[Sensors] Improvements in shared buffer managing

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

Index: third_party/WebKit/Source/modules/sensor/SensorProxy.cpp
diff --git a/third_party/WebKit/Source/modules/sensor/SensorProxy.cpp b/third_party/WebKit/Source/modules/sensor/SensorProxy.cpp
index be0321dbded63426673baa7397231115a552cd72..513b0030629e6cdd8257b99460d3ba38953119a7 100644
--- a/third_party/WebKit/Source/modules/sensor/SensorProxy.cpp
+++ b/third_party/WebKit/Source/modules/sensor/SensorProxy.cpp
@@ -101,8 +101,29 @@ const device::mojom::blink::SensorConfiguration* SensorProxy::defaultConfig()
 
 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() {
@@ -133,8 +154,9 @@ void SensorProxy::onSensorCreated(SensorInitParamsPtr 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);
@@ -148,8 +170,8 @@ void SensorProxy::onSensorCreated(SensorInitParamsPtr params,
 
   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();