Update the current frame count in a safe place

Source/modules/webaudio/AudioContext.h

Index: Source/modules/webaudio/AudioContext.h
diff --git a/Source/modules/webaudio/AudioContext.h b/Source/modules/webaudio/AudioContext.h
index 4ce5c232b9851c55acb611b78d09b0acacea1e68..4c6facfedaa22255740098bc5cfad724228c0030 100644
--- a/Source/modules/webaudio/AudioContext.h
+++ b/Source/modules/webaudio/AudioContext.h
@@ -106,7 +106,12 @@ public:
     virtual bool hasPendingActivity() const override;
 
     AudioDestinationNode* destination() { return m_destinationNode.get(); }
+    // currentSampleFrame() returns the current sample frame. It should only be called from the
+    // audio thread.
     size_t currentSampleFrame() const { return m_destinationNode->currentSampleFrame(); }
+    // sampleFrame() is like currentSampleFrame() but must be called from the main thread to get the
+    // sample frame. It might be slightly behind curentSampleFrame() due to locking.
+    size_t sampleFrame() const;

[hongchan, 2014/12/03 18:50:47]

Just a thought - can't this be shadowCurrentSampleFrame()?

I think the method name is a bit general for its behavior and having 'shadow' in
the method name clarifies it.

[Raymond Toy, 2014/12/03 19:04:44]

I'm terrible at names.  m_shadowCurrentSampleFrame isn't a great name, and I
think shadowCurrentSampleFrame() seems to reflect the implementation too much
because it's really meant to be the currentSampleFrame of the destination that
the main thread can use.

Maybe sampleFrame() should be called currentSampleFrame() and
currentSampleFrame() should be renamed to destinationCurrentSampleFrame() or
currentDestinationSampleFrame()?
  

[hongchan, 2014/12/03 19:13:29]

I agree that it is too much.

How about using the word 'cached' in the issue description instead? I think it
is a good name for the frame value introduced in this patch.

currentSampleFrame() -> m_currentSampleFrame (no change)
cachedSampleFrame() -> m_cachedSampleFrame

I believe the name 'cached' implies that it is behind the actual current sample
time and also thread-safe. Maybe a tad better than 'shadow' or 'destination.'

[Raymond Toy, 2014/12/03 19:34:21]

Sounds good.

     double currentTime() const { return m_destinationNode->currentTime(); }
     float sampleRate() const { return m_destinationNode->sampleRate(); }
     String state() const;
@@ -396,6 +401,9 @@ private:
     GC_PLUGIN_IGNORE("http://crbug.com/404527")
     HashSet<AudioNode*> m_deferredCountModeChange;
 
+    // Follows the destinations currentSampleFrame, but might be slightly behind due to locking.
+    size_t m_shadowCurrentSampleFrame;
+
     // This is considering 32 is large enough for multiple channels audio.
     // It is somewhat arbitrary and could be increased if necessary.
     enum { MaxNumberOfChannels = 32 };