Description: fix sweep step for tainted QObject JavaScript wrappers
 Currently, whenever the garbage collector runs, it will destroy all
 valid tainted wrappers.
 .
 Only null or undefined wrappers will be preserved in the
 m_multiplyWrappedQObjects map.
 .
 It seems like "!" was overlooked in
 3b5d37ce3841c4bfdf1c629d33f0e33b881b47fb. Prior to that change, it
 was "!it.value()->markBit()", so calling erase() in the then branch
 did make sense. But with "!it.value().isNullOrUndefined()", erase()
 will be called for every valid wrapper, which is the opposite what we
 want.
Origin: upstream, https://code.qt.io/cgit/qt/qtdeclarative.git/commit/?id=e6b2f88d892dcf39
Last-Update: 2022-02-13

--- a/src/qml/memory/qv4mm.cpp
+++ b/src/qml/memory/qv4mm.cpp
@@ -981,7 +981,7 @@ void MemoryManager::sweep(bool lastSweep
 
     if (MultiplyWrappedQObjectMap *multiplyWrappedQObjects = engine->m_multiplyWrappedQObjects) {
         for (MultiplyWrappedQObjectMap::Iterator it = multiplyWrappedQObjects->begin(); it != multiplyWrappedQObjects->end();) {
-            if (!it.value().isNullOrUndefined())
+            if (it.value().isNullOrUndefined())
                 it = multiplyWrappedQObjects->erase(it);
             else
                 ++it;
--- a/tests/auto/qml/qjsengine/tst_qjsengine.cpp
+++ b/tests/auto/qml/qjsengine/tst_qjsengine.cpp
@@ -102,6 +102,7 @@ private slots:
     void valueConversion_RegularExpression();
     void castWithMultipleInheritance();
     void collectGarbage();
+    void collectGarbageNestedWrappersTwoEngines();
     void gcWithNestedDataStructure();
     void stacktrace();
     void numberParsing_data();
@@ -1809,6 +1810,44 @@ void tst_QJSEngine::collectGarbage()
     QVERIFY(ptr.isNull());
 }
 
+class TestObjectContainer : public QObject
+{
+    Q_OBJECT
+    Q_PROPERTY(QObject *dummy MEMBER m_dummy CONSTANT)
+
+public:
+    TestObjectContainer() : m_dummy(new QObject(this)) {}
+
+private:
+    QObject *m_dummy;
+};
+
+void tst_QJSEngine::collectGarbageNestedWrappersTwoEngines()
+{
+    QJSEngine engine1;
+    QJSEngine engine2;
+
+    TestObjectContainer container;
+    QQmlEngine::setObjectOwnership(&container, QQmlEngine::CppOwnership);
+
+    engine1.globalObject().setProperty("foobar", engine1.newQObject(&container));
+    engine2.globalObject().setProperty("foobar", engine2.newQObject(&container));
+
+    engine1.evaluate("foobar.dummy.baz = 42");
+    engine2.evaluate("foobar.dummy.baz = 43");
+
+    QCOMPARE(engine1.evaluate("foobar.dummy.baz").toInt(), 42);
+    QCOMPARE(engine2.evaluate("foobar.dummy.baz").toInt(), 43);
+
+    engine1.collectGarbage();
+    engine2.collectGarbage();
+
+    // The GC should not collect dummy object wrappers neither in engine1 nor engine2, we
+    // verify that by checking whether the baz property still has its previous value.
+    QCOMPARE(engine1.evaluate("foobar.dummy.baz").toInt(), 42);
+    QCOMPARE(engine2.evaluate("foobar.dummy.baz").toInt(), 43);
+}
+
 void tst_QJSEngine::gcWithNestedDataStructure()
 {
     // The GC must be able to traverse deeply nested objects, otherwise this
--- a/tests/auto/qml/qv4mm/tst_qv4mm.cpp
+++ b/tests/auto/qml/qv4mm/tst_qv4mm.cpp
@@ -76,10 +76,10 @@ void tst_qv4mm::multiWrappedQObjects()
         QCOMPARE(engine1.memoryManager->m_pendingFreedObjectWrapperValue.size(), 1);
         QCOMPARE(engine2.memoryManager->m_pendingFreedObjectWrapperValue.size(), 0);
 
-        // Moves the additional WeakValue from m_multiplyWrappedQObjects to
-        // m_pendingFreedObjectWrapperValue. It's still alive after all.
+        // The additional WeakValue from m_multiplyWrappedQObjects hasn't been moved
+        // to m_pendingFreedObjectWrapperValue yet. It's still alive after all.
         engine1.memoryManager->runGC();
-        QCOMPARE(engine1.memoryManager->m_pendingFreedObjectWrapperValue.size(), 2);
+        QCOMPARE(engine1.memoryManager->m_pendingFreedObjectWrapperValue.size(), 1);
 
         // engine2 doesn't own the object as engine1 was the first to wrap it above.
         // Therefore, no effect here.