Bug 1286766 - make MediaPrefs::MDSMSuspendBackgroundVideoDelay() safe to read off main thread using Preferences::AddAtomicUintVarCache(). r=gerald,kamidphish

MozReview-Commit-ID: FAplUTE07l0

--HG--
extra : rebase_source : 729a56f13c68621d03313b460d95e90796e66412

dom/media/MediaDecoderStateMachine.cpp

@@ -206,15 +206,11 @@ static void InitVideoQueuePrefs() {
 
 // Delay, in milliseconds, that tabs needs to be in background before video
 // decoding is suspended.
-static TimeDuration sSuspendBackgroundVideoDelay;
-
-static void
-InitSuspendBackgroundPref()
+static TimeDuration
+SuspendBackgroundVideoDelay()
 {
-  MOZ_ASSERT(NS_IsMainThread(), "Must be on main thread.");
-
-  sSuspendBackgroundVideoDelay = TimeDuration::FromMilliseconds(
-      MediaPrefs::MDSMSuspendBackgroundVideoDelay());
+  return TimeDuration::FromMilliseconds(
+    MediaPrefs::MDSMSuspendBackgroundVideoDelay());
 }
 
 #define INIT_WATCHABLE(name, val) \
@@ -294,7 +290,6 @@ MediaDecoderStateMachine::MediaDecoderStateMachine(MediaDecoder* aDecoder,
   NS_ASSERTION(NS_IsMainThread(), "Should be on main thread.");
 
   InitVideoQueuePrefs();
-  InitSuspendBackgroundPref();
 
   mBufferingWait = IsRealTime() ? 0 : 15;
   mLowDataThresholdUsecs = IsRealTime() ? 0 : detail::LOW_DATA_THRESHOLD_USECS;
@@ -1351,7 +1346,7 @@ void MediaDecoderStateMachine::VisibilityChanged()
 
   // Start timer to trigger suspended decoding state when going invisible.
   if (!mIsVisible) {
-    TimeStamp target = TimeStamp::Now() + sSuspendBackgroundVideoDelay;
+    TimeStamp target = TimeStamp::Now() + SuspendBackgroundVideoDelay();
 
     RefPtr<MediaDecoderStateMachine> self = this;
     mVideoDecodeSuspendTimer.Ensure(target,

dom/media/MediaPrefs.cpp

@@ -69,4 +69,11 @@ void MediaPrefs::PrefAddVarCache(float* aVariable,
   Preferences::AddFloatVarCache(aVariable, aPref, aDefault);
 }
 
+void MediaPrefs::PrefAddVarCache(AtomicUint32* aVariable,
+                                 const char* aPref,
+                                 uint32_t aDefault)
+{
+  Preferences::AddAtomicUintVarCache(aVariable, aPref, aDefault);
+}
+
 } // namespace mozilla

dom/media/MediaPrefs.h

@@ -10,6 +10,8 @@
 #include "AndroidBridge.h"
 #endif
 
+#include "mozilla/Atomics.h"
+
 // First time MediaPrefs::GetSingleton() needs to be called on the main thread,
 // before any of the methods accessing the values are used, but after
 // the Preferences system has been initialized.
@@ -33,7 +35,7 @@ public:                                                                       \
 static const Type& Name() { MOZ_ASSERT(SingletonExists()); return GetSingleton().mPref##Name.mValue; } \
 private:                                                                      \
 static const char* Get##Name##PrefName() { return Pref; }                     \
-static Type Get##Name##PrefDefault() { return Default; }                      \
+static StripAtomic<Type> Get##Name##PrefDefault() { return Default; }         \
 PrefTemplate<Type, Get##Name##PrefDefault, Get##Name##PrefName> mPref##Name
 
 // Custom Definitions.
@@ -47,10 +49,24 @@ template<class T> class StaticAutoPtr;
 
 class MediaPrefs final
 {
+  typedef Atomic<uint32_t, Relaxed> AtomicUint32;
+
+  template <typename T>
+  struct StripAtomicImpl {
+    typedef T Type;
+  };
+
+  template <typename T, MemoryOrdering Order>
+  struct StripAtomicImpl<Atomic<T, Order>> {
+    typedef T Type;
+  };
+
+  template <typename T>
+  using StripAtomic = typename StripAtomicImpl<T>::Type;
 
 private:
   // Since we cannot use const char*, use a function that returns it.
-  template <class T, T Default(), const char* Pref()>
+  template <class T, StripAtomic<T> Default(), const char* Pref()>
   class PrefTemplate
   {
   public:
@@ -114,7 +130,7 @@ private:
 
   // MediaDecoderStateMachine
   DECL_MEDIA_PREF("media.suspend-bkgnd-video.enabled",        MDSMSuspendBackgroundVideoEnabled, bool, false);
-  DECL_MEDIA_PREF("media.suspend-bkgnd-video.delay-ms",       MDSMSuspendBackgroundVideoDelay, uint32_t, SUSPEND_BACKGROUND_VIDEO_DELAY_MS);
+  DECL_MEDIA_PREF("media.suspend-bkgnd-video.delay-ms",       MDSMSuspendBackgroundVideoDelay, AtomicUint32, SUSPEND_BACKGROUND_VIDEO_DELAY_MS);
 
   // WebSpeech
   DECL_MEDIA_PREF("media.webspeech.synth.force_global_queue", WebSpeechForceGlobal, bool, false);
@@ -156,6 +172,7 @@ private:
   static void PrefAddVarCache(int32_t*, const char*, int32_t);
   static void PrefAddVarCache(uint32_t*, const char*, uint32_t);
   static void PrefAddVarCache(float*, const char*, float);
+  static void PrefAddVarCache(AtomicUint32*, const char*, uint32_t);
 
   static void AssertMainThread();