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>

1 files changed, 3 insertions, 3 deletions

diff --git a/src/qml/qml/qqmlengine.cpp b/src/qml/qml/qqmlengine.cpp
index c2f8459fb1..0d9326b9a9 100644
--- a/src/qml/qml/qqmlengine.cpp
+++ b/src/qml/qml/qqmlengine.cpp
@@ -2518,7 +2518,7 @@ QJSValue QQmlEnginePrivate::singletonInstance<QJSValue>(const QQmlType &type)
             // should behave identically to QML singleton types.
             q->setContextForObject(o, new QQmlContext(q->rootContext(), q));
         }
-        singletonInstances.insert(type, value);
+        singletonInstances.convertAndInsert(v4engine(), type, &value);
 
     } else if (siinfo->qobjectCallback) {
         QObject *o = siinfo->qobjectCallback(q, q);
@@ -2536,12 +2536,12 @@ QJSValue QQmlEnginePrivate::singletonInstance<QJSValue>(const QQmlType &type)
         // should behave identically to QML singleton types.
         q->setContextForObject(o, new QQmlContext(q->rootContext(), q));
         value = q->newQObject(o);
-        singletonInstances.insert(type, value);
+        singletonInstances.convertAndInsert(v4engine(), type, &value);
     } else if (!siinfo->url.isEmpty()) {
         QQmlComponent component(q, siinfo->url, QQmlComponent::PreferSynchronous);
         QObject *o = component.beginCreate(q->rootContext());
         value = q->newQObject(o);
-        singletonInstances.insert(type, value);
+        singletonInstances.convertAndInsert(v4engine(), type, &value);
         component.completeCreate();
     }