[sensors] Introduce asynchronous way to create sensors.

device/generic_sensor/sensor_provider_impl.cc

Index: device/generic_sensor/sensor_provider_impl.cc
diff --git a/device/generic_sensor/sensor_provider_impl.cc b/device/generic_sensor/sensor_provider_impl.cc
index 2cdb1c5a32d5b32ac3c3b97070d0a3a2f0983d33..51ad6cac2a1a81b731a485a4a96febef08fc65a5 100644
--- a/device/generic_sensor/sensor_provider_impl.cc
+++ b/device/generic_sensor/sensor_provider_impl.cc
@@ -32,7 +32,7 @@ void SensorProviderImpl::Create(mojom::SensorProviderRequest request) {
 }
 
 SensorProviderImpl::SensorProviderImpl(PlatformSensorProvider* provider)
-    : provider_(provider) {
+    : provider_(provider), weak_ptr_factory_(this) {
   DCHECK(provider_);
 }
 
@@ -49,10 +49,25 @@ void SensorProviderImpl::GetSensor(mojom::SensorType type,
 
   scoped_refptr<PlatformSensor> sensor = provider_->GetSensor(type);
   if (!sensor) {
-    sensor = provider_->CreateSensor(
-        type, mojom::SensorInitParams::kReadBufferSize, GetBufferOffset(type));
+    PlatformSensorProviderBase::CreateSensorCallback cb = base::Bind(
+        &SensorProviderImpl::SensorCreated, weak_ptr_factory_.GetWeakPtr(),
+        type, base::Passed(&cloned_handle), base::Passed(&sensor_request),
+        callback);
+    provider_->CreateSensor(type, mojom::SensorInitParams::kReadBufferSize,
+                            GetBufferOffset(type), cb);

[timvolodine, 2016/09/30 18:50:48]

you mentioned this is 'synchronous' on android/mac i.e. the callback is executed
immediately, are there any potential re-entrancy issues, if for example I call
GetSensor inside the GetSensorCallback?

[maksims (do not use this acc), 2016/10/03 12:58:11]

It's technically possible, but it would be considered as a programming error if
one calls getSensor on the same process using the same sensor_provider_impl. Do
you think this should be fixed on this level?

[timvolodine, 2016/10/03 17:44:12]

I think this should be addressed to some extent to avoid potential future
issues.. I would either use a PostTask (instead of immediate callback
invocation) or maybe a dcheck with a comment to avoid reentrancy. But handling
everything in async way seems the easiest approach (probably without much if any
overhead).

[maksims (do not use this acc), 2016/10/04 07:21:34]

Sure. As per discussion with shalamov@, we had the same conclusion (I mean using
PostTask to run the callback.

+    return;
   }
 
+  SensorCreated(type, std::move(cloned_handle), std::move(sensor_request),
+                callback, std::move(sensor));
+}
+
+void SensorProviderImpl::SensorCreated(
+    mojom::SensorType type,
+    mojo::ScopedSharedBufferHandle cloned_handle,
+    mojom::SensorRequest sensor_request,
+    const GetSensorCallback& callback,
+    scoped_refptr<PlatformSensor> sensor) {
   if (!sensor) {
     callback.Run(nullptr, nullptr);
     return;