SkImage_Raster's pixels are always immutable.

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
 #define SkPixelRef_DEFINED
 
@@ skipping 134 matching lines @@
     /**
      *  Change the info's AlphaType. Note that this does not automatically
      *  invalidate the generation ID. If the pixel values themselves have
      *  changed, then you must explicitly call notifyPixelsChanged() as well.
      */
     void changeAlphaType(SkAlphaType at);
 
     /** Returns true if this pixelref is marked as immutable, meaning that the
         contents of its pixels will not change for the lifetime of the pixelref.
     */
-    bool isImmutable() const { return fIsImmutable; }
+    bool isImmutable() const { return fMutability != kMutable; }
 
     /** Marks this pixelref is immutable, meaning that the contents of its
         pixels will not change for the lifetime of the pixelref. This state can
         be set on a pixelref, but it cannot be cleared once it is set.
     */
     void setImmutable();
 
     /** Return the optional URI string associated with this pixelref. May be
         null.
     */
@@ skipping 193 matching lines @@
 #ifdef SK_BUILD_FOR_ANDROID_FRAMEWORK
     const uint32_t fStableID;
 #endif
 
     SkTDArray<GenIDChangeListener*> fGenIDChangeListeners;  // pointers are owned
 
     SkString    fURI;
 
     // Set true by caches when they cache content that's derived from the current pixels.
     SkAtomic<bool> fAddedToCache;
-    // can go from false to true, but never from true to false
-    bool fIsImmutable;
+
+    enum {
+        kMutable,               // PixelRefs begin mutable.
+        kTemporarilyImmutable,  // Considered immutable, but can revert to mutable.
+        kImmutable,             // Once set to this state, it never leaves.
+    } fMutability : 8;          // easily fits inside a byte
+
     // only ever set in constructor, const after that
     bool fPreLocked;
 
     void needsNewGenID();
     void callGenIDChangeListeners();
 
     void setMutex(SkBaseMutex* mutex);
 
+    void setTemporarilyImmutable();
+    void restoreMutability();
+    friend class SkSurface_Raster;   // For the two methods above.
+
     // 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 SkRefCnt INHERITED;
 };
 
 class SkPixelRefFactory : public SkRefCnt {
 public:
     /**
      *  Allocate a new pixelref matching the specified ImageInfo, allocating
      *  the memory for the pixels. If the ImageInfo requires a ColorTable,
      *  the pixelref will ref() the colortable.
      *  On failure return NULL.
      */
     virtual SkPixelRef* create(const SkImageInfo&, size_t rowBytes, SkColorTable*) = 0;
 };
 
 #endif