Add garbage collector prologue callback to qv8engine

This commit provides a generic way to manage persistent handles
created by QML so that circular references don't cause leaks, by
utilising v8's garbage collector callbacks.

Change-Id: Ia898197fdf5d86b90915b835ce3e532f7d400de4
Reviewed-on: http://codereview.qt.nokia.com/3688
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Aaron Kennedy <aaron.kennedy@nokia.com>

3 files changed, 252 insertions, 0 deletions

diff --git a/src/declarative/qml/v8/qv8engine.cpp b/src/declarative/qml/v8/qv8engine.cpp
index 5b74f48170..6f584174c9 100644
--- a/src/declarative/qml/v8/qv8engine.cpp
+++ b/src/declarative/qml/v8/qv8engine.cpp
@@ -43,6 +43,7 @@
 
 #include "qv8contextwrapper_p.h"
 #include "qv8valuetypewrapper_p.h"
+#include "qv8gccallback_p.h"
 #include "qv8include_p.h"
 #include "../../../3rdparty/javascriptcore/DateMath.h"
 
@@ -134,6 +135,7 @@ QV8Engine::QV8Engine(QJSEngine* qq, QJSEngine::ContextOwnership ownership)
     v8::Context::Scope context_scope(m_context);
 
     v8::V8::SetUserObjectComparisonCallbackFunction(ObjectComparisonCallback);
+    QV8GCCallback::registerGcPrologueCallback();
 
     m_stringWrapper.init();
     m_contextWrapper.init(this);
@@ -2293,5 +2295,142 @@ void QV8Engine::emitSignalHandlerException()
     emit q->signalHandlerException(scriptValueFromInternal(uncaughtException()));
 }
 
+QThreadStorage<QV8GCCallback::ThreadData *> QV8GCCallback::threadData;
+void QV8GCCallback::initializeThreadData()
+{
+    QV8GCCallback::ThreadData *newThreadData = new QV8GCCallback::ThreadData;
+    threadData.setLocalData(newThreadData);
+}
+
+void QV8GCCallback::registerGcPrologueCallback()
+{
+    if (!threadData.hasLocalData())
+        initializeThreadData();
+
+    QV8GCCallback::ThreadData *td = threadData.localData();
+    if (!td->gcPrologueCallbackRegistered) {
+        td->gcPrologueCallbackRegistered = true;
+        v8::V8::AddGCPrologueCallback(QV8GCCallback::garbageCollectorPrologueCallback, v8::kGCTypeMarkSweepCompact);
+    }
+}
+
+QV8GCCallback::Node::Node(PrologueCallback callback)
+    : prologueCallback(callback)
+{
+}
+
+QV8GCCallback::Node::~Node()
+{
+    node.remove();
+}
+
+QV8GCCallback::Referencer::Referencer()
+{
+    v8::HandleScope handleScope;
+    v8::Handle<v8::Context> context = v8::Context::New();
+    v8::Context::Scope contextScope(context);
+    strongReferencer = qPersistentNew(v8::Object::New());
+}
+
+void QV8GCCallback::Referencer::addRelationship(QObject *object, QObject *other)
+{
+    bool handleShouldBeStrong = false;
+    v8::Persistent<v8::Object> *implicitOwner = findOwnerAndStrength(object, &handleShouldBeStrong);
+    v8::Persistent<v8::Value> handle = QDeclarativeData::get(other, true)->v8object;
+    if (handleShouldBeStrong) {
+        v8::V8::AddImplicitReferences(strongReferencer, &handle, 1);
+    } else if (!implicitOwner->IsEmpty()) {
+        v8::V8::AddImplicitReferences(*implicitOwner, &handle, 1);
+    }
+}
+
+void QV8GCCallback::Referencer::addRelationship(QObject *object, v8::Persistent<v8::Value> handle)
+{
+    if (handle.IsEmpty())
+        return;
+
+    bool handleShouldBeStrong = false;
+    v8::Persistent<v8::Object> *implicitOwner = findOwnerAndStrength(object, &handleShouldBeStrong);
+    if (handleShouldBeStrong) {
+        v8::V8::AddImplicitReferences(strongReferencer, &handle, 1);
+    } else if (!implicitOwner->IsEmpty()) {
+        v8::V8::AddImplicitReferences(*implicitOwner, &handle, 1);
+    }
+}
+
+v8::Persistent<v8::Object> *QV8GCCallback::Referencer::findOwnerAndStrength(QObject *object, bool *shouldBeStrong)
+{
+    QObject *parent = object->parent();
+    if (!parent) {
+        // if the object has JS ownership, the object's v8object owns the lifetime of the persistent value.
+        if (QDeclarativeEngine::objectOwnership(object) == QDeclarativeEngine::JavaScriptOwnership) {
+            *shouldBeStrong = false;
+            return &(QDeclarativeData::get(object)->v8object);
+        }
+
+        // no parent, and has CPP ownership - doesn't have an implicit parent.
+        *shouldBeStrong = true;
+        return 0;
+    }
+
+    // if it is owned by CPP, it's root parent may still be owned by JS.
+    // in that case, the owner of the persistent handle is the root parent's v8object.
+    while (parent->parent())
+        parent = parent->parent();
+
+    if (QDeclarativeEngine::objectOwnership(parent) == QDeclarativeEngine::JavaScriptOwnership) {
+        // root parent is owned by JS.  It's v8object owns the persistent value in question.
+        *shouldBeStrong = false;
+        return &(QDeclarativeData::get(parent)->v8object);
+    } else {
+        // root parent has CPP ownership.  The persistent value should not be made weak.
+        *shouldBeStrong = true;
+        return 0;
+    }
+}
+
+/*
+   Ensure that each persistent handle is strong if it has CPP ownership
+   and has no implicitly JS owned object owner in its parent chain, and
+   weak otherwise.
+
+   Any weak handle whose parent object is still alive will have an implicit
+   reference (between the parent and the handle) added, so that it will
+   not be collected.
+
+   Note that this callback is registered only for kGCTypeMarkSweepCompact
+   collection cycles, as it is during collection cycles of that type
+   in which weak persistent handle callbacks are called when required.
+ */
+void QV8GCCallback::garbageCollectorPrologueCallback(v8::GCType, v8::GCCallbackFlags)
+{
+    if (!threadData.hasLocalData())
+        return;
+
+    QV8GCCallback::ThreadData *td = threadData.localData();
+    QV8GCCallback::Node *currNode = td->gcCallbackNodes.first();
+
+    while (currNode) {
+        // The client which adds itself to the list is responsible
+        // for maintaining the correct implicit references in the
+        // specified callback.
+        currNode->prologueCallback(&td->referencer, currNode);
+        currNode = td->gcCallbackNodes.next(currNode);
+    }
+}
+
+void QV8GCCallback::addGcCallbackNode(QV8GCCallback::Node *node)
+{
+    if (!threadData.hasLocalData())
+        initializeThreadData();
+
+    QV8GCCallback::ThreadData *td = threadData.localData();
+    td->gcCallbackNodes.insert(node);
+}
+
+QV8GCCallback::ThreadData::~ThreadData()
+{
+}
+
 QT_END_NAMESPACE
 
diff --git a/src/declarative/qml/v8/qv8gccallback_p.h b/src/declarative/qml/v8/qv8gccallback_p.h
new file mode 100644
index 0000000000..095c0e5bac
--- /dev/null
+++ b/src/declarative/qml/v8/qv8gccallback_p.h
@@ -0,0 +1,112 @@
+/****************************************************************************
+**
+** Copyright (C) 2011 Nokia Corporation and/or its subsidiary(-ies).
+** All rights reserved.
+** Contact: Nokia Corporation (qt-info@nokia.com)
+**
+** This file is part of the QtDeclarative module of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:LGPL$
+** GNU Lesser General Public License Usage
+** This file may be used under the terms of the GNU Lesser General Public
+** License version 2.1 as published by the Free Software Foundation and
+** appearing in the file LICENSE.LGPL included in the packaging of this
+** file. Please review the following information to ensure the GNU Lesser
+** General Public License version 2.1 requirements will be met:
+** http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.
+**
+** In addition, as a special exception, Nokia gives you certain additional
+** rights. These rights are described in the Nokia Qt LGPL Exception
+** version 1.1, included in the file LGPL_EXCEPTION.txt in this package.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU General
+** Public License version 3.0 as published by the Free Software Foundation
+** and appearing in the file LICENSE.GPL included in the packaging of this
+** file. Please review the following information to ensure the GNU General
+** Public License version 3.0 requirements will be met:
+** http://www.gnu.org/copyleft/gpl.html.
+**
+** Other Usage
+** Alternatively, this file may be used in accordance with the terms and
+** conditions contained in a signed written agreement between you and Nokia.
+**
+**
+**
+**
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+#ifndef QV8GCCALLBACK_P_H
+#define QV8GCCALLBACK_P_H
+
+//
+//  W A R N I N G
+//  -------------
+//
+// This file is not part of the Qt API.  It exists purely as an
+// implementation detail.  This header file may change from version to
+// version without notice, or even be removed.
+//
+// We mean it.
+//
+
+#include <QtCore/qobject.h>
+#include <QtCore/qthreadstorage.h>
+#include <private/qv8_p.h>
+#include <private/qintrusivelist_p.h>
+
+QT_BEGIN_NAMESPACE
+
+class Q_AUTOTEST_EXPORT QV8GCCallback
+{
+private:
+    class ThreadData;
+public:
+    static void garbageCollectorPrologueCallback(v8::GCType, v8::GCCallbackFlags);
+    static void registerGcPrologueCallback();
+
+    class Referencer {
+    public:
+        ~Referencer() {}
+        void addRelationship(QObject *object, v8::Persistent<v8::Value> handle);
+        void addRelationship(QObject *object, QObject *other);
+    private:
+        Referencer();
+        static v8::Persistent<v8::Object> *findOwnerAndStrength(QObject *qobjectOwner, bool *shouldBeStrong);
+        v8::Persistent<v8::Object> strongReferencer;
+        friend class QV8GCCallback::ThreadData;
+    };
+
+    class Node {
+    public:
+        typedef void (*PrologueCallback)(Referencer *r, Node *node);
+        Node(PrologueCallback callback);
+        ~Node();
+
+        QIntrusiveListNode node;
+        PrologueCallback prologueCallback;
+    };
+
+    static void addGcCallbackNode(Node *node);
+
+private:
+    class ThreadData {
+    public:
+        ThreadData() : gcPrologueCallbackRegistered(false) { }
+        ~ThreadData();
+        Referencer referencer;
+        bool gcPrologueCallbackRegistered;
+        QIntrusiveList<Node, &Node::node> gcCallbackNodes;
+    };
+
+    static void initializeThreadData();
+    static QThreadStorage<ThreadData *> threadData;
+};
+
+QT_END_NAMESPACE
+
+#endif // QV8GCCALLBACK_P_H
+
diff --git a/src/declarative/qml/v8/v8.pri b/src/declarative/qml/v8/v8.pri
index 175efd6b6c..4b642179d3 100644
--- a/src/declarative/qml/v8/v8.pri
+++ b/src/declarative/qml/v8/v8.pri
@@ -8,6 +8,7 @@ HEADERS += \
     $$PWD/qv8debug_p.h \
     $$PWD/qv8stringwrapper_p.h \
     $$PWD/qv8engine_p.h \
+    $$PWD/qv8gccallback_p.h \
     $$PWD/qv8contextwrapper_p.h \
     $$PWD/qv8qobjectwrapper_p.h \
     $$PWD/qv8typewrapper_p.h \