Fix race condition in DataFetcherSharedMemoryBase

content/browser/device_orientation/data_fetcher_shared_memory_default.cc

Index: content/browser/device_orientation/data_fetcher_shared_memory_default.cc
diff --git a/content/browser/device_orientation/data_fetcher_shared_memory_default.cc b/content/browser/device_orientation/data_fetcher_shared_memory_default.cc
index 9f9a07874b7a2dafbcc4afe7d0864d9577af45c1..45d2296c01eff65e417ac5fd93a30aeffa742152 100644
--- a/content/browser/device_orientation/data_fetcher_shared_memory_default.cc
+++ b/content/browser/device_orientation/data_fetcher_shared_memory_default.cc
@@ -12,7 +12,8 @@ namespace {
 
 static bool SetMotionBuffer(content::DeviceMotionHardwareBuffer* buffer,
     bool enabled) {
-  DCHECK(buffer);
+  if (!buffer)
+    return false;
   buffer->seqlock.WriteBegin();
   buffer->data.allAvailableSensorsAreActive = enabled;
   buffer->seqlock.WriteEnd();
@@ -29,14 +30,13 @@ DataFetcherSharedMemory::DataFetcherSharedMemory() {
 DataFetcherSharedMemory::~DataFetcherSharedMemory() {
 }
 
-bool DataFetcherSharedMemory::Start(ConsumerType consumer_type) {
-  void* buffer = GetSharedMemoryBuffer(consumer_type);
+bool DataFetcherSharedMemory::Start(ConsumerType consumer_type, void* buffer) {
   DCHECK(buffer);
 
   switch (consumer_type) {
     case CONSUMER_TYPE_MOTION:
-      return SetMotionBuffer(
-          static_cast<DeviceMotionHardwareBuffer*>(buffer), true);
+      motion_buffer_ = static_cast<DeviceMotionHardwareBuffer*>(buffer);
+      return SetMotionBuffer(motion_buffer_, true);
     case CONSUMER_TYPE_ORIENTATION:
       NOTIMPLEMENTED();
       break;
@@ -47,13 +47,10 @@ bool DataFetcherSharedMemory::Start(ConsumerType consumer_type) {
 }
 
 bool DataFetcherSharedMemory::Stop(ConsumerType consumer_type) {
-  void* buffer = GetSharedMemoryBuffer(consumer_type);
-  DCHECK(buffer);
 
   switch (consumer_type) {
     case CONSUMER_TYPE_MOTION:
-      return SetMotionBuffer(
-          static_cast<DeviceMotionHardwareBuffer*>(buffer), false);
+      return SetMotionBuffer(motion_buffer_, false);
     case CONSUMER_TYPE_ORIENTATION:
       NOTIMPLEMENTED();
       break;