Make ObjectGraph wait for concurrent sweeper before using mark bits

runtime/vm/object_graph.cc

Index: runtime/vm/object_graph.cc
diff --git a/runtime/vm/object_graph.cc b/runtime/vm/object_graph.cc
index f50046896face8357917abecee537691a61c7f61..b07fe0a90637f15965a6e46660fa3154451c1a8e 100644
--- a/runtime/vm/object_graph.cc
+++ b/runtime/vm/object_graph.cc
@@ -147,11 +147,6 @@ class Unmarker : public ObjectVisitor {
   }
 
   static void UnmarkAll(Isolate* isolate) {
-    PageSpace* old_space = isolate->heap()->old_space();
-    MonitorLocker ml(old_space->tasks_lock());
-    while (old_space->tasks() > 0) {
-      ml.Wait();
-    }
     Unmarker unmarker(isolate);
     isolate->heap()->VisitObjects(&unmarker);
   }
@@ -177,6 +172,11 @@ ObjectGraph::~ObjectGraph() {
 
 void ObjectGraph::IterateObjects(ObjectGraph::Visitor* visitor) {
   NoSafepointScope no_safepoint_scope_;
+  PageSpace* old_space = isolate()->heap()->old_space();
+  MonitorLocker ml(old_space->tasks_lock());
+  while (old_space->tasks() > 0) {
+    ml.Wait();
+  }
   Stack stack(isolate());
   isolate()->VisitObjectPointers(&stack, false, false);
   stack.TraverseGraph(visitor);
@@ -187,6 +187,11 @@ void ObjectGraph::IterateObjects(ObjectGraph::Visitor* visitor) {
 void ObjectGraph::IterateObjectsFrom(const Object& root,
                                      ObjectGraph::Visitor* visitor) {
   NoSafepointScope no_safepoint_scope_;
+  PageSpace* old_space = isolate()->heap()->old_space();
+  MonitorLocker ml(old_space->tasks_lock());
+  while (old_space->tasks() > 0) {
+    ml.Wait();
+  }
   Stack stack(isolate());
   RawObject* root_raw = root.raw();
   stack.VisitPointer(&root_raw);