TaskScheduler: Add SequenceToken and ScopedSetSequenceTokenForCurrentThread

base/sequence_token.cc

Index: base/sequence_token.cc
diff --git a/base/sequence_token.cc b/base/sequence_token.cc
new file mode 100644
index 0000000000000000000000000000000000000000..b3b57bd04c3f4f3e2287ef77dbe16de4de35561f
--- /dev/null
+++ b/base/sequence_token.cc
@@ -0,0 +1,60 @@
+// Copyright 2016 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.
+
+#include "base/sequence_token.h"
+
+#include "base/atomic_sequence_num.h"
+#include "base/lazy_instance.h"
+#include "base/logging.h"
+#include "base/threading/thread_local.h"
+
+namespace base {
+namespace internal {
+
+namespace {
+
+base::StaticAtomicSequenceNumber g_sequence_token_generator;
+
+LazyInstance<ThreadLocalPointer<ScopedSetCurrentSequenceToken>>::Leaky
+    tls_current_sequence_token = LAZY_INSTANCE_INITIALIZER;
+
+}  // namespace
+
+bool SequenceToken::operator==(const SequenceToken& other) const {
+  return token_ == other.token_;
+}
+
+bool SequenceToken::operator!=(const SequenceToken& other) const {
+  return !(*this == other);
+}
+
+bool SequenceToken::IsValid() const {
+  return token_ != kInvalidSequenceToken;
+}
+
+SequenceToken SequenceToken::Create() {
+  return SequenceToken(g_sequence_token_generator.GetNext());
+}
+
+SequenceToken SequenceToken::GetCurrent() {
+  const ScopedSetCurrentSequenceToken* current_sequence_token =
+      tls_current_sequence_token.Get().Get();
+  return current_sequence_token ? current_sequence_token->token_
+                                : SequenceToken();
+}
+
+ScopedSetCurrentSequenceToken::ScopedSetCurrentSequenceToken(
+    const SequenceToken& token)
+    : token_(token) {
+  DCHECK(!tls_current_sequence_token.Get().Get());
+  tls_current_sequence_token.Get().Set(this);
+}
+
+ScopedSetCurrentSequenceToken::~ScopedSetCurrentSequenceToken() {
+  DCHECK_EQ(tls_current_sequence_token.Get().Get(), this);
+  tls_current_sequence_token.Get().Set(nullptr);
+}
+
+}  // namespace internal
+}  // namespace base