Adds Media.Audio.InputDevicePropertyChangedMac UMA stat on Mac

media/audio/mac/audio_low_latency_input_mac.h

Index: media/audio/mac/audio_low_latency_input_mac.h
diff --git a/media/audio/mac/audio_low_latency_input_mac.h b/media/audio/mac/audio_low_latency_input_mac.h
index 6d2d8f5e5830fa4ac344a4807df6750de8959ae3..08d1775723c6dff716ee544dce80f556b7f6723d 100644
--- a/media/audio/mac/audio_low_latency_input_mac.h
+++ b/media/audio/mac/audio_low_latency_input_mac.h
@@ -38,8 +38,7 @@
 
 #include <AudioUnit/AudioUnit.h>
 #include <CoreAudio/CoreAudio.h>
-#include <stddef.h>
-#include <stdint.h>
+#include <map>
 
 #include "base/atomicops.h"
 #include "base/cancelable_callback.h"
@@ -94,6 +93,8 @@ class MEDIA_EXPORT AUAudioInputStream
   size_t requested_buffer_size() const { return number_of_frames_; }
 
  private:
+  static const AudioObjectPropertyAddress kDeviceChangePropertyAddress;
+
   // Callback functions called on a real-time priority I/O thread from the audio
   // unit. These methods are called when recorded audio is available.
   static OSStatus DataIsAvailable(void* context,
@@ -111,6 +112,23 @@ class MEDIA_EXPORT AUAudioInputStream
   OSStatus Provide(UInt32 number_of_frames, AudioBufferList* io_data,
                    const AudioTimeStamp* time_stamp);
 
+  // Callback functions called on different system threads from the Core Audio
+  // framework. These methods are called when device properties are changed.
+  static OSStatus OnDevicePropertyChanged(
+      AudioObjectID object_id,
+      UInt32 num_addresses,
+      const AudioObjectPropertyAddress addresses[],
+      void* context);
+  OSStatus DevicePropertyChanged(AudioObjectID object_id,
+                                 UInt32 num_addresses,
+                                 const AudioObjectPropertyAddress addresses[]);
+
+  // Registers OnDevicePropertyChanged() to receive notifications when device
+  // properties changes.
+  void RegisterDeviceChangeListener();
+  // Stop listening for changes in device properties.
+  void DeRegisterDeviceChangeListener();
+
   // Gets the fixed capture hardware latency and store it during initialization.
   // Returns 0 if not available.
   double GetHardwareLatency();
@@ -143,6 +161,10 @@ class MEDIA_EXPORT AUAudioInputStream
   // Adds extra UMA stats when it has been detected that startup failed.
   void AddHistogramsForFailedStartup();
 
+  // Scans the map of all available property changes (notification types) and
+  // filters out some that make sense to add to UMA stats.
+  void AddDevicePropertyChangesToUMA(const std::string& name);
+
   // Verifies that Open(), Start(), Stop() and Close() are all called on the
   // creating thread which is the main browser thread (CrBrowserMain) on Mac.
   base::ThreadChecker thread_checker_;
@@ -218,6 +240,13 @@ class MEDIA_EXPORT AUAudioInputStream
   // Set to true once when AudioUnitRender() succeeds for the first time.
   bool audio_unit_render_has_worked_;
 
+  // Maps unique representations of device property notification types and
+  // number of times we have been notified about a change in such a type.

[tommi (sloooow) - chröme, 2016/02/19 15:17:33]

can you add a note about what thread is allowed to touch it?

[henrika (OOO until Aug 14), 2016/02/22 12:32:52]

Tommi, FYI, each callback can come from "any" thread it seems like. My guess is
that the inner implementation uses a serial dispatch queue ("Serial queues (also
known as private dispatch queues) execute one task at a time in the order in
which they are added to the queue. The currently executing task runs on a
distinct thread (which can vary from task to task) that is managed by the
dispatch queue. Serial queues are often used to synchronize access to a specific
resource.". There is no documentation that states it but I have never found any
examples from Apple that uses locks in these callbacks. 

But, hhm., I do read it from the creating thread while active. Hence, will add a
lock.

[tommi (sloooow) - chröme, 2016/02/22 14:16:43]

If we don't need a lock, I'd prefer to not have a lock.  My question is more
about us understanding and documenting why it's safe to not use a lock.

+  std::map<UInt32, int> device_property_changes_map_;
+
+  // Set to true when we are listening for changes in device properties.
+  bool device_listener_is_active_;

[tommi (sloooow) - chröme, 2016/02/19 15:17:33]

modified/checked on what threads?

[henrika (OOO until Aug 14), 2016/02/22 12:32:52]

Done, also uses DCHECK to ensure proper calling.

+
   DISALLOW_COPY_AND_ASSIGN(AUAudioInputStream);
 };