[Impeller] render empty filled paths without crashing. (#51713)

Fixes flutter/flutter#145823

If we try to render a solid filled empty size path, the subpath division code creates an obscene number of vertices that hits overflow errors. Just No-op instead.

Author: Jonah Williams

impeller/entity/contents/color_source_contents.h

@@ -137,6 +137,9 @@ class ColorSourceContents : public Contents {
 
       GeometryResult stencil_geometry_result =
           GetGeometry()->GetPositionBuffer(renderer, entity, pass);
+      if (stencil_geometry_result.vertex_buffer.vertex_count == 0u) {
+        return true;
+      }
       pass.SetVertexBuffer(std::move(stencil_geometry_result.vertex_buffer));
       options.primitive_type = stencil_geometry_result.type;
 
@@ -182,6 +185,9 @@ class ColorSourceContents : public Contents {
             ? geometry.GetPositionUVBuffer(texture_coverage, effect_transform,
                                            renderer, entity, pass)
             : geometry.GetPositionBuffer(renderer, entity, pass);
+    if (geometry_result.vertex_buffer.vertex_count == 0u) {
+      return true;
+    }
     pass.SetVertexBuffer(std::move(geometry_result.vertex_buffer));
     options.primitive_type = geometry_result.type;
 

impeller/entity/entity_unittests.cc

@@ -2813,6 +2813,51 @@ TEST_P(EntityTest, FailOnValidationError) {
       "");
 }
 
+TEST_P(EntityTest, CanComputeGeometryForEmptyPathsWithoutCrashing) {
+  PathBuilder builder = {};
+  builder.AddRect(Rect::MakeLTRB(0, 0, 0, 0));
+  Path path = builder.TakePath();
+
+  EXPECT_TRUE(path.GetBoundingBox()->IsEmpty());
+
+  auto geom = Geometry::MakeFillPath(path);
+
+  Entity entity;
+  RenderTarget target =
+      GetContentContext()->GetRenderTargetCache()->CreateOffscreen(
+          *GetContext(), {1, 1}, 1u);
+  testing::MockRenderPass render_pass(GetContext(), target);
+  auto position_result =
+      geom->GetPositionBuffer(*GetContentContext(), entity, render_pass);
+
+  auto uv_result =
+      geom->GetPositionUVBuffer(Rect::MakeLTRB(0, 0, 100, 100), Matrix(),
+                                *GetContentContext(), entity, render_pass);
+
+  EXPECT_EQ(position_result.vertex_buffer.vertex_count, 0u);
+  EXPECT_EQ(uv_result.vertex_buffer.vertex_count, 0u);
+
+  EXPECT_EQ(geom->GetResultMode(), GeometryResult::Mode::kNormal);
+}
+
+TEST_P(EntityTest, CanRenderEmptyPathsWithoutCrashing) {
+  PathBuilder builder = {};
+  builder.AddRect(Rect::MakeLTRB(0, 0, 0, 0));
+  Path path = builder.TakePath();
+
+  EXPECT_TRUE(path.GetBoundingBox()->IsEmpty());
+
+  auto contents = std::make_shared<SolidColorContents>();
+  contents->SetGeometry(Geometry::MakeFillPath(path));
+  contents->SetColor(Color::Red());
+
+  Entity entity;
+  entity.SetTransform(Matrix::MakeScale(GetContentScale()));
+  entity.SetContents(contents);
+
+  ASSERT_TRUE(OpenPlaygroundHere(std::move(entity)));
+}
+
 }  // namespace testing
 }  // namespace impeller
 

impeller/entity/geometry/fill_path_geometry.cc

@@ -6,6 +6,7 @@
 
 #include "fml/logging.h"
 #include "impeller/core/formats.h"
+#include "impeller/core/vertex_buffer.h"
 #include "impeller/entity/contents/content_context.h"
 #include "impeller/entity/geometry/geometry.h"
 
@@ -20,8 +21,22 @@ GeometryResult FillPathGeometry::GetPositionBuffer(
     const Entity& entity,
     RenderPass& pass) const {
   auto& host_buffer = renderer.GetTransientsBuffer();
-  VertexBuffer vertex_buffer;
 
+  const auto& bounding_box = path_.GetBoundingBox();
+  if (bounding_box.has_value() && bounding_box->IsEmpty()) {
+    return GeometryResult{
+        .type = PrimitiveType::kTriangle,
+        .vertex_buffer =
+            VertexBuffer{
+                .vertex_buffer = {},
+                .vertex_count = 0,
+                .index_type = IndexType::k16bit,
+            },
+        .transform = pass.GetOrthographicTransform() * entity.GetTransform(),
+    };
+  }
+
+  VertexBuffer vertex_buffer;
   if constexpr (!ContentContext::kEnableStencilThenCover) {
     if (!path_.IsConvex()) {
       auto tesselation_result = renderer.GetTessellator()->Tessellate(
@@ -79,6 +94,20 @@ GeometryResult FillPathGeometry::GetPositionUVBuffer(
     RenderPass& pass) const {
   using VS = TextureFillVertexShader;
 
+  const auto& bounding_box = path_.GetBoundingBox();
+  if (bounding_box.has_value() && bounding_box->IsEmpty()) {
+    return GeometryResult{
+        .type = PrimitiveType::kTriangle,
+        .vertex_buffer =
+            VertexBuffer{
+                .vertex_buffer = {},
+                .vertex_count = 0,
+                .index_type = IndexType::k16bit,
+            },
+        .transform = pass.GetOrthographicTransform() * entity.GetTransform(),
+    };
+  }
+
   auto uv_transform =
       texture_coverage.GetNormalizingTransform() * effect_transform;
 
@@ -139,7 +168,9 @@ GeometryResult FillPathGeometry::GetPositionUVBuffer(
 }
 
 GeometryResult::Mode FillPathGeometry::GetResultMode() const {
-  if (!ContentContext::kEnableStencilThenCover || path_.IsConvex()) {
+  const auto& bounding_box = path_.GetBoundingBox();
+  if (!ContentContext::kEnableStencilThenCover || path_.IsConvex() ||
+      (bounding_box.has_value() && bounding_box->IsEmpty())) {
     return GeometryResult::Mode::kNormal;
   }
 

impeller/entity/geometry/geometry_unittests.cc

@@ -2,11 +2,14 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+#include <memory>
 #include "flutter/testing/testing.h"
+#include "impeller/entity/contents/content_context.h"
 #include "impeller/entity/geometry/geometry.h"
 #include "impeller/entity/geometry/stroke_path_geometry.h"
 #include "impeller/geometry/geometry_asserts.h"
 #include "impeller/geometry/path_builder.h"
+#include "impeller/renderer/testing/mocks.h"
 
 inline ::testing::AssertionResult SolidVerticesNear(
     std::vector<impeller::SolidFillVertexShader::PerVertexData> a,

impeller/tessellator/tessellator.cc

@@ -179,6 +179,7 @@ Path::Polyline Tessellator::CreateTempPolyline(const Path& path,
 std::vector<Point> Tessellator::TessellateConvex(const Path& path,
                                                  Scalar tolerance) {
   FML_DCHECK(point_buffer_);
+
   std::vector<Point> output;
   point_buffer_->clear();
   auto polyline =