Store a QV4::ReturnedValue in QJSValue

Being careful, we can now save primitive values inline. We use the heap
pointer of QV4::Value as either QString* or QV4::Value* for complex
types. We cannot store persistent managed QV4::Value without the double
indirection as those need to be allocated in a special place.

The generic QVariant case is not supported anymore. The only place where
it was actually needed were the stream operators for QJSValue. Those
were fundamentally broken:

* A managed QJSValue saved and loaded from a stream was converted to a
  QVariant-type QJSValue
* QVariant-type QJSValues were not callable, could not be objects or
  arrays, or any of the special types.
* Cyclic references were forcibly broken when saving to a data stream.

In general the support for saving and loading of managed types to/from
a data stream was so abysmally bad that we don't lose much by dropping
it.

[ChangeLog][QML][Important Behavior Changes] When saving a QJSValue to a
QDataStream only primitive values or strings will be retained. Support
for objects and arrays was incomplete and unreliable already before. It
cannot work correctly as we don't necessarily have a JavaScript heap
when loading a QJSValue from a stream. Therefore, we don't have a proper
place to keep any managed values. Using QVariant to keep them instead is
a bad idea because QVariant cannot represent everything a QJSValue can
contain.

Fixes: QTBUG-75174
Change-Id: I75697670639bca8d4b1668763d7020c4cf871bda
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

2 files changed, 13 insertions, 9 deletions

diff --git a/src/qmlmodels/qqmldelegatemodel.cpp b/src/qmlmodels/qqmldelegatemodel.cpp
index 095b66b379..08c91d172e 100644
--- a/src/qmlmodels/qqmldelegatemodel.cpp
+++ b/src/qmlmodels/qqmldelegatemodel.cpp
@@ -48,6 +48,7 @@
 #include <private/qqmlchangeset_p.h>
 #include <private/qqmlengine_p.h>
 #include <private/qqmlcomponent_p.h>
+#include <private/qjsvalue_p.h>
 
 #include <private/qv4value_p.h>
 #include <private/qv4functionobject_p.h>
@@ -2656,8 +2657,10 @@ void QQmlDelegateModelGroupPrivate::emitChanges(QV4::ExecutionEngine *v4)
 {
     Q_Q(QQmlDelegateModelGroup);
     if (isChangedConnected() && !changeSet.isEmpty()) {
-        emit q->changed(QJSValue(v4, engineData(v4)->array(v4, changeSet.removes())),
-                        QJSValue(v4, engineData(v4)->array(v4, changeSet.inserts())));
+        emit q->changed(QJSValuePrivate::fromReturnedValue(
+                            engineData(v4)->array(v4, changeSet.removes())),
+                        QJSValuePrivate::fromReturnedValue(
+                            engineData(v4)->array(v4, changeSet.inserts())));
     }
     if (changeSet.difference() != 0)
         emit q->countChanged();
@@ -2874,7 +2877,7 @@ QJSValue QQmlDelegateModelGroup::get(int index)
     QV4::ScopedObject p(scope, model->m_cacheMetaType->modelItemProto.value());
     o->setPrototypeOf(p);
 
-    return QJSValue(v4, o->asReturnedValue());
+    return QJSValuePrivate::fromReturnedValue(o->asReturnedValue());
 }
 
 bool QQmlDelegateModelGroupPrivate::parseIndex(const QV4::Value &value, int *index, Compositor::Group *group) const
diff --git a/src/qmlmodels/qqmllistmodel.cpp b/src/qmlmodels/qqmllistmodel.cpp
index e58fc19178..0d45d03fa6 100644
--- a/src/qmlmodels/qqmllistmodel.cpp
+++ b/src/qmlmodels/qqmllistmodel.cpp
@@ -589,7 +589,7 @@ void ListModel::set(int elementIndex, QV4::Object *object, QVector<int> *roles)
             const ListLayout::Role &r = m_layout->getRoleOrCreate(propertyName, ListLayout::Role::Function);
             QV4::ScopedFunctionObject func(scope, f);
             QJSValue jsv;
-            QJSValuePrivate::setValue(&jsv, v4, func);
+            QJSValuePrivate::setValue(&jsv, func);
             roleIndex = e->setFunctionProperty(r, jsv);
         } else if (QV4::Object *o = propertyValue->as<QV4::Object>()) {
             if (QV4::QObjectWrapper *wrapper = o->as<QV4::QObjectWrapper>()) {
@@ -1434,7 +1434,7 @@ int ListElement::setJsProperty(const ListLayout::Role &role, const QV4::Value &d
     } else if (d.as<QV4::FunctionObject>()) {
         QV4::ScopedFunctionObject f(scope, d);
         QJSValue jsv;
-        QJSValuePrivate::setValue(&jsv, eng, f);
+        QJSValuePrivate::setValue(&jsv, f);
         roleIndex = setFunctionProperty(role, jsv);
     } else if (d.isObject()) {
         QV4::ScopedObject o(scope, d);
@@ -1638,7 +1638,8 @@ PropertyKey ModelObjectOwnPropertyKeyIterator::next(const Object *o, Property *p
                 auto size = recursiveListModel->count();
                 auto array = ScopedArrayObject{scope, v4->newArrayObject(size)};
                 for (auto i = 0; i < size; i++) {
-                    array->arrayPut(i, QJSValuePrivate::convertedToValue(v4, recursiveListModel->get(i)));
+                    array->arrayPut(i, QJSValuePrivate::convertToReturnedValue(
+                                        v4, recursiveListModel->get(i)));
                 }
                 pd->value = array;
             } else {
@@ -2570,7 +2571,7 @@ QJSValue QQmlListModel::get(int index) const
         }
     }
 
-    return QJSValue(engine(), result->asReturnedValue());
+    return QJSValuePrivate::fromReturnedValue(result->asReturnedValue());
 }
 
 /*!
@@ -2592,7 +2593,7 @@ QJSValue QQmlListModel::get(int index) const
 void QQmlListModel::set(int index, const QJSValue &value)
 {
     QV4::Scope scope(engine());
-    QV4::ScopedObject object(scope, QJSValuePrivate::getValue(&value));
+    QV4::ScopedObject object(scope, QJSValuePrivate::asReturnedValue(&value));
 
     if (!object) {
         qmlWarning(this) << tr("set: value is not an object");
@@ -2791,7 +2792,7 @@ bool QQmlListModelParser::applyProperty(
                 if (v4->hasException)
                     v4->catchException();
                 else
-                    QJSValuePrivate::setValue(&v, v4, result->asReturnedValue());
+                    QJSValuePrivate::setValue(&v, result->asReturnedValue());
                 value.setValue<QJSValue>(v);
             } else {
                 QByteArray script = scriptStr.toUtf8();