Fix QLabel crash if buddy's lifetime is too short

If buddy is deleted then QLabel will still try to deliver the
shortcut event to it, in QLabel::event(), and cras.

Besides connecting to destroyed() which alone fixes the crash,
also hardened it and guarded against dereferencing null buddy,
in the unlikely event of someone adding features to QLabel.

The second part is suitable for backporting to the LTS branches.

Task-Id: QTBUG-66841
Change-Id: Ib6a36a9f99cee31afe1f88eb77f5b6cafb8b1086
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

3 files changed, 22 insertions, 1 deletions

diff --git a/src/widgets/widgets/qlabel.cpp b/src/widgets/widgets/qlabel.cpp
index ee4825d3e5..046acfeea9 100644
--- a/src/widgets/widgets/qlabel.cpp
+++ b/src/widgets/widgets/qlabel.cpp
@@ -963,7 +963,9 @@ bool QLabel::event(QEvent *e)
     if (type == QEvent::Shortcut) {
         QShortcutEvent *se = static_cast<QShortcutEvent *>(e);
         if (se->shortcutId() == d->shortcutId) {
-            QWidget * w = d->buddy;
+            QWidget *w = d->buddy;
+            if (!w)
+                return QFrame::event(e);
             if (w->focusPolicy() != Qt::NoFocus)
                 w->setFocus(Qt::ShortcutFocusReason);
 #if QT_CONFIG(abstractbutton)
@@ -1163,7 +1165,15 @@ void QLabelPrivate::updateLabel()
 void QLabel::setBuddy(QWidget *buddy)
 {
     Q_D(QLabel);
+
+    if (d->buddy)
+        disconnect(d->buddy, SIGNAL(destroyed()), this, SLOT(_q_buddyDeleted()));
+
     d->buddy = buddy;
+
+    if (buddy)
+        connect(buddy, SIGNAL(destroyed()), this, SLOT(_q_buddyDeleted()));
+
     if (d->isTextLabel) {
         if (d->shortcutId)
             releaseShortcut(d->shortcutId);
@@ -1204,6 +1214,13 @@ void QLabelPrivate::updateShortcut()
     shortcutId = q->grabShortcut(QKeySequence::mnemonic(text));
 }
 
+
+void QLabelPrivate::_q_buddyDeleted()
+{
+    Q_Q(QLabel);
+    q->setBuddy(nullptr);
+}
+
 #endif // QT_NO_SHORTCUT
 
 #if QT_CONFIG(movie)
diff --git a/src/widgets/widgets/qlabel.h b/src/widgets/widgets/qlabel.h
index e1cc333a1c..2f5db5a7d3 100644
--- a/src/widgets/widgets/qlabel.h
+++ b/src/widgets/widgets/qlabel.h
@@ -158,6 +158,9 @@ private:
 #endif
     Q_PRIVATE_SLOT(d_func(), void _q_linkHovered(const QString &))
 
+#ifndef QT_NO_SHORTCUT
+    Q_PRIVATE_SLOT(d_func(), void _q_buddyDeleted())
+#endif
     friend class QTipLabel;
     friend class QMessageBoxPrivate;
     friend class QBalloonTip;
diff --git a/src/widgets/widgets/qlabel_p.h b/src/widgets/widgets/qlabel_p.h
index e05a5b5c35..59188563a9 100644
--- a/src/widgets/widgets/qlabel_p.h
+++ b/src/widgets/widgets/qlabel_p.h
@@ -89,6 +89,7 @@ public:
 #endif
 #ifndef QT_NO_SHORTCUT
     void updateShortcut();
+    void _q_buddyDeleted();
 #endif
     inline bool needTextControl() const {
         return isTextLabel