QAbstractItemModelPrivate: add resetting member

This allows QQmlDelegateModel to know if a QAbstractItemModel subclass
is in the process of a reset, which it can't know if beginResetModel
was called in the model's constructor.

As an added bonus, it also allows us to warn the user if they call
endResetModel with a previous call to beginResetModel.

Task-number: QTBUG-125053
Task-number: QTBUG-127340
Pick-to: 6.5
Change-Id: I7d1fb983e9bf868c48472624ad945ae158115943
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>
(cherry picked from commit 9d8663c)
(cherry picked from commit 2ea3abe)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/itemmodels/qabstractitemmodel.cpp

@@ -3395,6 +3395,13 @@ void QAbstractItemModel::endMoveColumns()
 */
 void QAbstractItemModel::beginResetModel()
 {
+    Q_D(QAbstractItemModel);
+    if (d->resetting) {
+        qWarning() << "beginResetModel called on" << this << "without calling endResetModel first";
+        // Warn, but don't return early in case user code relies on the incorrect behavior.
+    }
+
+    d->resetting = true;
     emit modelAboutToBeReset(QPrivateSignal());
 }
 
@@ -3412,8 +3419,14 @@ void QAbstractItemModel::beginResetModel()
 void QAbstractItemModel::endResetModel()
 {
     Q_D(QAbstractItemModel);
+    if (!d->resetting) {
+        qWarning() << "endResetModel called on" << this << "without calling beginResetModel first";
+        // Warn, but don't return early in case user code relies on the incorrect behavior.
+    }
+
     d->invalidatePersistentIndexes();
     resetInternalData();
+    d->resetting = false;
     emit modelReset(QPrivateSignal());
 }
 

src/corelib/itemmodels/qabstractitemmodel_p.h

@@ -45,6 +45,8 @@ class Q_CORE_EXPORT QAbstractItemModelPrivate : public QObjectPrivate
     QAbstractItemModelPrivate();
     ~QAbstractItemModelPrivate();
 
+    static const QAbstractItemModelPrivate *get(const QAbstractItemModel *model) { return model->d_func(); }
+
     void removePersistentIndexData(QPersistentModelIndexData *data);
     void movePersistentIndexes(const QList<QPersistentModelIndexData *> &indexes, int change, const QModelIndex &parent,
                                Qt::Orientation orientation);
@@ -115,6 +117,8 @@ class Q_CORE_EXPORT QAbstractItemModelPrivate : public QObjectPrivate
         void insertMultiAtEnd(const QModelIndex& key, QPersistentModelIndexData *data);
     } persistent;
 
+    bool resetting = false;
+
     static const QHash<int,QByteArray> &defaultRoleNames();
     static bool isVariantLessThan(const QVariant &left, const QVariant &right,
                                   Qt::CaseSensitivity cs = Qt::CaseSensitive, bool isLocaleAware = false);