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, 3 insertions, 0 deletions

diff --git a/src/script/api/qscriptengine.cpp b/src/script/api/qscriptengine.cpp
index 8b3b844..3b1cb9d 100644
--- a/src/script/api/qscriptengine.cpp
+++ b/src/script/api/qscriptengine.cpp
@@ -1253,6 +1253,9 @@ void QScriptEnginePrivate::setContextFlags(JSC::ExecState *exec, uint flags)
 }
 
 
+// This function is called by JSC after all objects reachable by JSC itself
+// have been processed (see JSC::Heap::markRoots()).
+// Here we should mark additional objects managed by QtScript.
 void QScriptEnginePrivate::mark(JSC::MarkStack& markStack)
 {
     Q_Q(QScriptEngine);