Bug 1114840 - Dynamically compute preroll thresholds. r=cpearce

Currently, the preroll threshold ends up higher than the ample threshold in the
audio-only case where we slash the audio thresholds by a factor of 8. The best
way to avoid these sorts of bugs is to compute the values dynamically.

dom/media/MediaDecoderStateMachine.cpp

@@ -232,9 +232,6 @@ MediaDecoderStateMachine::MediaDecoderStateMachine(MediaDecoder* aDecoder,
   mBufferingWait = mScheduler->IsRealTime() ? 0 : 30;
   mLowDataThresholdUsecs = mScheduler->IsRealTime() ? 0 : LOW_DATA_THRESHOLD_USECS;
 
-  mVideoPrerollFrames = mScheduler->IsRealTime() ? 0 : mAmpleVideoFrames / 2;
-  mAudioPrerollUsecs = mScheduler->IsRealTime() ? 0 : LOW_AUDIO_USECS * 2;
-
 #ifdef XP_WIN
   // Ensure high precision timers are enabled on Windows, otherwise the state
   // machine thread isn't woken up at reliable intervals to set the next frame,
@@ -638,7 +635,7 @@ MediaDecoderStateMachine::DecodeVideo()
     // some frames before enabling the keyframe skip logic on video.
     if (mIsVideoPrerolling &&
         (static_cast<uint32_t>(VideoQueue().GetSize())
-          >= mVideoPrerollFrames * mPlaybackRate))
+          >= VideoPrerollFrames() * mPlaybackRate))
     {
       mIsVideoPrerolling = false;
     }
@@ -698,7 +695,7 @@ MediaDecoderStateMachine::DecodeAudio()
     // only just started up the decode loop, so wait until we've decoded
     // some audio data before enabling the keyframe skip logic on audio.
     if (mIsAudioPrerolling &&
-        GetDecodedAudioDuration() >= mAudioPrerollUsecs * mPlaybackRate) {
+        GetDecodedAudioDuration() >= AudioPrerollUsecs() * mPlaybackRate) {
       mIsAudioPrerolling = false;
     }
   }

dom/media/MediaDecoderStateMachine.h

@@ -89,6 +89,7 @@ hardware (via AudioStream).
 #include "MediaDecoderReader.h"
 #include "MediaDecoderOwner.h"
 #include "MediaMetadataManager.h"
+#include "MediaDecoderStateMachineScheduler.h"
 
 class nsITimer;
 
@@ -943,8 +944,17 @@ protected:
   // unneccessarily if we start playing as soon as the first sample is
   // decoded. These two fields store how many video frames and audio
   // samples we must consume before are considered to be finished prerolling.
-  uint32_t mAudioPrerollUsecs;
-  uint32_t mVideoPrerollFrames;
+  uint32_t AudioPrerollUsecs() const
+  {
+    if (mScheduler->IsRealTime()) {
+      return 0;
+    }
+
+    uint32_t result = mLowAudioThresholdUsecs * 2;
+    MOZ_ASSERT(result <= mAmpleAudioThresholdUsecs, "Prerolling will never finish");
+    return result;
+  }
+  uint32_t VideoPrerollFrames() const { return mScheduler->IsRealTime() ? 0 : mAmpleVideoFrames / 2; }
 
   // This temporarily stores the first frame we decode after we seek.
   // This is so that if we hit end of stream while we're decoding to reach

dom/media/moz.build

@@ -102,6 +102,7 @@ EXPORTS += [
     'MediaDecoderOwner.h',
     'MediaDecoderReader.h',
     'MediaDecoderStateMachine.h',
+    'MediaDecoderStateMachineScheduler.h',
     'MediaInfo.h',
     'MediaMetadataManager.h',
     'MediaPromise.h',