[heap] Filter out stale left-trimmed handles for scavenges

src/heap/heap.cc

Index: src/heap/heap.cc
diff --git a/src/heap/heap.cc b/src/heap/heap.cc
index 1a768fbc920ec6116bda538e741d205ce41744ae..817a856ba98c547936393eb4322e69d6cf93752a 100644
--- a/src/heap/heap.cc
+++ b/src/heap/heap.cc
@@ -3136,6 +3136,10 @@ FixedArrayBase* Heap::LeftTrimFixedArray(FixedArrayBase* object,
   DCHECK(!lo_space()->Contains(object));
   DCHECK(object->map() != fixed_cow_array_map());
 
+  // Ensure that the no handle-scope has more than one pointer to the same
+  // backing-store.
+  SLOW_DCHECK(CountHandlesForObject(object) <= 1);
+
   STATIC_ASSERT(FixedArrayBase::kMapOffset == 0);
   STATIC_ASSERT(FixedArrayBase::kLengthOffset == kPointerSize);
   STATIC_ASSERT(FixedArrayBase::kHeaderSize == 2 * kPointerSize);
@@ -5659,6 +5663,32 @@ void Heap::PrintHandles() {
 
 #endif
 
+#ifdef ENABLE_SLOW_DCHECKS
+
+class CountHandleVisitor : public ObjectVisitor {
+ public:
+  explicit CountHandleVisitor(Object* object) : object_(object) {}
+
+  void VisitPointers(Object** start, Object** end) override {
+    for (Object** p = start; p < end; p++) {
+      if (object_ == reinterpret_cast<Object*>(*p)) count_++;
+    }
+  }
+
+  int count() { return count_; }
+
+ private:
+  Object* object_;
+  int count_ = 0;
+};
+
+int Heap::CountHandlesForObject(Object* object) {
+  CountHandleVisitor v(object);
+  isolate_->handle_scope_implementer()->Iterate(&v);
+  return v.count();
+}
+#endif
+
 class CheckHandleCountVisitor : public ObjectVisitor {
  public:
   CheckHandleCountVisitor() : handle_count_(0) {}