Fix crash caused by dereferencing collected v8 data

If a var property of a QObject is read after the v8 data associated
with the qobject has been deleted but prior to the DeferredDelete
event being processed, the varProperties array will be null and
a crash will occur.

This patch ensures that we check for this condition in both the
access and set codepaths for var properties, and also ensures
that an object which has previously been queued for deletion cannot
be referenced in JS.

Finally, it adds a unit test to ensure that we don't regress.

Task-number: QTBUG-24748
Change-Id: Idde384ca01e18f4dcf9e376e9379f2c5eb410e14
Reviewed-by: Michael Brasser <michael.brasser@nokia.com>

4 files changed, 30 insertions, 13 deletions

diff --git a/src/qml/qml/qqmldata_p.h b/src/qml/qml/qqmldata_p.h
index e4ba44583d..09d1a23510 100644
--- a/src/qml/qml/qqmldata_p.h
+++ b/src/qml/qml/qqmldata_p.h
@@ -78,8 +78,8 @@ class Q_QML_EXPORT QQmlData : public QAbstractDeclarativeData
 public:
     QQmlData()
         : ownMemory(true), ownContext(false), indestructible(true), explicitIndestructibleSet(false), 
-          hasTaintedV8Object(false), notifyList(0), context(0), outerContext(0), bindings(0), 
-          nextContextObject(0), prevContextObject(0), bindingBitsSize(0), bindingBits(0), 
+          hasTaintedV8Object(false), isQueuedForDeletion(false), notifyList(0), context(0), outerContext(0),
+          bindings(0), nextContextObject(0), prevContextObject(0), bindingBitsSize(0), bindingBits(0),
           lineNumber(0), columnNumber(0), deferredComponent(0), deferredIdx(0), v8objectid(0), 
           propertyCache(0), guards(0), extendedData(0) {
           init(); 
@@ -110,7 +110,8 @@ public:
     quint32 indestructible:1;
     quint32 explicitIndestructibleSet:1;
     quint32 hasTaintedV8Object:1;
-    quint32 dummy:27;
+    quint32 isQueuedForDeletion:1;
+    quint32 dummy:26;
 
     struct NotifyList {
         quint64 connectionMask;
diff --git a/src/qml/qml/qqmlvmemetaobject.cpp b/src/qml/qml/qqmlvmemetaobject.cpp
index ecfde203c6..cc4ba091ce 100644
--- a/src/qml/qml/qqmlvmemetaobject.cpp
+++ b/src/qml/qml/qqmlvmemetaobject.cpp
@@ -832,15 +832,17 @@ v8::Handle<v8::Value> QQmlVMEMetaObject::readVarProperty(int id)
 {
     Q_ASSERT(id >= firstVarPropertyIndex);
 
-    ensureVarPropertiesAllocated();
-    return varProperties->Get(id - firstVarPropertyIndex);
+    if (ensureVarPropertiesAllocated())
+        return varProperties->Get(id - firstVarPropertyIndex);
+    return v8::Handle<v8::Value>();
 }
 
 QVariant QQmlVMEMetaObject::readPropertyAsVariant(int id)
 {
     if (id >= firstVarPropertyIndex) {
-        ensureVarPropertiesAllocated();
-        return QQmlEnginePrivate::get(ctxt->engine)->v8engine()->toVariant(varProperties->Get(id - firstVarPropertyIndex), -1);
+        if (ensureVarPropertiesAllocated())
+            return QQmlEnginePrivate::get(ctxt->engine)->v8engine()->toVariant(varProperties->Get(id - firstVarPropertyIndex), -1);
+        return QVariant();
     } else {
         if (data[id].dataType() == QMetaType::QObjectStar) {
             return QVariant::fromValue(data[id].asQObject());
@@ -853,7 +855,8 @@ QVariant QQmlVMEMetaObject::readPropertyAsVariant(int id)
 void QQmlVMEMetaObject::writeVarProperty(int id, v8::Handle<v8::Value> value)
 {
     Q_ASSERT(id >= firstVarPropertyIndex);
-    ensureVarPropertiesAllocated();
+    if (!ensureVarPropertiesAllocated())
+        return;
 
     // Importantly, if the current value is a scarce resource, we need to ensure that it
     // gets automatically released by the engine if no other references to it exist.
@@ -882,7 +885,8 @@ void QQmlVMEMetaObject::writeVarProperty(int id, v8::Handle<v8::Value> value)
 void QQmlVMEMetaObject::writeProperty(int id, const QVariant &value)
 {
     if (id >= firstVarPropertyIndex) {
-        ensureVarPropertiesAllocated();
+        if (!ensureVarPropertiesAllocated())
+            return;
 
         // Importantly, if the current value is a scarce resource, we need to ensure that it
         // gets automatically released by the engine if no other references to it exist.
@@ -1029,10 +1033,17 @@ void QQmlVMEMetaObject::setVMEProperty(int index, v8::Handle<v8::Value> v)
     return writeVarProperty(index - propOffset, v);
 }
 
-void QQmlVMEMetaObject::ensureVarPropertiesAllocated()
+bool QQmlVMEMetaObject::ensureVarPropertiesAllocated()
 {
     if (!varPropertiesInitialized)
         allocateVarPropertiesArray();
+
+    // in some situations, the QObject's v8object (and associated v8 data,
+    // such as the varProperties array) will have been cleaned up, but the
+    // QObject ptr will not yet have been deleted (eg, waiting on deleteLater).
+    // In this situation, the varProperties handle will be (and should remain)
+    // empty.
+    return !varProperties.IsEmpty();
 }
 
 // see also: QV8GCCallback::garbageCollectorPrologueCallback()
diff --git a/src/qml/qml/qqmlvmemetaobject_p.h b/src/qml/qml/qqmlvmemetaobject_p.h
index 84c88fdd7b..1b5ceb8203 100644
--- a/src/qml/qml/qqmlvmemetaobject_p.h
+++ b/src/qml/qml/qqmlvmemetaobject_p.h
@@ -195,7 +195,7 @@ private:
     static void VarPropertiesWeakReferenceCallback(v8::Persistent<v8::Value> object, void* parameter);
     static void GcPrologueCallback(QV8GCCallback::Node *node);
     inline void allocateVarPropertiesArray();
-    inline void ensureVarPropertiesAllocated();
+    inline bool ensureVarPropertiesAllocated();
 
     void connectAlias(int aliasId);
     QBitArray aConnected;
diff --git a/src/qml/qml/v8/qv8qobjectwrapper.cpp b/src/qml/qml/v8/qv8qobjectwrapper.cpp
index a483346dd1..3faea2c97b 100644
--- a/src/qml/qml/v8/qv8qobjectwrapper.cpp
+++ b/src/qml/qml/v8/qv8qobjectwrapper.cpp
@@ -883,8 +883,10 @@ static void WeakQObjectReferenceCallback(v8::Persistent<v8::Value> handle, void
         QQmlData *ddata = QQmlData::get(object, false);
         if (ddata) {
             ddata->v8object.Clear();
-            if (!object->parent() && !ddata->indestructible)
+            if (!object->parent() && !ddata->indestructible) {
+                ddata->isQueuedForDeletion = true;
                 object->deleteLater();
+            }
         }
     }
 
@@ -1043,12 +1045,15 @@ v8::Handle<v8::Value> QV8QObjectWrapper::newQObject(QObject *object)
 
     if (QObjectPrivate::get(object)->wasDeleted)
        return v8::Null();
-    
+
     QQmlData *ddata = QQmlData::get(object, true);
 
     if (!ddata) 
         return v8::Undefined();
 
+    if (ddata->isQueuedForDeletion)
+        return v8::Null();
+
     if (ddata->v8objectid == m_id && !ddata->v8object.IsEmpty()) {
         // We own the v8object 
         return v8::Local<v8::Object>::New(ddata->v8object);