Avoid adding widget to its own layout

Widgets and layouts added or inserted to a layout are checked for:
- Not being NULL
- Not being the parent widget of a layout or the layout itself,
  respectively

Without this commit, adding a widget to its own layout would result in a
CPU-hogging infinite loop. Now, a warning is written to stderr and the
add or insert function call is ignored.

The checks are implemented as public functions of QLayoutPrivate and
thus accessible in QLayout's descendants to be used in various
"addWidget", "insertWidget", etc functions.

Unlike 'classical' layouts like QGridLayout, QFormLayout does indeed
accept widgets that are NULL. To not break this behavior, any call for
the check functions first tests if the widget or layout, respectively,
to test is NULL or not and calls the check only in the latter case.

Automated tests for QBoxLayout, QGridLayout, and QFormLayout were added.
For an unpatched Qt 5.3, each of those automated tests will freeze as
explained in QTBUG-40609. For a fixed version, warning messages about
invalid parameters to addWidget/addLayout/... calls will be read by
QTest::ignoreMessage, resulting in a passed test.

Change-Id: I1522d5727e643da3f7c025755975aca9f482676d
Task-number: QTBUG-40609
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

1 files changed, 41 insertions, 0 deletions

diff --git a/src/widgets/kernel/qlayout.cpp b/src/widgets/kernel/qlayout.cpp
index eb21a580b1..778514f47a 100644
--- a/src/widgets/kernel/qlayout.cpp
+++ b/src/widgets/kernel/qlayout.cpp
@@ -858,6 +858,47 @@ void QLayoutPrivate::reparentChildWidgets(QWidget *mw)
 }
 
 /*!
+    Returns \c true if the \a widget can be added to the \a layout;
+    otherwise returns \c false.
+*/
+bool QLayoutPrivate::checkWidget(QWidget *widget) const
+{
+    Q_Q(const QLayout);
+    if (!widget) {
+        qWarning("QLayout: Cannot add a null widget to %s/%s", q->metaObject()->className(),
+                  qPrintable(q->objectName()));
+        return false;
+    }
+    if (widget == q->parentWidget()) {
+        qWarning("QLayout: Cannot add parent widget %s/%s to its child layout %s/%s",
+                  widget->metaObject()->className(), qPrintable(widget->objectName()),
+                  q->metaObject()->className(), qPrintable(q->objectName()));
+        return false;
+    }
+    return true;
+}
+
+/*!
+    Returns \c true if the \a otherLayout can be added to the \a layout;
+    otherwise returns \c false.
+*/
+bool QLayoutPrivate::checkLayout(QLayout *otherLayout) const
+{
+    Q_Q(const QLayout);
+    if (!otherLayout) {
+        qWarning("QLayout: Cannot add a null layout to %s/%s", q->metaObject()->className(),
+                  qPrintable(q->objectName()));
+        return false;
+    }
+    if (otherLayout == q) {
+        qWarning("QLayout: Cannot add layout %s/%s to itself", q->metaObject()->className(),
+                  qPrintable(q->objectName()));
+        return false;
+    }
+    return true;
+}
+
+/*!
     This function is called from \c addWidget() functions in
     subclasses to add \a w as a managed widget of a layout.