cppgc: Add composite object tracing to Visitor

This allows embedding objects in each other and recursively trace
through them.

Bug: chromium:1056170
Change-Id: I4e4ae4c1669109c01003cb6b69797cf271a74033
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2198977
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67841}

BUILD.gn

@@ -4107,6 +4107,7 @@ v8_source_set("cppgc_base") {
     "src/heap/cppgc/stack.h",
     "src/heap/cppgc/sweeper.cc",
     "src/heap/cppgc/sweeper.h",
+    "src/heap/cppgc/visitor.cc",
     "src/heap/cppgc/worklist.h",
   ]
 

include/cppgc/visitor.h

@@ -35,7 +35,7 @@ class Visitor {
 
   template <typename T>
   void Trace(const WeakMember<T>& weak_member) {
-    static_assert(sizeof(T), "T must be fully defined");
+    static_assert(sizeof(T), "Pointee type must be fully defined.");
     static_assert(internal::IsGarbageCollectedType<T>::value,
                   "T must be GarabgeCollected or GarbageCollectedMixin type");
 
@@ -81,6 +81,18 @@ class Visitor {
                   &HandleWeak<WeakPersistent>, &p);
   }
 
+  template <typename T>
+  void Trace(const T& object) {
+#if V8_ENABLE_CHECKS
+    // This object is embedded in potentially multiple nested objects. The
+    // outermost object must not be in construction as such objects are (a) not
+    // processed immediately, and (b) only processed conservatively if not
+    // otherwise possible.
+    CheckObjectNotInConstruction(&object);
+#endif  // V8_ENABLE_CHECKS
+    TraceTrait<T>::Trace(this, &object);
+  }
+
   template <typename T, void (T::*method)(const LivenessBroker&)>
   void RegisterWeakCallbackMethod(const T* obj) {
     RegisterWeakCallback(&WeakCallbackMethodDelegate<T, method>, obj);
@@ -121,7 +133,7 @@ class Visitor {
 
   template <typename T>
   void Trace(const T* t) {
-    static_assert(sizeof(T), "T must be fully defined");
+    static_assert(sizeof(T), "Pointee type must be fully defined.");
     static_assert(internal::IsGarbageCollectedType<T>::value,
                   "T must be GarabgeCollected or GarbageCollectedMixin type");
     if (!t) {
@@ -130,6 +142,10 @@ class Visitor {
     Visit(t, TraceTrait<T>::GetTraceDescriptor(t));
   }
 
+#if V8_ENABLE_CHECKS
+  V8_EXPORT void CheckObjectNotInConstruction(const void* address);
+#endif  // V8_ENABLE_CHECKS
+
   friend class internal::VisitorBase;
 };
 

src/heap/cppgc/visitor.cc

@@ -0,0 +1,16 @@
+// Copyright 2020 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "src/heap/cppgc/visitor.h"
+
+namespace cppgc {
+
+#ifdef V8_ENABLE_CHECKS
+void Visitor::CheckObjectNotInConstruction(const void* address) {
+  // TODO(chromium:1056170): |address| is an inner pointer of an object. Check
+  // that the object is not in construction.
+}
+#endif  // V8_ENABLE_CHECKS
+
+}  // namespace cppgc

test/unittests/heap/cppgc/visitor-unittest.cc

@@ -228,5 +228,34 @@ TEST_F(VisitorTest, DispatchRegisterWeakCallbackMethod) {
   EXPECT_EQ(1u, WeakCallbackDispatcher::callback_callcount);
 }
 
+namespace {
+
+class Composite final {
+ public:
+  static size_t callback_callcount;
+  Composite() { callback_callcount = 0; }
+  void Trace(Visitor* visitor) const { callback_callcount++; }
+};
+
+size_t Composite::callback_callcount;
+
+class GCedWithComposite final : public GarbageCollected<GCedWithComposite> {
+ public:
+  void Trace(Visitor* visitor) const { visitor->Trace(composite); }
+
+  Composite composite;
+};
+
+}  // namespace
+
+TEST_F(VisitorTest, DispatchToCompositeObject) {
+  Member<GCedWithComposite> ref =
+      MakeGarbageCollected<GCedWithComposite>(GetHeap());
+  DispatchingVisitor visitor(ref, ref);
+  EXPECT_EQ(0u, Composite::callback_callcount);
+  visitor.Trace(ref);
+  EXPECT_EQ(1u, Composite::callback_callcount);
+}
+
 }  // namespace internal
 }  // namespace cppgc