Add a SafepointMutexLocker class so that it is possible to have scopes inside the MutexLocker which…

runtime/vm/lockers.h

 // 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.
 
 #ifndef VM_LOCKERS_H_
 #define VM_LOCKERS_H_
 
 #include "platform/assert.h"
 #include "vm/allocation.h"
 #include "vm/globals.h"
@@ skipping 53 matching lines @@
   void NotifyAll() {
     monitor_->NotifyAll();
   }
 
  private:
   Monitor* const monitor_;
 
   DISALLOW_COPY_AND_ASSIGN(MonitorLocker);
 };
 
+
+// SafepointMutexLocker objects are used in code where the locks are
+// more coarse grained and a safepoint operation could be potentially
+// triggered while holding this lock. This ensures that other threads
+// which try to acquire the same lock will be marked as being at a
+// safepoint when they are blocked.
+class SafepointMutexLocker : public ValueObject {
+ public:
+  explicit SafepointMutexLocker(Mutex* mutex);
+  virtual ~SafepointMutexLocker() {
+    mutex_->Unlock();
+  }
+
+ private:
+  Mutex* const mutex_;
+
+  DISALLOW_COPY_AND_ASSIGN(SafepointMutexLocker);
+};
+
 }  // namespace dart
 
 
 #endif  // VM_LOCKERS_H_