Fix memory leak when lazily binding QScriptValue to an engine

Avoid the engine's list of free script values from growing
without bounds.

When a QScriptValue is initially not bound, its private will
be allocated from the normal heap (and not from the engine's
pool of privates, because there is no engine at this point).

But when a value is later bound (e.g. by setting it as a
property of an object, or by passing it as argument to
QScriptValue::call()) and is subsequently destroyed, its
private will be handed to the engine, which will add it to
its free-list (hence the memory is not freed). This
allocation/deallocation asymmetry causes this list go keep
growing.

The solution is to limit the size of the free-list, and free
the memory of the private immediately when the list has
reached a certain size.

Task-number: QTBUG-8400
Reviewed-by: Olivier Goffart

2 files changed, 11 insertions, 3 deletions

diff --git a/src/script/api/qscriptengine.cpp b/src/script/api/qscriptengine.cpp
index 1bd7377..1199263 100644
--- a/src/script/api/qscriptengine.cpp
+++ b/src/script/api/qscriptengine.cpp
@@ -768,7 +768,7 @@ static QScriptValue __setupPackage__(QScriptContext *ctx, QScriptEngine *eng)
 } // namespace QScript
 
 QScriptEnginePrivate::QScriptEnginePrivate()
-    : registeredScriptValues(0), freeScriptValues(0),
+    : registeredScriptValues(0), freeScriptValues(0), freeScriptValuesCount(0),
       registeredScriptStrings(0), inEval(false)
 {
     qMetaTypeId<QScriptValue>();
diff --git a/src/script/api/qscriptengine_p.h b/src/script/api/qscriptengine_p.h
index 6780b2c..401d6d2 100644
--- a/src/script/api/qscriptengine_p.h
+++ b/src/script/api/qscriptengine_p.h
@@ -255,6 +255,8 @@ public:
     int agentLineNumber;
     QScriptValuePrivate *registeredScriptValues;
     QScriptValuePrivate *freeScriptValues;
+    static const int maxFreeScriptValues = 256;
+    int freeScriptValuesCount;
     QScriptStringPrivate *registeredScriptStrings;
     QHash<int, QScriptTypeInfo*> m_typeInfos;
     int processEventsInterval;
@@ -377,6 +379,7 @@ inline QScriptValuePrivate *QScriptEnginePrivate::allocateScriptValuePrivate(siz
     if (freeScriptValues) {
         QScriptValuePrivate *p = freeScriptValues;
         freeScriptValues = p->next;
+        --freeScriptValuesCount;
         return p;
     }
     return reinterpret_cast<QScriptValuePrivate*>(qMalloc(size));
@@ -384,8 +387,13 @@ inline QScriptValuePrivate *QScriptEnginePrivate::allocateScriptValuePrivate(siz
 
 inline void QScriptEnginePrivate::freeScriptValuePrivate(QScriptValuePrivate *p)
 {
-    p->next = freeScriptValues;
-    freeScriptValues = p;
+    if (freeScriptValuesCount < maxFreeScriptValues) {
+        p->next = freeScriptValues;
+        freeScriptValues = p;
+        ++freeScriptValuesCount;
+    } else {
+        qFree(p);
+    }
 }
 
 inline void QScriptEnginePrivate::registerScriptValue(QScriptValuePrivate *value)