Ensure QObject wrappers are garbage-collected if appropriate

Address the "###" comment.
When QScriptEngine::newQObject() is called with the option
PreferExistingWrapperObject, the resulting wrapper object is cached
by the engine, in case it will be needed later (e.g., by a subsequent
newQObject() call for the same QObject, with the same wrap options).

But if a QObject wrapper object is only referenced by the QtScript
internals (i.e., not reachable from the JSC stack/heap, or kept in a
QScriptValue), the wrapper should not be kept alive if the ownership
is ScriptOwnership, or if the ownership is AutoOwnership and the C++
object has no parent.

If the wrapper is marked in that case, it won't get collected, and
hence the C++ object will be kept alive, too. In practice, QtScript
appears to leak memory (the objects will only be destroyed when the
engine is destroyed).

Our copy of JSC doesn't have a concept of weak references; the
ClientData callback in the JSC markRoots() function (which causes
QScriptEnginePrivate::mark() to be called) was moved to the end. This
enables the wrapper and connection marking logic to determine whether
a wrapper can be safely discarded (if it hasn't been marked by JSC by
this point, it must be a weak reference).

Task-number: QTBUG-21993
Change-Id: I61f6aafc91f080b80d3f5859148e645b80d9b653
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>

1 files changed, 5 insertions, 5 deletions

diff --git a/src/3rdparty/javascriptcore/JavaScriptCore/runtime/Collector.cpp b/src/3rdparty/javascriptcore/JavaScriptCore/runtime/Collector.cpp
index 36269ff..d5adbd7 100644
--- a/src/3rdparty/javascriptcore/JavaScriptCore/runtime/Collector.cpp
+++ b/src/3rdparty/javascriptcore/JavaScriptCore/runtime/Collector.cpp
@@ -1151,11 +1151,6 @@ void Heap::markRoots()
     // Mark explicitly registered roots.
     markProtectedObjects(markStack);
 
-#if QT_BUILD_SCRIPT_LIB
-    if (m_globalData->clientData)
-        m_globalData->clientData->mark(markStack);
-#endif
-
     // Mark misc. other roots.
     if (m_markListSet && m_markListSet->size())
         MarkedArgumentBuffer::markLists(markStack, *m_markListSet);
@@ -1167,6 +1162,11 @@ void Heap::markRoots()
     if (m_globalData->firstStringifierToMark)
         JSONObject::markStringifiers(markStack, m_globalData->firstStringifierToMark);
 
+#if QT_BUILD_SCRIPT_LIB
+    if (m_globalData->clientData)
+        m_globalData->clientData->mark(markStack);
+#endif
+
     markStack.drain();
     markStack.compact();