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, 15 insertions, 1 deletions

diff --git a/src/qml/qml/qqmlengine_p.h b/src/qml/qml/qqmlengine_p.h
index fe42d4e347..23c69651ed 100644
--- a/src/qml/qml/qqmlengine_p.h
+++ b/src/qml/qml/qqmlengine_p.h
@@ -67,6 +67,7 @@
 #include <private/qrecyclepool_p.h>
 #include <private/qfieldlist_p.h>
 #include <private/qv4engine_p.h>
+#include <private/qjsvalue_p.h>
 
 #include <QtCore/qlist.h>
 #include <QtCore/qpair.h>
@@ -285,7 +286,20 @@ public:
     }
 
 private:
-    QHash<QQmlType, QJSValue> singletonInstances;
+    class SingletonInstances : private QHash<QQmlType, QJSValue>
+    {
+    public:
+        void convertAndInsert(QV4::ExecutionEngine *engine, const QQmlType &type, QJSValue *value)
+        {
+            QJSValuePrivate::manageStringOnV4Heap(engine, value);
+            insert(type, *value);
+        }
+
+        using QHash<QQmlType, QJSValue>::value;
+        using QHash<QQmlType, QJSValue>::take;
+    };
+
+    SingletonInstances singletonInstances;
     QHash<int, QQmlGadgetPtrWrapper *> cachedValueTypeInstances;
 
     // These members must be protected by a QQmlEnginePrivate::Locker as they are required by