TaskScheduler: Add SequenceToken and ScopedSetSequenceTokenForCurrentThread

base/sequence_token.cc

+// 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 {
+
+base::StaticAtomicSequenceNumber g_sequence_token_generator;
+
+LazyInstance<ThreadLocalPointer<ScopedSetSequenceTokenForCurrentThread>>::Leaky
+    tls_current_sequence_token = LAZY_INSTANCE_INITIALIZER;
+
+}  // namespace
+
+bool SequenceToken::operator==(const SequenceToken& other) const {
+  return token_ == other.token_ && token_ != kInvalidSequenceToken;
+}
+
+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::GetForCurrentThread() {
+  const ScopedSetSequenceTokenForCurrentThread* current_sequence_token =
+      tls_current_sequence_token.Get().Get();
+  return current_sequence_token ? current_sequence_token->token_
+                                : SequenceToken();
+}
+
+ScopedSetSequenceTokenForCurrentThread::ScopedSetSequenceTokenForCurrentThread(
+    const SequenceToken& token)
+    : token_(token) {
+  DCHECK(!tls_current_sequence_token.Get().Get());
+  tls_current_sequence_token.Get().Set(this);
+}
+
+ScopedSetSequenceTokenForCurrentThread::
+    ~ScopedSetSequenceTokenForCurrentThread() {
+  DCHECK_EQ(tls_current_sequence_token.Get().Get(), this);
+  tls_current_sequence_token.Get().Set(nullptr);
+}
+
+}  // namespace base