Fix interaction between head room and appending in arrays

We reserve space on both ends of the JS array for appending
and prepending. Make sure they interact well with each other
and don't cause any memory corruption.

Task-number: QTBUG-34853
Change-Id: I184280178690e3cb12ab9b199a8436b32383af38
Reviewed-by: Simon Hausmann <[email protected]>

4 files changed, 20 insertions, 7 deletions

diff --git a/src/qml/jsruntime/qv4arrayobject.cpp b/src/qml/jsruntime/qv4arrayobject.cpp
index 5422bff800..a0f0345b8b 100644
--- a/src/qml/jsruntime/qv4arrayobject.cpp
+++ b/src/qml/jsruntime/qv4arrayobject.cpp
@@ -572,6 +572,7 @@ ReturnedValue ArrayPrototype::method_unshift(CallContext *ctx)
                 --instance->arrayOffset;
                 --instance->arrayData;
                 ++instance->arrayDataLen;
+                ++instance->arrayAlloc;
                 if (instance->arrayAttributes) {
                     --instance->arrayAttributes;
                     *instance->arrayAttributes = Attr_Data;
diff --git a/src/qml/jsruntime/qv4object.cpp b/src/qml/jsruntime/qv4object.cpp
index 099b5a5480..df2f6ca3f4 100644
--- a/src/qml/jsruntime/qv4object.cpp
+++ b/src/qml/jsruntime/qv4object.cpp
@@ -1319,19 +1319,17 @@ void Object::arrayReserve(uint n)
             off = arrayOffset;
         }
         arrayAlloc = qMax(n, 2*arrayAlloc);
-        Property *newArrayData = new Property[arrayAlloc];
+        Property *newArrayData = new Property[arrayAlloc + off];
         if (arrayData) {
-            memcpy(newArrayData, arrayData, sizeof(Property)*arrayDataLen);
+            memcpy(newArrayData + off, arrayData, sizeof(Property)*arrayDataLen);
             delete [] (arrayData - off);
         }
-        arrayData = newArrayData;
+        arrayData = newArrayData + off;
         if (sparseArray) {
             for (uint i = arrayFreeList; i < arrayAlloc; ++i) {
                 arrayData[i].value = Primitive::emptyValue();
                 arrayData[i].value = Primitive::fromInt32(i + 1);
             }
-        } else {
-            arrayOffset = 0;
         }
 
         if (arrayAttributes) {
@@ -1354,7 +1352,9 @@ void Object::ensureArrayAttributes()
         return;
 
     flags &= ~SimpleArray;
-    arrayAttributes = new PropertyAttributes[arrayAlloc];
+    uint off = sparseArray ? 0 : arrayOffset;
+    arrayAttributes = new PropertyAttributes[arrayAlloc + off];
+    arrayAttributes += off;
     for (uint i = 0; i < arrayDataLen; ++i)
         arrayAttributes[i] = Attr_Data;
     for (uint i = arrayDataLen; i < arrayAlloc; ++i)
diff --git a/src/qml/jsruntime/qv4object_p.h b/src/qml/jsruntime/qv4object_p.h
index 0d5955a0e7..daef18d4e2 100644
--- a/src/qml/jsruntime/qv4object_p.h
+++ b/src/qml/jsruntime/qv4object_p.h
@@ -210,7 +210,6 @@ struct Q_QML_EXPORT Object: Managed {
             delete [] arrayAttributes;
             arrayAttributes = newAttrs + arrayOffset;
         }
-        arrayAlloc += arrayOffset;
     }
 
 public:
diff --git a/tests/auto/qml/qqmlecmascript/tst_qqmlecmascript.cpp b/tests/auto/qml/qqmlecmascript/tst_qqmlecmascript.cpp
index 95f03d2c5b..7c1eb7d54b 100644
--- a/tests/auto/qml/qqmlecmascript/tst_qqmlecmascript.cpp
+++ b/tests/auto/qml/qqmlecmascript/tst_qqmlecmascript.cpp
@@ -307,6 +307,7 @@ private slots:
     void updateCall();
     void numberParsing();
     void stringParsing();
+    void push_and_shift();
     void qtbug_32801();
     void thisObject();
     void qtbug_33754();
@@ -7318,6 +7319,18 @@ void tst_qqmlecmascript::stringParsing()
     }
 }
 
+void tst_qqmlecmascript::push_and_shift()
+{
+    QJSEngine e;
+    const QString program =
+            "var array = []; "
+            "for (var i = 0; i < 10000; i++) {"
+            "    array.push(5); array.unshift(5); array.push(5);"
+            "}"
+            "array.length;";
+    QVERIFY(e.evaluate(program).toNumber() == 30000);
+}
+
 void tst_qqmlecmascript::qtbug_32801()
 {
     QQmlComponent component(&engine, testFileUrl("qtbug_32801.qml"));