Add SensorReadingSharedBufferReader class to read sensor data

This CL adds a SensorReadingSharedBufferReader class at
//services/device/public/cpp/generic_sensor so that it can be
used by both //content/renderer/device_sensors and Blink code.

Bug: 727788
Change-Id: I9d03b64e5cf191738e87bbe29fd7611654f3fd5b
Reviewed-on: https://chromium-review.googlesource.com/541685Reviewed-by: Tim Volodine <timvolodine@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Alexander Shalamov <alexander.shalamov@intel.com>
Commit-Queue: Jun Cai <juncai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486865}

services/device/public/cpp/generic_sensor/BUILD.gn

@@ -8,6 +8,8 @@ source_set("generic_sensor") {
     "platform_sensor_configuration.h",
     "sensor_reading.cc",
     "sensor_reading.h",
+    "sensor_reading_shared_buffer_reader.cc",
+    "sensor_reading_shared_buffer_reader.h",
   ]
 
   public_deps = [

services/device/public/cpp/generic_sensor/sensor_reading_shared_buffer_reader.cc

+// Copyright 2017 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 "services/device/public/cpp/generic_sensor/sensor_reading_shared_buffer_reader.h"
+
+#include "device/base/synchronization/shared_memory_seqlock_buffer.h"
+
+namespace {
+
+constexpr int kMaxReadAttemptsCount = 10;
+
+}  // namespace
+
+namespace device {
+
+SensorReadingSharedBufferReader::SensorReadingSharedBufferReader(
+    const SensorReadingSharedBuffer* buffer)
+    : buffer_(buffer) {}
+
+SensorReadingSharedBufferReader::~SensorReadingSharedBufferReader() = default;
+
+bool SensorReadingSharedBufferReader::GetReading(SensorReading* result) {
+  int read_attempts = 0;
+  while (!TryReadFromBuffer(result)) {
+    // Only try to read this many times before failing to avoid waiting here
+    // very long in case of contention with the writer.
+    if (++read_attempts == kMaxReadAttemptsCount) {
+      // Failed to successfully read, presumably because the hardware
+      // thread was taking unusually long. Data in |result| was not updated
+      // and was simply left what was there before.
+      return false;
+    }
+  }
+
+  return true;
+}
+
+bool SensorReadingSharedBufferReader::TryReadFromBuffer(SensorReading* result) {
+  auto version = buffer_->seqlock.value().ReadBegin();
+  temp_reading_data_ = buffer_->reading;
+  if (buffer_->seqlock.value().ReadRetry(version))
+    return false;
+  *result = temp_reading_data_;
+  return true;
+}
+
+}  // namespace device

services/device/public/cpp/generic_sensor/sensor_reading_shared_buffer_reader.h

+// Copyright 2017 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 SERVICES_DEVICE_PUBLIC_CPP_GENERIC_SENSOR_SENSOR_READING_SHARED_BUFFER_READER_H_
+#define SERVICES_DEVICE_PUBLIC_CPP_GENERIC_SENSOR_SENSOR_READING_SHARED_BUFFER_READER_H_
+
+#include "base/macros.h"
+#include "services/device/public/cpp/generic_sensor/sensor_reading.h"
+
+namespace device {
+
+class SensorReadingSharedBufferReader {
+ public:
+  // TODO(juncai): Maybe pass a mojo::ScopedSharedBufferHandle to the
+  // constructor.
+  // https://crbug.com/742408
+  explicit SensorReadingSharedBufferReader(
+      const SensorReadingSharedBuffer* buffer);
+  ~SensorReadingSharedBufferReader();
+
+  // Get sensor reading from shared buffer.
+  bool GetReading(SensorReading* result);
+
+ private:
+  bool TryReadFromBuffer(SensorReading* result);
+
+  const SensorReadingSharedBuffer* buffer_;
+  SensorReading temp_reading_data_;
+
+  DISALLOW_COPY_AND_ASSIGN(SensorReadingSharedBufferReader);
+};
+
+}  // namespace device
+
+#endif  // SERVICES_DEVICE_PUBLIC_CPP_GENERIC_SENSOR_SENSOR_READING_SHARED_BUFFER_READER_H_

third_party/WebKit/Source/modules/sensor/SensorProxy.cpp

@@ -114,15 +114,12 @@ const SensorConfiguration* SensorProxy::DefaultConfig() const {
 
 void SensorProxy::UpdateSensorReading() {
   DCHECK(ShouldProcessReadings());
-  int read_attempts = 0;
-  const int kMaxReadAttemptsCount = 10;
   device::SensorReading reading_data;
-  while (!TryReadFromBuffer(reading_data)) {
-    if (++read_attempts == kMaxReadAttemptsCount) {
+  if (!shared_buffer_handle_->is_valid() ||
+      !shared_buffer_reader_->GetReading(&reading_data)) {
     HandleSensorError();
     return;
   }
-  }
 
   if (reading_.timestamp != reading_data.timestamp) {
     DCHECK_GT(reading_data.timestamp, reading_.timestamp)
@@ -202,6 +199,11 @@ void SensorProxy::OnSensorCreated(SensorInitParamsPtr params,
     HandleSensorError();
     return;
   }
+
+  const auto* buffer = static_cast<const device::SensorReadingSharedBuffer*>(
+      shared_buffer_.get());
+  shared_buffer_reader_.reset(
+      new device::SensorReadingSharedBufferReader(buffer));
   frequency_limits_.first = params->minimum_frequency;
   frequency_limits_.second = params->maximum_frequency;
 
@@ -254,19 +256,6 @@ void SensorProxy::OnRemoveConfigurationCompleted(double frequency,
   UpdatePollingStatus();
 }
 
-bool SensorProxy::TryReadFromBuffer(device::SensorReading& result) {
-  DCHECK(IsInitialized());
-  const ReadingBuffer* buffer =
-      static_cast<const ReadingBuffer*>(shared_buffer_.get());
-  const device::OneWriterSeqLock& seqlock = buffer->seqlock.value();
-  auto version = seqlock.ReadBegin();
-  auto reading_data = buffer->reading;
-  if (seqlock.ReadRetry(version))
-    return false;
-  result = reading_data;
-  return true;
-}
-
 void SensorProxy::OnPollingTimer(TimerBase*) {
   UpdateSensorReading();
 }

third_party/WebKit/Source/modules/sensor/SensorProxy.h

@@ -14,6 +14,7 @@
 #include "platform/heap/Handle.h"
 #include "platform/wtf/Vector.h"
 #include "services/device/public/cpp/generic_sensor/sensor_reading.h"
+#include "services/device/public/cpp/generic_sensor/sensor_reading_shared_buffer_reader.h"
 #include "services/device/public/interfaces/sensor.mojom-blink.h"
 #include "services/device/public/interfaces/sensor_provider.mojom-blink.h"
 
@@ -112,8 +113,6 @@ class SensorProxy final : public GarbageCollectedFinalized<SensorProxy>,
       bool result);
   void OnRemoveConfigurationCompleted(double frequency, bool result);
 
-  bool TryReadFromBuffer(device::SensorReading& result);
-
   void OnPollingTimer(TimerBase*);
 
   // Returns 'true' if readings should be propagated to Observers
@@ -141,6 +140,8 @@ class SensorProxy final : public GarbageCollectedFinalized<SensorProxy>,
   State state_;
   mojo::ScopedSharedBufferHandle shared_buffer_handle_;
   mojo::ScopedSharedBufferMapping shared_buffer_;
+  std::unique_ptr<device::SensorReadingSharedBufferReader>
+      shared_buffer_reader_;
   bool suspended_;
   device::SensorReading reading_;
   std::pair<double, double> frequency_limits_;