QVariant: introduce ShouldDeleteVariantData flag

This flag is used in QSequentialIterable and QAssociativeIterable to indicate
that the data pointer in VariantData should be deleted after the variant has
been constructed.

The use case for this is
https://codereview.qt-project.org/c/qt/qtdeclarative/+/284151, where we have
a proxy iterator and cannot easily return a pointer to already owned data, as
it is hard to manage its lifetime in the iterator. In contrast, it is clear
that we can release the memory in the QSequentialIterable functions, as it has
already been copied into the QVariant there.

Change-Id: I2b33497d991cd4f752153e0ebda767b82e4bb851
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

3 files changed, 117 insertions, 20 deletions

diff --git a/src/corelib/kernel/qvariant.cpp b/src/corelib/kernel/qvariant.cpp
index 84ad555f34..a1e1c71d12 100644
--- a/src/corelib/kernel/qvariant.cpp
+++ b/src/corelib/kernel/qvariant.cpp
@@ -4520,15 +4520,24 @@ QSequentialIterable::const_iterator QSequentialIterable::end() const
     return it;
 }
 
+static const QVariant variantFromVariantDataHelper(const QtMetaTypePrivate::VariantData &d) {
+    QVariant v;
+    if (d.metaTypeId == qMetaTypeId<QVariant>())
+        v =  *reinterpret_cast<const QVariant*>(d.data);
+    else
+        v = QVariant(d.metaTypeId, d.data, d.flags & ~QVariantConstructionFlags::ShouldDeleteVariantData);
+    if (d.flags & QVariantConstructionFlags::ShouldDeleteVariantData)
+        QMetaType::destroy(d.metaTypeId, const_cast<void *>(d.data));
+    return v;
+}
+
 /*!
     Returns the element at position \a idx in the container.
 */
 QVariant QSequentialIterable::at(int idx) const
 {
     const QtMetaTypePrivate::VariantData d = m_impl.at(idx);
-    if (d.metaTypeId == qMetaTypeId<QVariant>())
-        return *reinterpret_cast<const QVariant*>(d.data);
-    return QVariant(d.metaTypeId, d.data, d.flags);
+    return variantFromVariantDataHelper(d);
 }
 
 /*!
@@ -4605,9 +4614,7 @@ QSequentialIterable::const_iterator::operator=(const const_iterator &other)
 const QVariant QSequentialIterable::const_iterator::operator*() const
 {
     const QtMetaTypePrivate::VariantData d = m_impl.getCurrent();
-    if (d.metaTypeId == qMetaTypeId<QVariant>())
-        return *reinterpret_cast<const QVariant*>(d.data);
-    return QVariant(d.metaTypeId, d.data, d.flags);
+    return variantFromVariantDataHelper(d);
 }
 
 /*!
@@ -4939,10 +4946,7 @@ QAssociativeIterable::const_iterator::operator=(const const_iterator &other)
 const QVariant QAssociativeIterable::const_iterator::operator*() const
 {
     const QtMetaTypePrivate::VariantData d = m_impl.getCurrentValue();
-    QVariant v(d.metaTypeId, d.data, d.flags);
-    if (d.metaTypeId == qMetaTypeId<QVariant>())
-        return *reinterpret_cast<const QVariant*>(d.data);
-    return v;
+    return variantFromVariantDataHelper(d);
 }
 
 /*!
@@ -4951,10 +4955,7 @@ const QVariant QAssociativeIterable::const_iterator::operator*() const
 const QVariant QAssociativeIterable::const_iterator::key() const
 {
     const QtMetaTypePrivate::VariantData d = m_impl.getCurrentKey();
-    QVariant v(d.metaTypeId, d.data, d.flags);
-    if (d.metaTypeId == qMetaTypeId<QVariant>())
-        return *reinterpret_cast<const QVariant*>(d.data);
-    return v;
+    return variantFromVariantDataHelper(d);
 }
 
 /*!
@@ -4962,11 +4963,7 @@ const QVariant QAssociativeIterable::const_iterator::key() const
 */
 const QVariant QAssociativeIterable::const_iterator::value() const
 {
-    const QtMetaTypePrivate::VariantData d = m_impl.getCurrentValue();
-    QVariant v(d.metaTypeId, d.data, d.flags);
-    if (d.metaTypeId == qMetaTypeId<QVariant>())
-        return *reinterpret_cast<const QVariant*>(d.data);
-    return v;
+    return operator*();
 }
 
 /*!
diff --git a/src/corelib/kernel/qvariant_p.h b/src/corelib/kernel/qvariant_p.h
index 3d87beac83..b8b63b5e6f 100644
--- a/src/corelib/kernel/qvariant_p.h
+++ b/src/corelib/kernel/qvariant_p.h
@@ -105,6 +105,11 @@ inline T *v_cast(QVariant::Private *d, T * = nullptr)
 
 #endif
 
+enum QVariantConstructionFlags : uint {
+    Default = 0x0,
+    PointerType = 0x1,
+    ShouldDeleteVariantData = 0x2 // only used in Q*Iterable
+};
 
 //a simple template that avoids to allocate 2 memory chunks when creating a QVariant
 template <class T> class QVariantPrivateSharedEx : public QVariant::PrivateShared
diff --git a/tests/auto/corelib/kernel/qvariant/tst_qvariant.cpp b/tests/auto/corelib/kernel/qvariant/tst_qvariant.cpp
index edb15a8db2..e719871128 100644
--- a/tests/auto/corelib/kernel/qvariant/tst_qvariant.cpp
+++ b/tests/auto/corelib/kernel/qvariant/tst_qvariant.cpp
@@ -31,6 +31,7 @@
 #include <QtTest/QtTest>
 
 #include <qvariant.h>
+#include <QtCore/private/qvariant_p.h>
 #include <qbitarray.h>
 #include <qbytearraylist.h>
 #include <qdatetime.h>
@@ -276,7 +277,8 @@ private slots:
     void nullConvert();
 
     void accessSequentialContainerKey();
-
+    void shouldDeleteVariantDataWorksForSequential();
+    void shouldDeleteVariantDataWorksForAssociative();
     void fromStdVariant();
     void qt4UuidDataStream();
 
@@ -4990,6 +4992,99 @@ void tst_QVariant::accessSequentialContainerKey()
     QCOMPARE(nameResult, QStringLiteral("Seven"));
 }
 
+void tst_QVariant::shouldDeleteVariantDataWorksForSequential()
+{
+    QCOMPARE(instanceCount, 0);
+    {
+        QtMetaTypePrivate::QSequentialIterableImpl iterator {};
+        iterator._iteratorCapabilities = QtMetaTypePrivate::RandomAccessCapability |
+                                         QtMetaTypePrivate::BiDirectionalCapability |
+                                         QtMetaTypePrivate::ForwardCapability;
+        iterator._metaType_flags = QVariantConstructionFlags::ShouldDeleteVariantData;
+
+        iterator._size = [](const void *) {return 1;};
+        iterator._metaType_id = qMetaTypeId<MyType>();
+        iterator._moveToBegin = [](const void *, void **) {};
+        iterator._moveToEnd = [](const void *, void **) {};
+        iterator._advance = [](void **, int) {};
+        iterator._destroyIter = [](void **){};
+        iterator._equalIter = [](void * const *, void * const *){return true; /*all iterators are nullptr*/};
+        iterator._destroyIter = [](void **){};
+        iterator._at = [](const void *, int ) -> void const * {
+            MyType mytype {1, "eins"};
+            return QMetaType::create(qMetaTypeId<MyType>(), &mytype);
+        };
+        iterator._get = [](void * const *, int, uint) -> QtMetaTypePrivate::VariantData {
+            MyType mytype {2, "zwei"};
+            return {qMetaTypeId<MyType>(), QMetaType::create(qMetaTypeId<MyType>(), &mytype),  QVariantConstructionFlags::ShouldDeleteVariantData};
+        };
+        QSequentialIterable iterable {iterator};
+        QVariant value1 = iterable.at(0);
+        QVERIFY(value1.canConvert<MyType>());
+        QCOMPARE(value1.value<MyType>().number, 1);
+        QVariant value2 = *iterable.begin();
+        QVERIFY(value2.canConvert<MyType>());
+        QCOMPARE(value2.value<MyType>().number, 2);
+    }
+    QCOMPARE(instanceCount, 0);
+}
+
+void tst_QVariant::shouldDeleteVariantDataWorksForAssociative()
+{
+    QCOMPARE(instanceCount, 0);
+    {
+        QtMetaTypePrivate::QAssociativeIterableImpl iterator {};
+        iterator._metaType_flags_key = QVariantConstructionFlags::ShouldDeleteVariantData;
+        iterator._metaType_flags_value = QVariantConstructionFlags::ShouldDeleteVariantData;
+
+        iterator._size = [](const void *) {return 1;};
+        iterator._metaType_id_value = qMetaTypeId<MyType>();
+        iterator._metaType_id_key = qMetaTypeId<MyType>();
+        iterator._begin = [](const void *, void **) {};
+        iterator._end = [](const void *, void **) {};
+        iterator._advance = [](void **, int) {};
+        iterator._destroyIter = [](void **){};
+        iterator._equalIter = [](void * const *, void * const *){return true; /*all iterators are nullptr*/};
+        iterator._destroyIter = [](void **){};
+        iterator._find = [](const void *, const void *, void **iterator ) -> void  {
+            (*iterator) =  reinterpret_cast<void *>(quintptr(42));
+        };
+        iterator._getKey = [](void * const *iterator, int, uint) -> QtMetaTypePrivate::VariantData {
+            MyType mytype {1, "key"};
+            if (reinterpret_cast<quintptr>(*iterator) == 42) {
+                mytype.number = 42;
+                mytype.text = "find_key";
+            }
+            return {qMetaTypeId<MyType>(), QMetaType::create(qMetaTypeId<MyType>(), &mytype),  QVariantConstructionFlags::ShouldDeleteVariantData};
+        };
+        iterator._getValue = [](void * const *iterator, int, uint) -> QtMetaTypePrivate::VariantData {
+            MyType mytype {2, "value"};
+            if (reinterpret_cast<quintptr>(*iterator) == 42) {
+                mytype.number = 42;
+                mytype.text = "find_value";
+            }
+            return {qMetaTypeId<MyType>(), QMetaType::create(qMetaTypeId<MyType>(), &mytype),  QVariantConstructionFlags::ShouldDeleteVariantData};
+        };
+        QAssociativeIterable iterable {iterator};
+        auto it = iterable.begin();
+        QVariant value1 = it.key();
+        QVERIFY(value1.canConvert<MyType>());
+        QCOMPARE(value1.value<MyType>().number, 1);
+        QCOMPARE(value1.value<MyType>().text, "key");
+        QVariant value2 = it.value();
+        QVERIFY(value2.canConvert<MyType>());
+        QCOMPARE(value2.value<MyType>().number, 2);
+        auto findIt = iterable.find(QVariant::fromValue(MyType {}));
+        value1 = findIt.key();
+        QCOMPARE(value1.value<MyType>().number, 42);
+        QCOMPARE(value1.value<MyType>().text, "find_key");
+        value2 = findIt.value();
+        QCOMPARE(value2.value<MyType>().number, 42);
+        QCOMPARE(value2.value<MyType>().text, "find_value");
+    }
+    QCOMPARE(instanceCount, 0);
+}
+
 void tst_QVariant::fromStdVariant()
 {
 #if __has_include(<variant>) && __cplusplus >= 201703L