Skip TransformCanvas calls for FunctorDrawables

Fix issue where attempts to render overscroll content
into the alpha 8 mask would crash if there are WebView
related Functors in the scene.

Provide implementations for getTypeName in SkDrawable instances
of FunctorDrawable.

Fixes: 203960959
Test: manual
Change-Id: Idf321d9bfcbe2b8ba1eb205eadcec95e55865305

libs/hwui/pipeline/skia/FunctorDrawable.h

@@ -34,6 +34,8 @@ namespace skiapipeline {
  */
 class FunctorDrawable : public SkDrawable {
 public:
+    constexpr static const char* const TYPE_NAME = "FunctorDrawable";
+
     FunctorDrawable(int functor, SkCanvas* canvas)
             : mBounds(canvas->getLocalClipBounds())
             , mWebViewHandle(WebViewFunctorManager::instance().handleFor(functor)) {}
@@ -48,6 +50,8 @@ public:
         mWebViewHandle->onRemovedFromTree();
     }
 
+    const char* getTypeName() const override { return TYPE_NAME; }
+
 protected:
     virtual SkRect onGetBounds() override { return mBounds; }
 

libs/hwui/pipeline/skia/TransformCanvas.cpp

@@ -14,6 +14,8 @@
  * limitations under the License.
  */
 #include "TransformCanvas.h"
+
+#include "FunctorDrawable.h"
 #include "HolePunch.h"
 #include "SkData.h"
 #include "SkDrawable.h"
@@ -35,8 +37,18 @@ void TransformCanvas::onDrawAnnotation(const SkRect& rect, const char* key, SkDa
 }
 
 void TransformCanvas::onDrawDrawable(SkDrawable* drawable, const SkMatrix* matrix) {
+    // TransformCanvas filters all drawing commands while maintaining the current
+    // clip stack and transformation. We need to draw most SkDrawables, since their
+    // draw calls may call methods that affect the clip stack and transformation. (Any
+    // actual draw commands will then be filtered out.) But FunctorDrawables are used
+    // as leaf nodes which issue self-contained OpenGL/Vulkan commands. These won't
+    // affect the clip stack + transformation, and in some cases cause problems (e.g. if
+    // the surface only has an alpha channel). See b/203960959
+    const auto* drawableName = drawable->getTypeName();
+    if (drawableName == nullptr || strcmp(drawableName, FunctorDrawable::TYPE_NAME) != 0) {
         drawable->draw(this, matrix);
     }
+}
 
 bool TransformCanvas::onFilter(SkPaint& paint) const {
     return false;