Prevent synchronous image change notifications during paint

third_party/WebKit/Source/platform/graphics/BitmapImage.cpp

 /*
  * Copyright (C) 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2004, 2005, 2006, 2008 Apple 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
@@ skipping 481 matching lines @@
             if (time < frameAfterNextStartTime)
                 break;
 
             // Yes; skip over it without notifying our observers.
             if (!internalAdvanceAnimation(true))
                 return;
             m_desiredFrameStartTime = frameAfterNextStartTime;
             nextFrame = frameAfterNext;
         }
 
-        // Draw the next frame immediately.  Note that m_desiredFrameStartTime
+        // Post a task to advance the frame immediately. m_desiredFrameStartTime
         // may be in the past, meaning the next time through this function we'll
         // kick off the next advancement sooner than this frame's duration would
         // suggest.
-        if (internalAdvanceAnimation(false)) {
-            // The image region has been marked dirty, but once we return to our
-            // caller, draw() will clear it, and nothing will cause the
-            // animation to advance again.  We need to start the timer for the
-            // next frame running, or the animation can hang.  (Compare this
-            // with when advanceAnimation() is called, and the region is dirtied
-            // while draw() is not in the callstack, meaning draw() gets called
-            // to update the region and thus startAnimation() is reached again.)
-            // NOTE: For large images with slow or heavily-loaded systems,
-            // throwing away data as we go (see destroyDecodedData()) means we
-            // can spend so much time re-decoding data above that by the time we
-            // reach here we're behind again.  If we let startAnimation() run
-            // the catch-up code again, we can get long delays without painting
-            // as we race the timer, or even infinite recursion.  In this
-            // situation the best we can do is to simply change frames as fast
-            // as possible, so force startAnimation() to set a zero-delay timer
-            // and bail out if we're not caught up.
-            startAnimation(DoNotCatchUp);
-        }
+        m_frameTimer = adoptPtr(new Timer<BitmapImage>(this, &BitmapImage::advanceAnimationWithoutCatchUp));

[chrishtr, 2016/06/04 06:39:49]

It seems it might be possible for startAnimation() to be called again (via
draw() or some other call site) before
this timer fires. That seems bad..

[pdr., 2016/06/04 15:05:34]

It's safe because startAnimation bails out if there's an active timer.

+        m_frameTimer->startOneShot(0, BLINK_FROM_HERE);
     }
 }
 
 void BitmapImage::stopAnimation()
 {
     // This timer is used to animate all occurrences of this image.  Don't invalidate
     // the timer unless all renderers have stopped drawing.
     m_frameTimer.reset();
 }
 
@@ skipping 26 matching lines @@
 }
 
 void BitmapImage::advanceAnimation(Timer<BitmapImage>*)
 {
     internalAdvanceAnimation(false);
     // At this point the image region has been marked dirty, and if it's
     // onscreen, we'll soon make a call to draw(), which will call
     // startAnimation() again to keep the animation moving.
 }
 
+void BitmapImage::advanceAnimationWithoutCatchUp(Timer<BitmapImage>*)
+{
+    if (internalAdvanceAnimation(false))
+        startAnimation(DoNotCatchUp);
+}
+
 bool BitmapImage::internalAdvanceAnimation(bool skippingFrames)
 {
     // Stop the animation.
     stopAnimation();
 
     // See if anyone is still paying attention to this animation.  If not, we don't
     // advance and will remain suspended at the current frame until the animation is resumed.
     if (!skippingFrames && getImageObserver()->shouldPauseAnimation(this))
         return false;
 
@@ skipping 19 matching lines @@
 
     // We need to draw this frame if we advanced to it while not skipping, or if
     // while trying to skip frames we hit the last frame and thus had to stop.
     if (skippingFrames != advancedAnimation)
         getImageObserver()->animationAdvanced(this);
 
     return advancedAnimation;
 }
 
 } // namespace blink