cppgc: Add test for handling objects conservatively

The test ensures that in-construction objects that have been found
through a write barrier are properly processed (marked + trace) when
finalizing the collection conservatively with a different stack.

This is a test for https://crrev.com/c/2744074

Bug: chromium:1056170
Change-Id: I8099bca1fb9025a315a8f0a3530aac822d1c45d2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2745334Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73306}

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

@@ -4,10 +4,13 @@
 
 #include "src/heap/cppgc/marker.h"
 
+#include <memory>
+
 #include "include/cppgc/allocation.h"
 #include "include/cppgc/internal/pointer-policies.h"
 #include "include/cppgc/member.h"
 #include "include/cppgc/persistent.h"
+#include "include/cppgc/trace-trait.h"
 #include "src/heap/cppgc/heap-object-header.h"
 #include "src/heap/cppgc/marking-visitor.h"
 #include "src/heap/cppgc/stats-collector.h"
@@ -212,6 +215,7 @@ TEST_F(MarkerTest, NestedObjectsOnStackAreMarked) {
 }
 
 namespace {
+
 class GCedWithCallback : public GarbageCollected<GCedWithCallback> {
  public:
   template <typename Callback>
@@ -219,8 +223,19 @@ class GCedWithCallback : public GarbageCollected<GCedWithCallback> {
     callback(this);
   }
 
-  void Trace(Visitor*) const {}
+  template <typename Callback>
+  GCedWithCallback(Callback callback, GCed* gced) : gced_(gced) {
+    callback(this);
+  }
+
+  void Trace(Visitor* visitor) const { visitor->Trace(gced_); }
+
+  GCed* gced() const { return gced_; }
+
+ private:
+  Member<GCed> gced_;
 };
+
 }  // namespace
 
 TEST_F(MarkerTest, InConstructionObjectIsEventuallyMarkedEmptyStack) {
@@ -254,6 +269,63 @@ TEST_F(MarkerTest, InConstructionObjectIsEventuallyMarkedNonEmptyStack) {
       });
 }
 
+namespace {
+
+// Storage that can be used to hide a pointer from the GC. Only useful when
+// dealing with the stack separately.
+class GCObliviousObjectStorage final {
+ public:
+  GCObliviousObjectStorage()
+      : storage_(std::make_unique<const void*>(nullptr)) {}
+
+  template <typename T>
+  void set_object(T* t) {
+    *storage_.get() = TraceTrait<T>::GetTraceDescriptor(t).base_object_payload;
+  }
+
+  const void* object() const { return *storage_; }
+
+ private:
+  std::unique_ptr<const void*> storage_;
+};
+
+V8_NOINLINE void RegisterInConstructionObject(
+    AllocationHandle& allocation_handle, Visitor& v,
+    GCObliviousObjectStorage& storage) {
+  // Create deeper stack to avoid finding any temporary reference in the caller.
+  char space[500];
+  USE(space);
+  MakeGarbageCollected<GCedWithCallback>(
+      allocation_handle,
+      [&visitor = v, &storage](GCedWithCallback* obj) {
+        Member<GCedWithCallback> member(obj);
+        // Adds GCedWithCallback to in-construction objects.
+        visitor.Trace(member);
+        EXPECT_FALSE(HeapObjectHeader::FromPayload(obj).IsMarked());
+        // The inner object GCed is only found if GCedWithCallback is processed.
+        storage.set_object(obj->gced());
+      },
+      // Initializing store does not trigger a write barrier.
+      MakeGarbageCollected<GCed>(allocation_handle));
+}
+
+}  // namespace
+
+TEST_F(MarkerTest,
+       InConstructionObjectIsEventuallyMarkedDifferentNonEmptyStack) {
+  static const Marker::MarkingConfig config = {
+      MarkingConfig::CollectionType::kMajor,
+      MarkingConfig::StackState::kMayContainHeapPointers};
+  InitializeMarker(*Heap::From(GetHeap()), GetPlatformHandle().get(), config);
+
+  GCObliviousObjectStorage storage;
+  RegisterInConstructionObject(GetAllocationHandle(), marker()->Visitor(),
+                               storage);
+  EXPECT_FALSE(HeapObjectHeader::FromPayload(storage.object()).IsMarked());
+  marker()->FinishMarking(MarkingConfig::StackState::kMayContainHeapPointers);
+  EXPECT_TRUE(HeapObjectHeader::FromPayload(storage.object()).IsMarked());
+}
+
 TEST_F(MarkerTest, SentinelNotClearedOnWeakPersistentHandling) {
   static const Marker::MarkingConfig config = {
       MarkingConfig::CollectionType::kMajor,
@@ -290,7 +362,8 @@ class IncrementalMarkingTest : public testing::TestWithHeap {
       MarkingConfig::MarkingType::kIncremental};
 
   void FinishSteps(MarkingConfig::StackState stack_state) {
-    while (!SingleStep(stack_state)) {}
+    while (!SingleStep(stack_state)) {
+    }
   }
 
   void FinishMarking() {