Enable iterating over roots from helper threads by moving stack walking.

runtime/vm/heap.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/heap.h"
 
 #include "platform/assert.h"
 #include "platform/utils.h"
 #include "vm/flags.h"
 #include "vm/isolate.h"
@@ skipping 213 matching lines @@
 
 
 HeapIterationScope::HeapIterationScope()
     : StackResource(Thread::Current()),
       old_space_(isolate()->heap()->old_space()) {
   // It's not yet safe to iterate over a paged space while it's concurrently
   // sweeping, so wait for any such task to complete first.
   MonitorLocker ml(old_space_->tasks_lock());
 #if defined(DEBUG)
   // We currently don't support nesting of HeapIterationScopes.
-  ASSERT(!old_space_->is_iterating_);
-  old_space_->is_iterating_ = true;
+  ASSERT(old_space_->iterating_thread_ != thread());
 #endif
   while (old_space_->tasks() > 0) {
     ml.Wait();
   }
+#if defined(DEBUG)
+  ASSERT(old_space_->iterating_thread_ == NULL);
+  old_space_->iterating_thread_ = thread();
+#endif
   old_space_->set_tasks(1);
 }
 
 
 HeapIterationScope::~HeapIterationScope() {
   MonitorLocker ml(old_space_->tasks_lock());
 #if defined(DEBUG)
-  ASSERT(old_space_->is_iterating_);
-  old_space_->is_iterating_ = false;
+  ASSERT(old_space_->iterating_thread_ == thread());
+  old_space_->iterating_thread_ = NULL;
 #endif
   ASSERT(old_space_->tasks() == 1);
   old_space_->set_tasks(0);
   ml.Notify();
 }
 
 
 void Heap::IterateObjects(ObjectVisitor* visitor) const {
   // The visitor must not allocate from the heap.
   NoSafepointScope no_safepoint_scope_;
@@ skipping 528 matching lines @@
     heap->DisableGrowthControl();
 }
 
 
 NoHeapGrowthControlScope::~NoHeapGrowthControlScope() {
     Heap* heap = reinterpret_cast<Isolate*>(isolate())->heap();
     heap->SetGrowthControlState(current_growth_controller_state_);
 }
 
 }  // namespace dart