ensure that we call onUnlock only when we onLock succeeded

include/core/SkPixelRef.h

 
 /*
  * Copyright 2008 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 
 #ifndef SkPixelRef_DEFINED
@@ skipping 226 matching lines @@
     };
 
     // Takes ownership of listener.
     void addGenIDChangeListener(GenIDChangeListener* listener);
 
 protected:
     /** Called when the lockCount goes from 0 to 1. The caller will have already
         acquire a mutex for thread safety, so this method need not do that.
     */
     virtual void* onLockPixels(SkColorTable**) = 0;
-    /** Called when the lock count goes from 1 to 0. The caller will have
-        already acquire a mutex for thread safety, so this method need not do
-        that.
-    */
+ 
+    /**
+     *  Called when the lock count goes from 1 to 0. The caller will have
+     *  already acquire a mutex for thread safety, so this method need not do
+     *  that.
+     *
+     *  If the previous call to onLockPixels failed (i.e. returned NULL), then
+     *  the onUnlockPixels will NOT be called.
+     */
     virtual void onUnlockPixels() = 0;
 
     /** Default impl returns true */
     virtual bool onLockPixelsAreWritable() const;
 
     // returns false;
     virtual bool onImplementsDecodeInto();
     // returns false;
     virtual bool onDecodeInto(int pow2, SkBitmap* bitmap);
 
@@ skipping 59 matching lines @@
 
     // When copying a bitmap to another with the same shape and config, we can safely
     // clone the pixelref generation ID too, which makes them equivalent under caching.
     friend class SkBitmap;  // only for cloneGenID
     void cloneGenID(const SkPixelRef&);
 
     typedef SkFlattenable INHERITED;
 };
 
 #endif