Bug 1430948. P1 - add fuzzy mode to MediaTimer. r=jya

We will resolve the timer promise even if it is fired slightly before
the scheduled target. This is used by MDSM which doesn't require high-res
timers.

MozReview-Commit-ID: 1IAsG5gG9D7

--HG--
extra : rebase_source : 71c9bec9d196cd736effabb5b9da54c58b44044c
extra : source : 7b96fc15d13d54b78b0aebbf21bb16b08e4eca50

dom/media/MediaTimer.cpp

@@ -18,11 +18,12 @@ namespace mozilla {
 NS_IMPL_ADDREF(MediaTimer)
 NS_IMPL_RELEASE_WITH_DESTROY(MediaTimer, DispatchDestroy())
 
-MediaTimer::MediaTimer()
+MediaTimer::MediaTimer(bool aFuzzy)
   : mMonitor("MediaTimer Monitor")
   , mTimer(NS_NewTimer())
   , mCreationTimeStamp(TimeStamp::Now())
   , mUpdateScheduled(false)
+  , mFuzzy(aFuzzy)
 {
   TIMER_LOG("MediaTimer::MediaTimer");
 
@@ -114,6 +115,19 @@ MediaTimer::Update()
   UpdateLocked();
 }
 
+bool
+MediaTimer::IsExpired(const TimeStamp& aTarget, const TimeStamp& aNow)
+{
+  MOZ_ASSERT(OnMediaTimerThread());
+  mMonitor.AssertCurrentThreadOwns();
+  // Treat this timer as expired in fuzzy mode even if it is fired
+  // slightly (< 1ms) before the schedule target. So we don't need to schedule a
+  // timer with very small timeout again when the client doesn't need a high-res
+  // timer.
+  TimeStamp t = mFuzzy ? aTarget - TimeDuration::FromMilliseconds(1) : aTarget;
+  return t <= aNow;
+}
+
 void
 MediaTimer::UpdateLocked()
 {
@@ -125,7 +139,7 @@ MediaTimer::UpdateLocked()
 
   // Resolve all the promises whose time is up.
   TimeStamp now = TimeStamp::Now();
-  while (!mEntries.empty() && mEntries.top().mTimeStamp <= now) {
+  while (!mEntries.empty() && IsExpired(mEntries.top().mTimeStamp, now)) {
     mEntries.top().mPromise->Resolve(true, __func__);
     DebugOnly<TimeStamp> poppedTimeStamp = mEntries.top().mTimeStamp;
     mEntries.pop();

dom/media/MediaTimer.h

@@ -36,7 +36,7 @@ typedef MozPromise<bool, bool, /* IsExclusive = */ true> MediaTimerPromise;
 class MediaTimer
 {
 public:
-  MediaTimer();
+  explicit MediaTimer(bool aFuzzy = false);
 
   // We use a release with a custom Destroy().
   NS_IMETHOD_(MozExternalRefCountType) AddRef(void);
@@ -54,6 +54,7 @@ private:
   void ScheduleUpdate();
   void Update();
   void UpdateLocked();
+  bool IsExpired(const TimeStamp& aTarget, const TimeStamp& aNow);
 
   static void TimerCallback(nsITimer* aTimer, void* aClosure);
   void TimerFired();
@@ -111,13 +112,14 @@ private:
   }
 
   bool mUpdateScheduled;
+  const bool mFuzzy;
 };
 
 // Class for managing delayed dispatches on target thread.
 class DelayedScheduler {
 public:
-  explicit DelayedScheduler(AbstractThread* aTargetThread)
-    : mTargetThread(aTargetThread), mMediaTimer(new MediaTimer())
+  explicit DelayedScheduler(AbstractThread* aTargetThread, bool aFuzzy = false)
+    : mTargetThread(aTargetThread), mMediaTimer(new MediaTimer(aFuzzy))
   {
     MOZ_ASSERT(mTargetThread);
   }