When a surface is backed by an external render target force a copy on image snap

src/image/SkSurface_Gpu.cpp

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkSurface_Gpu.h"
 
 #include "GrResourceProvider.h"
@@ skipping 64 matching lines @@
     static const SkBudgeted kBudgeted = SkBudgeted::kNo;
     return SkSurface::NewRenderTarget(fDevice->context(), kBudgeted, info, sampleCount,
                                       &this->props());
 }
 
 SkImage* SkSurface_Gpu::onNewImageSnapshot(SkBudgeted budgeted, ForceCopyMode forceCopyMode) {
     GrRenderTarget* rt = fDevice->accessRenderTarget();
     SkASSERT(rt);
     GrTexture* tex = rt->asTexture();
     SkAutoTUnref<GrTexture> copy;
-    // TODO: Force a copy when the rt is an external resource.
-    if (kYes_ForceCopyMode == forceCopyMode || !tex) {
+    // If the original render target is a buffer originally created by the client, then we don't
+    // want to ever retarget the SkSurface at another buffer we create. Force a copy now to avoid
+    // copy-on-write.
+    if (kYes_ForceCopyMode == forceCopyMode || !tex || rt->resourcePriv().isExternal()) {
         GrSurfaceDesc desc = fDevice->accessRenderTarget()->desc();
         GrContext* ctx = fDevice->context();
         desc.fFlags = desc.fFlags & ~kRenderTarget_GrSurfaceFlag;
         copy.reset(ctx->textureProvider()->createTexture(desc, budgeted));
         if (!copy) {
             return nullptr;
         }
         if (!ctx->copySurface(copy, rt)) {
             return nullptr;
         }
@@ skipping 108 matching lines @@
     }
     SkAutoTUnref<SkGpuDevice> device(SkGpuDevice::Create(rt, props,
                                                          SkGpuDevice::kUninit_InitContents));
     if (!device) {
         return nullptr;
     }
     return new SkSurface_Gpu(device);
 }
 
 #endif