Alter SkXfermode's asFragmentProcessor & asXPFactory contracts

src/gpu/SkGr.cpp

 /*
  * Copyright 2010 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 
 #include "SkGr.h"
 
@@ skipping 471 matching lines @@
             SkAutoTUnref<const GrFragmentProcessor> cfFP(
                 colorFilter->asFragmentProcessor(context));
             if (cfFP) {
                 grPaint->addColorFragmentProcessor(cfFP);
             } else {
                 return false;
             }
         }
     }
 
-    SkXfermode* mode = skPaint.getXfermode();
-    GrXPFactory* xpFactory = nullptr;
-    SkXfermode::AsXPFactory(mode, &xpFactory);
-    SkSafeUnref(grPaint->setXPFactory(xpFactory));
+    // When the xfermode is null on the SkPaint (meaning kSrcOver) we need the XPFactory field on
+    // the GrPaint to also be null (also kSrcOver).
+    SkASSERT(!grPaint->getXPFactory());
+    SkXfermode* xfermode = skPaint.getXfermode();
+    if (xfermode) {
+        // SafeUnref in case a new xfermode is added that returns null. 
+        // In such cases we will fall back to kSrcOver_Mode.
+        SkSafeUnref(grPaint->setXPFactory(xfermode->asXPFactory()));
+    }
 
 #ifndef SK_IGNORE_GPU_DITHER
     if (skPaint.isDither() && grPaint->numColorFragmentProcessors() > 0) {
         grPaint->addColorFragmentProcessor(GrDitherEffect::Create())->unref();
     }
 #endif
     return true;
 }
 
 bool SkPaintToGrPaint(GrContext* context, const SkPaint& skPaint, const SkMatrix& viewM,
@@ skipping 123 matching lines @@
             SkErrorInternals::SetError( kInvalidPaint_SkError,
                                         "Sorry, I don't understand the filtering "
                                         "mode you asked for.  Falling back to "
                                         "MIPMaps.");
             textureFilterMode = GrTextureParams::kMipMap_FilterMode;
             break;
 
     }
     return textureFilterMode;
 }