From dc2358e98b8ddab532866a403ffc09d1162ad0f9 Mon Sep 17 00:00:00 2001
From: Ulf Hermann <ulf.hermann@qt.io>
Date: Tue, 13 Jan 2026 11:53:27 +0100
Subject: [PATCH] QtQml: Do not clear objects' propertyCaches on last GC run

The property caches are not specific to the engine. The same object may
be exposed to other engines and still require its property cache. When
the clearing of the property caches on engine destruction was
introduced, the property caches were still engine-specific and we had no
choice but to clear them. Otherwise any further access would lead to a
dereference of a dangling pointer.

Furthermore, when clearing the JS wrapper for a QObject, check if it's
actually the wrapper being deleted. We don't want to clear some other
engine's wrapper.

Amends commit 749a7212e903d8e8c6f256edb1836b9449cc7fe1.
Amends commit c6b2dd879d02b21b18f80faab541f8f04286685a.

Pick-to: 6.11 6.10
Fixes: QTBUG-142514
Change-Id: I40bb1aeca65225d56cb1d2ff498f5f1722216a70
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
---

diff --git a/src/qml/jsruntime/qv4qobjectwrapper.cpp b/src/qml/jsruntime/qv4qobjectwrapper.cpp
index a8dfd50..e61867b 100644
--- a/src/qml/jsruntime/qv4qobjectwrapper.cpp
+++ b/src/qml/jsruntime/qv4qobjectwrapper.cpp
@@ -1583,10 +1583,9 @@
                     o->deleteLater();
             } else {
                 // If the object is C++-owned, we still have to release the weak reference we have
-                // to it.
-                ddata->jsWrapper.clear();
-                if (lastCall && ddata->propertyCache)
-                    ddata->propertyCache.reset();
+                // to it. If the "main" wrapper is not ours, we should leave it alone, though.
+                if (ddata->jsWrapper.as<QObjectWrapper>() == this)
+                    ddata->jsWrapper.clear();
             }
         }
     }
diff --git a/tests/auto/qml/qjsengine/tst_qjsengine.cpp b/tests/auto/qml/qjsengine/tst_qjsengine.cpp
index b56b241..11bc62e 100644
--- a/tests/auto/qml/qjsengine/tst_qjsengine.cpp
+++ b/tests/auto/qml/qjsengine/tst_qjsengine.cpp
@@ -1119,7 +1119,7 @@
         engine.newQObject(obj.data());
         QVERIFY(QQmlData::get(obj.data())->propertyCache);
     }
-    QVERIFY(!QQmlData::get(obj.data())->propertyCache);
+    QVERIFY(QQmlData::get(obj.data())->propertyCache);
 }
 
 void tst_QJSEngine::newQMetaObject() {
diff --git a/tests/auto/qml/qmlcppcodegen/data/CMakeLists.txt b/tests/auto/qml/qmlcppcodegen/data/CMakeLists.txt
index 88cc102..8949d00 100644
--- a/tests/auto/qml/qmlcppcodegen/data/CMakeLists.txt
+++ b/tests/auto/qml/qmlcppcodegen/data/CMakeLists.txt
@@ -326,6 +326,7 @@
     stringLength.qml
     stringToByteArray.qml
     structuredValueType.qml
+    multiEnginePropertyCache.qml
     takenumber.qml
     testlogger.js
     text.qml
diff --git a/tests/auto/qml/qmlcppcodegen/data/multiEnginePropertyCache.qml b/tests/auto/qml/qmlcppcodegen/data/multiEnginePropertyCache.qml
new file mode 100644
index 0000000..222b2f8
--- /dev/null
+++ b/tests/auto/qml/qmlcppcodegen/data/multiEnginePropertyCache.qml
@@ -0,0 +1,20 @@
+pragma Strict
+import QtQml
+
+QtObject {
+    id: root
+
+    property int foo: 0
+    onFooChanged: root.close1()
+
+    property int bar: 0
+    onBarChanged: close2()
+
+    function close1() {
+        console.log("close1")
+    }
+
+    function close2() {
+        console.log("close2")
+    }
+}
diff --git a/tests/auto/qml/qmlcppcodegen/tst_qmlcppcodegen.cpp b/tests/auto/qml/qmlcppcodegen/tst_qmlcppcodegen.cpp
index 3ce5627..8d98c02 100644
--- a/tests/auto/qml/qmlcppcodegen/tst_qmlcppcodegen.cpp
+++ b/tests/auto/qml/qmlcppcodegen/tst_qmlcppcodegen.cpp
@@ -280,6 +280,7 @@
     void stringLength();
     void stringToByteArray();
     void structuredValueType();
+    void multiEnginePropertyCache();
     void takeNumbers();
     void takeNumbers_data();
     void testIsnan();
@@ -5795,6 +5796,26 @@
     QCOMPARE(o->property("w2").value<WeatherModelUrl>(), w2);
 }
 
+void tst_QmlCppCodegen::multiEnginePropertyCache()
+{
+    QQmlEngine engine;
+    QQmlComponent c(&engine, QUrl(u"qrc:/qt/qml/TestTypes/multiEnginePropertyCache.qml"_s));
+    QVERIFY2(c.isReady(), qPrintable(c.errorString()));
+    std::unique_ptr<QObject> o(c.create());
+    QVERIFY(o);
+
+    {
+        QJSEngine other;
+        other.newQObject(o.get());
+    }
+
+    QTest::ignoreMessage(QtDebugMsg, "close1");
+    o->setProperty("foo", 1);
+
+    QTest::ignoreMessage(QtDebugMsg, "close2");
+    o->setProperty("bar", 2);
+}
+
 void tst_QmlCppCodegen::takeNumbers()
 {
     QFETCH(QByteArray, method);