[Sensors] Improvements in shared buffer managing

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

 // 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 "modules/sensor/SensorProxy.h"
 
 #include "core/frame/LocalFrame.h"
 #include "modules/sensor/SensorProviderProxy.h"
 #include "platform/mojo/MojoHelper.h"
 
@@ skipping 83 matching lines @@
 }
 
 const device::mojom::blink::SensorConfiguration* SensorProxy::defaultConfig()
     const {
   DCHECK(isInitialized());
   return m_defaultConfig.get();
 }
 
 void SensorProxy::updateInternalReading() {
   DCHECK(isInitialized());
-  Reading* reading = static_cast<Reading*>(m_sharedBuffer.get());
-  m_reading = *reading;
+
+  ReadingBuffer* readingBuffer =
+      static_cast<ReadingBuffer*>(m_sharedBuffer.get());
+  if (readingBuffer->data.timestamp == 0.0) {
+    // Shared buffer has not yet been updated.
+    // TODO(Mikhail): Should we schedule another update?
+    return;
+  }
+
+  device::OneWriterSeqLock& seqlock = readingBuffer->seqlock;
+
+  base::subtle::Atomic32 version;
+  Reading reading;
+  int readAttempts = 0;
+  const int kMaxReadAttemptsCount = 3;
+  do {
+    version = seqlock.ReadBegin();
+    reading = readingBuffer->data;
+    if (++readAttempts == kMaxReadAttemptsCount)
+      return;
+  } while (seqlock.ReadRetry(version));
+
+  m_reading = reading;
 }
 
 void SensorProxy::RaiseError() {
   handleSensorError();
 }
 
 void SensorProxy::SensorReadingChanged() {
   for (Observer* observer : m_observers)
     observer->onSensorReadingChanged();
 }
@@ skipping 10 matching lines @@
     observer->onSensorError();
 }
 
 void SensorProxy::onSensorCreated(SensorInitParamsPtr params,
                                   SensorClientRequest clientRequest) {
   DCHECK_EQ(Initializing, m_state);
   if (!params) {
     handleSensorError();
     return;
   }
+  const size_t kReadBufferSize = sizeof(ReadingBuffer);
 
-  DCHECK_EQ(0u, params->buffer_offset % SensorInitParams::kReadBufferSize);
+  DCHECK_EQ(0u, params->buffer_offset % kReadBufferSize);
 
   m_mode = params->mode;
   m_defaultConfig = std::move(params->default_configuration);
   if (!m_defaultConfig) {
     handleSensorError();
     return;
   }
 
   DCHECK(m_sensor.is_bound());
   m_clientBinding.Bind(std::move(clientRequest));
 
   m_sharedBufferHandle = std::move(params->memory);
   DCHECK(!m_sharedBuffer);
-  m_sharedBuffer = m_sharedBufferHandle->MapAtOffset(
-      SensorInitParams::kReadBufferSize, params->buffer_offset);
+  m_sharedBuffer =
+      m_sharedBufferHandle->MapAtOffset(kReadBufferSize, params->buffer_offset);
 
   if (!m_sharedBuffer) {
     handleSensorError();
     return;
   }
 
   m_state = Initialized;
   for (Observer* observer : m_observers)
     observer->onSensorInitialized();
 }
 
 }  // namespace blink