TaskScheduler: Add SequenceToken and ScopedSetSequenceTokenForCurrentThread

base/sequence_token.h

Index: base/sequence_token.h
diff --git a/base/sequence_token.h b/base/sequence_token.h
new file mode 100644
index 0000000000000000000000000000000000000000..8d1398ea3c592f082cddcc607733795382b4f028
--- /dev/null
+++ b/base/sequence_token.h
@@ -0,0 +1,65 @@
+// 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.
+
+#ifndef BASE_SEQUENCE_TOKEN_H_
+#define BASE_SEQUENCE_TOKEN_H_
+
+#include "base/base_export.h"
+#include "base/macros.h"
+
+namespace base {
+
+// A token that identifies a series of sequenced tasks (i.e. tasks that run one
+// at a time in posting order).
+class BASE_EXPORT SequenceToken {
+ public:
+  // Instantiates an invalid SequenceToken.
+  SequenceToken() = default;
+
+  // Explicitly allow copy.
+  SequenceToken(const SequenceToken& other) = default;
+  SequenceToken& operator=(const SequenceToken& other) = default;
+
+  // An invalid SequenceToken is not equal to any other SequenceToken, including
+  // other invalid SequenceTokens.
+  bool operator==(const SequenceToken& other) const;
+  bool operator!=(const SequenceToken& other) const;
+
+  // Returns true if this is a valid SequenceToken.
+  bool IsValid() const;
+
+  // Returns a valid SequenceToken which isn't equal to any previously returned
+  // SequenceToken.
+  static SequenceToken Create();
+
+  // Returns the SequenceToken associated with the task running on the current
+  // thread, as determined by the active ScopedSetSequenceTokenForCurrentThread
+  // if any.
+  static SequenceToken GetForCurrentThread();
+
+ private:
+  SequenceToken(int token) : token_(token) {}
+
+  static constexpr int kInvalidSequenceToken = -1;
+  int token_ = kInvalidSequenceToken;
+};
+
+// Throughout its lifetime, determines the value returned by
+// SequenceToken::GetForCurrentThread().
+class BASE_EXPORT ScopedSetSequenceTokenForCurrentThread {
+ public:
+  ScopedSetSequenceTokenForCurrentThread(const SequenceToken& token);
+  ~ScopedSetSequenceTokenForCurrentThread();
+
+ private:
+  friend class SequenceToken;
+
+  const SequenceToken token_;
+
+  DISALLOW_COPY_AND_ASSIGN(ScopedSetSequenceTokenForCurrentThread);
+};
+
+}  // namespace base
+
+#endif  // BASE_SEQUENCE_TOKEN_H_