Add ThreadBarrier; use in GCMarker and unit test

runtime/vm/thread_barrier_test.cc

Index: runtime/vm/thread_barrier_test.cc
diff --git a/runtime/vm/thread_barrier_test.cc b/runtime/vm/thread_barrier_test.cc
new file mode 100644
index 0000000000000000000000000000000000000000..4517e18d95766079341f680224e62207bd4b1d14
--- /dev/null
+++ b/runtime/vm/thread_barrier_test.cc
@@ -0,0 +1,59 @@
+// Copyright (c) 2015, 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 "platform/assert.h"
+#include "vm/random.h"
+#include "vm/thread_barrier.h"
+#include "vm/thread_pool.h"
+#include "vm/unit_test.h"
+
+namespace dart {
+
+class FuzzTask : public ThreadPool::Task {
+ public:
+  FuzzTask(intptr_t num_rounds,
+           ThreadBarrier* barrier,
+           uint64_t seed)
+    : num_rounds_(num_rounds),
+      barrier_(barrier),
+      rng_(seed) {
+  }
+
+  virtual void Run() {
+    for (intptr_t i = 0; i < num_rounds_; ++i) {
+      RandomSleep();
+      barrier_->Sync();
+    }
+    barrier_->Exit();
+  }
+
+ private:
+  void RandomSleep() {
+    int64_t ms = rng_.NextUInt32() % 4;
+    if (ms > 0) {
+      OS::Sleep(ms);
+    }
+  }
+
+  const intptr_t num_rounds_;
+  ThreadBarrier* barrier_;
+  Random rng_;
+};
+
+
+UNIT_TEST_CASE(ThreadBarrier) {
+  static const intptr_t kNumTasks = 5;
+  static const intptr_t kNumRounds = 500;
+
+  ThreadBarrier barrier(kNumTasks + 1);
+  for (intptr_t i = 0; i < kNumTasks; ++i) {
+    Dart::thread_pool()->Run(new FuzzTask(kNumRounds, &barrier, i + 1));
+  }
+  for (intptr_t i = 0; i < kNumRounds; ++i) {
+    barrier.Sync();
+  }
+  barrier.Exit();
+}
+
+}  // namespace dart