     public function setSerializeNull(bool $bool): self
     {
+        $this->assertMutable();
+
         $this->serializeNull = $bool;
 
         return $this;
@@ -142,6 +144,8 @@ public function getVisitingSet(): \SplObjectStorage
      */
     public function setInitialType(string $type): self
     {
+        $this->assertMutable();
+
         $this->initialType = $type;
         $this->setAttribute('initial_type', $type);
         return $this;

tests/Serializer/ContextTest.php

@@ -4,6 +4,8 @@
 
 namespace JMS\Serializer\Tests\Serializer;
 
+use JMS\Serializer\Exception\LogicException;
+use JMS\Serializer\GraphNavigator;
 use JMS\Serializer\Metadata\ClassMetadata;
 use JMS\Serializer\Metadata\PropertyMetadata;
 use JMS\Serializer\SerializationContext;
@@ -14,6 +16,8 @@
 use JMS\Serializer\Tests\Fixtures\Node;
 use JMS\Serializer\Tests\Fixtures\Publisher;
 use JMS\Serializer\Tests\Fixtures\VersionedObject;
+use JMS\Serializer\VisitorInterface;
+use Metadata\MetadataFactoryInterface;
 use PHPUnit\Framework\TestCase;
 
 class ContextTest extends TestCase
@@ -214,4 +218,20 @@ public function testSerializeNullOption()
         $context->setSerializeNull(true);
         self::assertTrue($context->shouldSerializeNull());
     }
+
+    public function testContextBecomesImmutableWhenStartingProcess()
+    {
+        $this->expectException(LogicException::class);
+        $this->expectExceptionMessage('This context was already initialized and is immutable; you cannot modify it anymore.');
+
+        $visitor = $this->createMock(VisitorInterface::class);
+        $navigator = $this->createMock(GraphNavigator::class);
+        $metadataFactory = $this->createMock(MetadataFactoryInterface::class);
+
+        $context = SerializationContext::create();
+
+        $context->initialize('json', $visitor, $navigator, $metadataFactory);
+
+        $context->setSerializeNull(false);
+    }
 }