From b080bdf56fe70df7905a69ad1c4626b1d028b049 Mon Sep 17 00:00:00 2001
From: Ferhat <ferhat@gmail.com>
Date: Tue, 15 Dec 2020 16:42:40 -0800
Subject: [PATCH] Prevent recycling of canvas multiple times (#23089)

---
 lib/web_ui/lib/src/engine/html/picture.dart   |  4 ++
 .../engine/surface/scene_builder_test.dart    | 40 +++++++++++++++++++
 2 files changed, 44 insertions(+)

diff --git a/lib/web_ui/lib/src/engine/html/picture.dart b/lib/web_ui/lib/src/engine/html/picture.dart
index fc40beeb75..4bdff64df2 100644
--- a/lib/web_ui/lib/src/engine/html/picture.dart
+++ b/lib/web_ui/lib/src/engine/html/picture.dart
@@ -314,6 +314,10 @@ class PersistedPicture extends PersistedLeafSurface {
       // painting. This removes all the setup work and scaffolding objects
       // that won't be useful for anything anyway.
       _recycleCanvas(oldCanvas);
+      if (oldSurface != null) {
+        // Make sure it doesn't get reused/recycled again.
+        oldSurface._canvas = null;
+      }
       if (rootElement != null) {
         domRenderer.clearDom(rootElement!);
       }
diff --git a/lib/web_ui/test/engine/surface/scene_builder_test.dart b/lib/web_ui/test/engine/surface/scene_builder_test.dart
index 9cb28a174a..e4e88a7050 100644
--- a/lib/web_ui/test/engine/surface/scene_builder_test.dart
+++ b/lib/web_ui/test/engine/surface/scene_builder_test.dart
@@ -588,6 +588,40 @@ void testMain() {
     expect(canvas2.width > unscaledWidth, true);
     expect(canvas2.height > unscaledHeight, true);
   });
+
+  test('Should recycle canvas once', () async {
+    final SurfaceSceneBuilder builder = SurfaceSceneBuilder();
+    final Picture picture1 = _drawPicture();
+    EngineLayer oldLayer = builder.pushClipRect(
+      const Rect.fromLTRB(10, 10, 300, 300),
+    );
+    builder.addPicture(Offset.zero, picture1);
+    builder.pop();
+    builder.build();
+
+    // Force update to scene which will utilize reuse code path.
+    final SurfaceSceneBuilder builder2 = SurfaceSceneBuilder();
+    EngineLayer oldLayer2 = builder2.pushClipRect(
+        const Rect.fromLTRB(5, 10, 300, 300),
+        oldLayer: oldLayer
+    );
+    builder2.addPicture(Offset.zero, _drawEmptyPicture());
+    builder2.pop();
+
+    html.HtmlElement contentAfterReuse = builder2.build().webOnlyRootElement;
+    expect(contentAfterReuse, isNotNull);
+
+    final SurfaceSceneBuilder builder3 = SurfaceSceneBuilder();
+    builder3.pushClipRect(
+        const Rect.fromLTRB(25, 10, 300, 300),
+        oldLayer: oldLayer2
+    );
+    builder3.addPicture(Offset.zero, _drawEmptyPicture());
+    builder3.pop();
+    // This build will crash if canvas gets recycled twice.
+    html.HtmlElement contentAfterReuse2 = builder3.build().webOnlyRootElement;
+    expect(contentAfterReuse2, isNotNull);
+  });
 }
 
 typedef TestLayerBuilder = EngineLayer Function(
@@ -761,6 +795,12 @@ Picture _drawPicture() {
   return recorder.endRecording();
 }
 
+Picture _drawEmptyPicture() {
+  final EnginePictureRecorder recorder = PictureRecorder();
+  recorder.beginRecording(const Rect.fromLTRB(0, 0, 400, 400));
+  return recorder.endRecording();
+}
+
 Picture _drawPathImagePath() {
   const double offsetX = 50;
   const double offsetY = 50;
-- 
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