Update the current frame count in a safe place

Source/modules/webaudio/AudioContext.h

 /*
  * Copyright (C) 2010, Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1.  Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 88 matching lines @@
     virtual void trace(Visitor*) override;
 
     bool isInitialized() const { return m_isInitialized; }
     bool isOfflineContext() { return m_isOfflineContext; }
 
     // Document notification
     virtual void stop() override final;
     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(); }
+    // cachedSampleFrame() 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 cachedSampleFrame() const;
     double currentTime() const { return m_destinationNode->currentTime(); }
     float sampleRate() const { return m_destinationNode->sampleRate(); }
     String state() const;
 
     AudioBuffer* createBuffer(unsigned numberOfChannels, size_t numberOfFrames, float sampleRate, ExceptionState&);
 
     // Asynchronous audio file data decoding.
     void decodeAudioData(DOMArrayBuffer*, AudioBufferCallback*, AudioBufferCallback*, ExceptionState&);
 
     AudioListener* listener() { return m_listener.get(); }
@@ skipping 270 matching lines @@
     AudioContextState m_contextState;
     void setContextState(AudioContextState);
 
     AsyncAudioDecoder m_audioDecoder;
 
     // Collection of nodes where the channel count mode has changed. We want the channel count mode
     // to change in the pre- or post-rendering phase so as not to disturb the running audio thread.
     GC_PLUGIN_IGNORE("http://crbug.com/404527")
     HashSet<AudioNode*> m_deferredCountModeChange;
 
+    // Follows the destination's currentSampleFrame, but might be slightly behind due to locking.
+    size_t m_cachedSampleFrame;
+
     // This is considering 32 is large enough for multiple channels audio.
     // It is somewhat arbitrary and could be increased if necessary.
     enum { MaxNumberOfChannels = 32 };
 };
 
 } // namespace blink
 
 #endif // AudioContext_h