[Sensors] Improvements in shared buffer managing

device/generic_sensor/public/cpp/sensor_reading.h

+// 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 DEVICE_GENERIC_SENSOR_PUBLIC_CPP_SENSOR_READING_H_
+#define DEVICE_GENERIC_SENSOR_PUBLIC_CPP_SENSOR_READING_H_
+
+#include "device/base/synchronization/one_writer_seqlock.h"
+
+namespace device {
+
+// This class is guarantied to have a fixed size of 64 bits on every platform.
+// It is introduce to simplify sensors shared buffer memory calculation.
+template <typename Data>
+class SensorReadingField {
+ public:
+  static_assert(sizeof(Data) <= sizeof(int64_t), "The field must be 64 bits.");

[Ken Rockot(use gerrit already), 2016/10/10 16:02:08]

nit: Assertion is really that the field size must be <= 64 bits

[Mikhail, 2016/10/10 18:29:20]

Done.

+  SensorReadingField() = default;
+  SensorReadingField(Data value) { storage.value = value; }
+  SensorReadingField& operator=(Data value) {
+    storage.value = value;
+    return *this;
+  }
+  Data& value() { return storage.value; }
+  operator Data() const { return storage.value; }
+
+ private:
+  union Storage {
+    int64_t unused;
+    Data value;
+    Storage() { new (&value) Data(); }

[Ken Rockot(use gerrit already), 2016/10/10 16:02:08]

This is subtly broken if Data has a non-trivial destructor. Two decent options I
see:

 1. define ~Storage() { value.~Data(); }, or

 2. don't use a union at all. We can just do:

#pragma pack(push, 1)
  static_assert(sizeof(Data) <= sizeof(int64_t), "...");
  Data data_;
  uint8_t padding_[sizeof(int64_t) - sizeof(Data)];
#pragma pack(pop)
};


Also please use |name_| convention for private members.

[Mikhail, 2016/10/10 18:29:20]

Done. Thanks for catching this!

+  };
+  Storage storage;
+};
+
+// This structure represents sensor reading data: timestamp and 3 values.
+struct SensorReading {
+  SensorReading();
+  SensorReading(const SensorReading& other);
+  SensorReadingField<double> timestamp;
+  SensorReadingField<double> values[3];
+};
+
+// This structure represents sensor reading buffer: sensor reading and seqlock
+// for synchronization.
+struct SensorReadingSharedBuffer {
+  SensorReadingSharedBuffer();
+  SensorReadingField<OneWriterSeqLock> seqlock;
+  SensorReading reading;
+};
+
+}  // namespace device
+
+#endif  // DEVICE_GENERIC_SENSOR_PUBLIC_CPP_SENSOR_READING_H_