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, 1 deletions

diff --git a/src/qmlworkerscript/qquickworkerscript.cpp b/src/qmlworkerscript/qquickworkerscript.cpp
index 20294d7ba7..705c6015a2 100644
--- a/src/qmlworkerscript/qquickworkerscript.cpp
+++ b/src/qmlworkerscript/qquickworkerscript.cpp
@@ -41,6 +41,7 @@
 #include "qquickworkerscript_p.h"
 #include <private/qqmlengine_p.h>
 #include <private/qqmlexpression_p.h>
+#include <private/qjsvalue_p.h>
 
 #include <QtCore/qcoreevent.h>
 #include <QtCore/qcoreapplication.h>
@@ -644,7 +645,8 @@ bool QQuickWorkerScript::event(QEvent *event)
         if (QQmlEngine *engine = qmlEngine(this)) {
             QV4::ExecutionEngine *v4 = engine->handle();
             WorkerDataEvent *workerEvent = static_cast<WorkerDataEvent *>(event);
-            emit message(QJSValue(v4, QV4::Serialize::deserialize(workerEvent->data(), v4)));
+            emit message(QJSValuePrivate::fromReturnedValue(
+                             QV4::Serialize::deserialize(workerEvent->data(), v4)));
         }
         return true;
     } else if (event->type() == (QEvent::Type)WorkerErrorEvent::WorkerError) {