Ensure that CompilationUnit instances are deleted in the engine thread

When using CompilationUnit with a QML engine, ensure that they are deleted in
the same thread as the QML engine.

Only the QML engine has a secondary thread (not a plain QJSEngine) and there it
may happen that the last refcount drops within the loader thread. For example
when the trimCache() is called within the loader. The destruction of the
CompilationUnit however is not safe to perform in a secondary thread.

Change-Id: Ia0105a8885ec97b0b2159e32e637adbd4e99f016
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>

src/qml/compiler/qv4compileddata.cpp

@@ -222,6 +222,17 @@ void CompilationUnit::markObjects(QV4::ExecutionEngine *e)
     }
 }
 
+void CompilationUnit::destroy()
+{
+    QQmlEngine *qmlEngine = 0;
+    if (engine)
+        qmlEngine = engine->qmlEngine();
+    if (qmlEngine)
+        QQmlEnginePrivate::deleteInEngineThread(qmlEngine, this);
+    else
+        delete this;
+}
+
 IdentifierHash<int> CompilationUnit::namedObjectsPerComponent(int componentObjectIndex)
 {
     auto it = namedObjectsPerComponentCache.find(componentObjectIndex);

src/qml/compiler/qv4compileddata_p.h

@@ -734,6 +734,7 @@ struct Q_QML_PRIVATE_EXPORT CompilationUnit : public QQmlRefCount
 
     void markObjects(QV4::ExecutionEngine *e);
 
+    void destroy() Q_DECL_OVERRIDE;
 protected:
     virtual void linkBackendToEngine(QV4::ExecutionEngine *engine) = 0;
 #endif // V4_BOOTSTRAP

src/qml/qml/qqmlcompileddata.cpp

@@ -60,14 +60,6 @@ QQmlCompiledData::QQmlCompiledData(QQmlEngine *engine)
     Q_ASSERT(engine);
 }
 
-void QQmlCompiledData::destroy()
-{
-    if (engine && compilationUnit && compilationUnit->engine)
-        QQmlEnginePrivate::deleteInEngineThread(engine, this);
-    else
-        delete this;
-}
-
 QQmlCompiledData::~QQmlCompiledData()
 {
 }

src/qml/qml/qqmlcompiler_p.h

@@ -91,9 +91,6 @@ public:
 
     QQmlRefPointer<QV4::CompiledData::CompilationUnit> compilationUnit;
 
-protected:
-    virtual void destroy(); // From QQmlRefCount
-
 private:
     QQmlCompiledData(const QQmlCompiledData &other);
     QQmlCompiledData &operator=(const QQmlCompiledData &other);