Prepare for white allocations during gc (2/9): QQmlVMEMetaObject

The QQmlVMEMetaObject stores its propertyAndMethodStorage as a weak
value. Consequently, no marking happens. The reason for using a weak
value is not quite clear, but for now, assume that there was a good
reason and manually mark the freshly allocated QV4::MemberData if
necessary.

Task-number: QTBUG-121910
Change-Id: Ie3e0fd7e036fe0805318dfc70891b28f70223851
Reviewed-by: Sami Shalayel <sami.shalayel@qt.io>
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

1 files changed, 6 insertions, 0 deletions

diff --git a/src/qml/qml/qqmlvmemetaobject.cpp b/src/qml/qml/qqmlvmemetaobject.cpp
index 6259b6c315..4a2c1c810c 100644
--- a/src/qml/qml/qqmlvmemetaobject.cpp
+++ b/src/qml/qml/qqmlvmemetaobject.cpp
@@ -430,6 +430,12 @@ QQmlVMEMetaObject::QQmlVMEMetaObject(QV4::ExecutionEngine *engine,
             uint size = compiledObject->nProperties + compiledObject->nFunctions;
             if (size) {
                 QV4::Heap::MemberData *data = QV4::MemberData::allocate(engine, size);
+                // we only have a weak reference below; if the VMEMetaObject is already marked
+                // (triggered by the allocate call above)
+                // we therefore might never mark the member data; consequently, mark it now
+                QV4::WriteBarrier::markCustom(engine, [data](QV4::MarkStack *ms) {
+                    data->mark(ms);
+                });
                 propertyAndMethodStorage.set(engine, data);
                 std::fill(data->values.values, data->values.values + data->values.size, QV4::Encode::undefined());
             }