[sensors] [mac] Implement ambient light sensor for macOS

content/common/one_writer_seqlock.h

-// Copyright 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 CONTENT_COMMON_ONE_WRITER_SEQLOCK_H_
-#define CONTENT_COMMON_ONE_WRITER_SEQLOCK_H_
-
-#include "base/atomicops.h"
-#include "base/macros.h"
-#include "base/threading/platform_thread.h"
-#include "content/common/content_export.h"
-
-namespace content {
-
-// This SeqLock handles only *one* writer and multiple readers. It may be
-// suitable for low-contention with relatively infrequent writes, and many
-// readers. See:
-//   http://en.wikipedia.org/wiki/Seqlock
-//   http://www.concurrencykit.org/doc/ck_sequence.html
-// This implementation is based on ck_sequence.h from http://concurrencykit.org.
-//
-// Currently this type of lock is used in two implementations (gamepad and
-// device motion, in particular see e.g. shared_memory_seqlock_buffer.h).
-// It may make sense to generalize this lock to multiple writers.
-//
-// You must be very careful not to operate on potentially inconsistent read
-// buffers. If the read must be retry'd, the data in the read buffer could
-// contain any random garbage. e.g., contained pointers might be
-// garbage, or indices could be out of range. Probably the only suitable thing
-// to do during the read loop is to make a copy of the data, and operate on it
-// only after the read was found to be consistent.
-class CONTENT_EXPORT OneWriterSeqLock {
- public:
-  OneWriterSeqLock();
-  base::subtle::Atomic32 ReadBegin();
-  bool ReadRetry(base::subtle::Atomic32 version);
-  void WriteBegin();
-  void WriteEnd();
-
- private:
-  base::subtle::Atomic32 sequence_;
-  DISALLOW_COPY_AND_ASSIGN(OneWriterSeqLock);
-};
-
-}  // namespace content
-
-#endif  // CONTENT_COMMON_ONE_WRITER_SEQLOCK_H_