Add a BarrierClosure.

base/barrier_closure.h

Index: base/barrier_closure.h
diff --git a/base/barrier_closure.h b/base/barrier_closure.h
new file mode 100644
index 0000000000000000000000000000000000000000..30f75ca7142ff368777c72b94849f1e3af084cef
--- /dev/null
+++ b/base/barrier_closure.h
@@ -0,0 +1,27 @@
+// Copyright (c) 2013 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef BASE_BARRIER_CLOSURE_H_
+#define BASE_BARRIER_CLOSURE_H_
+
+#include "base/base_export.h"
+#include "base/callback_forward.h"
+
+namespace base {
+
+// BarrierClosure executes |done_closure| after it has been invoked
+// |num_closures| times.
+//
+// If |num_closures| is 0, |done_closure| is executed immediately.
+//
+// BarrierClosure is thread-safe - the count of remaining closures is
+// maintained as a base::AtomicRefCount. |done_closure| will be run on
+// the thread that calls the final Run() on the returned closures. It will
+// be Reset() after that.

[awong, 2013/08/22 22:05:16]

I would say

|done_closure| is also Reset() on the final calling thread but due to the
refcounted nature of callback, it is hard to know thread resources will be
released on.

[groby-ooo-7-16, 2013/08/24 00:27:25]

Done.

+BASE_EXPORT base::Closure BarrierClosure(int num_closures,
+                                         const base::Closure& done_closure);
+
+}  // namespace base
+
+#endif  // BASE_BARRIER_CLOSURE_H_