Fix TreeModel emitting nested QAbstractItemModel signals

This led to an assert in QSFPM because it got confused by the
nested reset signals.

Detected by adding
new QAbstractItemModelTester(this,
QAbstractItemModelTester::FailureReportingMode::Fatal);
to the OutlineModel constructor, and just opening QtCreator on a small
qmake-based project.

Fixes: QTCREATORBUG-30035
Change-Id: I41dbc81b5a2275521ece6b865115e1428e07ecf7
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: hjk <hjk@qt.io>
(cherry picked from commit 9124833a63)
Reviewed-by: David Faure <david.faure@kdab.com>

src/libs/utils/treemodel.cpp

@@ -604,7 +604,7 @@ TreeItem::~TreeItem()
 {
     QTC_CHECK(m_parent == nullptr);
     QTC_CHECK(m_model == nullptr);
-    removeChildren();
+    removeChildrenSilently();
 }
 
 TreeItem *TreeItem::childAt(int pos) const
@@ -714,6 +714,13 @@ void TreeItem::removeChildren()
     }
 }
 
+void TreeItem::removeChildrenSilently()
+{
+    if (childCount() == 0)
+        return;
+    clear();
+}
+
 void TreeItem::sortChildren(const std::function<bool(const TreeItem *, const TreeItem *)> &cmp)
 {
     if (m_model) {
@@ -1060,6 +1067,13 @@ TreeItem *BaseTreeModel::rootItem() const
 }
 
 void BaseTreeModel::setRootItem(TreeItem *item)
+{
+    beginResetModel();
+    setRootItemInternal(item);
+    endResetModel();
+}
+
+void BaseTreeModel::setRootItemInternal(TreeItem *item)
 {
     QTC_ASSERT(item, return);
     QTC_ASSERT(item->m_model == nullptr, return);
@@ -1067,19 +1081,17 @@ void BaseTreeModel::setRootItem(TreeItem *item)
     QTC_ASSERT(item != m_root, return);
     QTC_CHECK(m_root);
 
-    beginResetModel();
     if (m_root) {
         QTC_CHECK(m_root->m_parent == nullptr);
         QTC_CHECK(m_root->m_model == this);
         // needs to be done explicitly before setting the model to 0, otherwise it might lead to a
         // crash inside a view or proxy model, especially if there are selected items
-        m_root->removeChildren();
+        m_root->removeChildrenSilently();
         m_root->m_model = nullptr;
         delete m_root;
     }
     m_root = item;
     item->propagateModel(this);
-    endResetModel();
 }
 
 void BaseTreeModel::setHeader(const QStringList &displays)

src/libs/utils/treemodel.h

@@ -39,6 +39,7 @@ public:
 
     void removeChildAt(int pos);
     void removeChildren();
+    void removeChildrenSilently();
     void sortChildren(const std::function<bool(const TreeItem *, const TreeItem *)> &cmp);
     void update();
     void updateAll();
@@ -180,7 +181,8 @@ protected:
     void clear();
 
     TreeItem *rootItem() const;
-    void setRootItem(TreeItem *item);
+    void setRootItem(TreeItem *item); // resets the model
+    void setRootItemInternal(TreeItem *item);
     TreeItem *itemForIndex(const QModelIndex &) const;
     QModelIndex indexForItem(const TreeItem *needle) const;
 

src/plugins/cppeditor/cppoutlinemodel.cpp

@@ -219,7 +219,7 @@ void OutlineModel::rebuild()
     auto root = new SymbolItem;
     if (m_cppDocument)
         buildTree(root, true);
-    setRootItem(root);
+    setRootItemInternal(root);
     endResetModel();
 }