Bug 1808632 - TaskbarPreviewCallback::Done(): modernization and error…

…-checking r=cmartin

Properly test for and handle errors in target-surface creation and
mapping.

(Additionally, perform some drive-by cleanup/modernization.)

Differential Revision: https://phabricator.services.mozilla.com/D166126

widget/windows/TaskbarPreview.cpp

@@ -345,37 +345,43 @@ TaskbarPreviewCallback::Done(nsISupports* aCanvas, bool aDrawBorder) {
   }
   RefPtr<gfxWindowsSurface> target = new gfxWindowsSurface(
       source->GetSize(), gfx::SurfaceFormat::A8R8G8B8_UINT32);
-  if (!target) {
+  if (target->CairoStatus() != CAIRO_STATUS_SUCCESS) {
     return NS_ERROR_FAILURE;
   }
 
-  RefPtr<gfx::DataSourceSurface> srcSurface = source->GetDataSurface();
+  using DataSrcSurf = gfx::DataSourceSurface;
+  RefPtr<DataSrcSurf> srcSurface = source->GetDataSurface();
   RefPtr<gfxImageSurface> imageSurface = target->GetAsImageSurface();
   if (!srcSurface || !imageSurface) {
     return NS_ERROR_FAILURE;
   }
 
-  gfx::DataSourceSurface::MappedSurface sourceMap;
-  srcSurface->Map(gfx::DataSourceSurface::READ, &sourceMap);
-  mozilla::gfx::CopySurfaceDataToPackedArray(
-      sourceMap.mData, imageSurface->Data(), srcSurface->GetSize(),
-      sourceMap.mStride, BytesPerPixel(srcSurface->GetFormat()));
-  srcSurface->Unmap();
+  if (DataSrcSurf::ScopedMap const sourceMap(srcSurface, DataSrcSurf::READ);
+      sourceMap.IsMapped()) {
+    mozilla::gfx::CopySurfaceDataToPackedArray(
+        sourceMap.GetData(), imageSurface->Data(), srcSurface->GetSize(),
+        sourceMap.GetStride(), BytesPerPixel(srcSurface->GetFormat()));
+  } else if (source->GetSize().IsEmpty()) {
+    // A zero-size source-surface probably shouldn't happen, but is harmless
+    // here. Fall through.
+  } else {
+    return NS_ERROR_FAILURE;
+  }
 
   HDC hDC = target->GetDC();
   HBITMAP hBitmap = (HBITMAP)GetCurrentObject(hDC, OBJ_BITMAP);
 
   DWORD flags = aDrawBorder ? DWM_SIT_DISPLAYFRAME : 0;
-  POINT pptClient = {0, 0};
   HRESULT hr;
   if (!mIsThumbnail) {
+    POINT pptClient = {0, 0};
     hr = DwmSetIconicLivePreviewBitmap(mPreview->PreviewWindow(), hBitmap,
                                        &pptClient, flags);
   } else {
     hr = DwmSetIconicThumbnail(mPreview->PreviewWindow(), hBitmap, flags);
   }
   MOZ_ASSERT(SUCCEEDED(hr));
-  (void)hr;
+  mozilla::Unused << hr;
   return NS_OK;
 }