Canvas: Guard context pointer against external deletion

You cannot keep the context when reparenting the canvas item. Use a
QPointer prevent dangling.

Task-number: QTBUG-73113
Change-Id: Ie7021c6f0bb0d09923eb358dc7e51d6727e74a7a
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

2 files changed, 88 insertions, 1 deletions

diff --git a/tests/auto/quick/qquickcanvasitem/data/tst_invalidContext.qml b/tests/auto/quick/qquickcanvasitem/data/tst_invalidContext.qml
new file mode 100644
index 0000000000..dd14aef433
--- /dev/null
+++ b/tests/auto/quick/qquickcanvasitem/data/tst_invalidContext.qml
@@ -0,0 +1,86 @@
+/****************************************************************************
+**
+** Copyright (C) 2019 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of the test suite of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:GPL-EXCEPT$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3 as published by the Free Software
+** Foundation with exceptions as appearing in the file LICENSE.GPL3-EXCEPT
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-3.0.html.
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+import QtQuick 2.4
+import QtTest 1.1
+
+Item {
+    id: root
+    width: 500
+    height: 500
+
+    Timer {
+        id: timer
+        interval: 1
+        running: true
+        repeat: true
+        onTriggered: {
+            if (myCanvas.parent == root) {
+                myCanvas.parent = null
+            } else {
+                myCanvas.parent = root
+            }
+        }
+    }
+
+    Canvas {
+        id: myCanvas
+        anchors.fill: parent
+        property var paintContext: null
+
+        function paint() {
+            paintContext.fillStyle = Qt.rgba(1, 0, 0, 1);
+            paintContext.fillRect(0, 0, width, height);
+            requestAnimationFrame(paint);
+        }
+
+        onAvailableChanged: {
+            if (available) {
+                paintContext = getContext("2d")
+                requestAnimationFrame(paint);
+            }
+        }
+    }
+
+    TestCase {
+        name: "invalidContext"
+        when: myCanvas.parent === null && myCanvas.paintContext !== null
+
+        function test_paintContextInvalid() {
+            verify(myCanvas.paintContext);
+            var caught = false;
+            try {
+                console.log(myCanvas.paintContext.fillStyle);
+            } catch(e) {
+                caught = true;
+            }
+            verify(caught);
+            timer.running = false
+        }
+    }
+}
diff --git a/tests/auto/quick/qquickcanvasitem/qquickcanvasitem.pro b/tests/auto/quick/qquickcanvasitem/qquickcanvasitem.pro
index 845128f9de..70e5a05f8d 100644
--- a/tests/auto/quick/qquickcanvasitem/qquickcanvasitem.pro
+++ b/tests/auto/quick/qquickcanvasitem/qquickcanvasitem.pro
@@ -52,7 +52,8 @@ OTHER_FILES += \
     data/transparent.png \
     data/transparent50.png \
     data/yellow.png \
-    data/yellow75.png
+    data/yellow75.png \
+    data/tst_invalidContext.qml
 
 
 CONFIG += insignificant_test # QTBUG-41043