Fabric: Using `shallowSourceNode()` inside `ViewShadowNode::cloneAndReplaceChild()`

Summary:
We have to call shallowSourceNode() in all cases of cloning which were not caused by UIManager instructions,
otherwise the diffing alogorith might produce incorrect mutation instructions.

Reviewed By: mdvacca

Differential Revision: D7503383

fbshipit-source-id: b33e5c39b7ba8cbd0f925fd29b3af379441a40a4

ReactCommon/fabric/core/shadownode/ShadowNode.cpp

@@ -109,7 +109,11 @@ void ShadowNode::clearSourceNode() {
 }
 
 void ShadowNode::shallowSourceNode() {
-  sourceNode_ = sourceNode_.lock()->getSourceNode();
+  ensureUnsealed();
+
+  auto sourceNode = sourceNode_.lock();
+  assert(sourceNode);
+  sourceNode_ = sourceNode->getSourceNode();
 }
 
 #pragma mark - Equality

ReactCommon/fabric/core/shadownode/ShadowNode.h

@@ -58,7 +58,15 @@ public:
   Tag getTag() const;
   Tag getRootTag() const;
   InstanceHandle getInstanceHandle() const;
+
+  /*
+   * Returns the node which was used as a prototype in clone constructor.
+   * The node is held as a weak reference so that the method may return
+   * `nullptr` in cases where the node was constructed using the explicit
+   * constructor or the node was already deallocated.
+   */
   SharedShadowNode getSourceNode() const;
+
   void sealRecursive() const;
 
 #pragma mark - Mutating Methods

ReactCommon/fabric/core/tests/ShadowNodeTest.cpp

@@ -94,3 +94,27 @@ TEST(ShadowNodeTest, handleShadowNodeMutation) {
   node5->clearSourceNode();
   ASSERT_EQ(node5->getSourceNode(), nullptr);
 }
+
+TEST(ShadowNodeTest, handleSourceNode) {
+  auto nodeFirstGeneration = std::make_shared<TestShadowNode>(9, 1, (void *)NULL);
+  auto nodeSecondGeneration = std::make_shared<TestShadowNode>(nodeFirstGeneration);
+  auto nodeThirdGeneration = std::make_shared<TestShadowNode>(nodeSecondGeneration);
+  auto nodeForthGeneration = std::make_shared<TestShadowNode>(nodeThirdGeneration);
+
+  // Ensure established shource nodes structure.
+  ASSERT_EQ(nodeForthGeneration->getSourceNode(), nodeThirdGeneration);
+  ASSERT_EQ(nodeThirdGeneration->getSourceNode(), nodeSecondGeneration);
+  ASSERT_EQ(nodeSecondGeneration->getSourceNode(), nodeFirstGeneration);
+
+  // Shallow source node for the forth generation node.
+  nodeForthGeneration->shallowSourceNode();
+  ASSERT_EQ(nodeForthGeneration->getSourceNode(), nodeSecondGeneration);
+
+  // Shallow it one more time.
+  nodeForthGeneration->shallowSourceNode();
+  ASSERT_EQ(nodeForthGeneration->getSourceNode(), nodeFirstGeneration);
+
+  // Ensure that 3th and 2nd were not affected.
+  ASSERT_EQ(nodeThirdGeneration->getSourceNode(), nodeSecondGeneration);
+  ASSERT_EQ(nodeSecondGeneration->getSourceNode(), nodeFirstGeneration);
+}

ReactCommon/fabric/view/ViewShadowNode.cpp

@@ -94,7 +94,21 @@ SharedLayoutableShadowNode ViewShadowNode::cloneAndReplaceChild(const SharedLayo
 
   auto viewShadowNodeChild = std::dynamic_pointer_cast<const ViewShadowNode>(child);
   assert(viewShadowNodeChild);
-  auto viewShadowNodeChildClone = std::make_shared<const ViewShadowNode>(viewShadowNodeChild);
+  auto viewShadowNodeChildClone = std::make_shared<ViewShadowNode>(viewShadowNodeChild);
+
+  // This is overloading of `SharedLayoutableShadowNode::cloneAndReplaceChild`,
+  // the method is used to clone some node as a preparation for future mutation
+  // caused by relayout.
+  // Because those changes are not requested by UIManager, they add a layer
+  // of node generation (between the committed stage and new proposed stage).
+  // That additional layer confuses the Diffing algorithm which uses
+  // `sourceNode` for referencing the previous (aka committed) stage
+  // of the tree to produce mutation instructions.
+  // In other words, if we don't compensate this change here,
+  // the Diffing algorithm will compare wrong trees
+  // ("new-but-not-laid-out-yet vs. new" instead of "committed vs. new").
+  viewShadowNodeChildClone->shallowSourceNode();
+
   ShadowNode::replaceChild(viewShadowNodeChild, viewShadowNodeChildClone);
   return std::static_pointer_cast<const LayoutableShadowNode>(viewShadowNodeChildClone);
 }