Re-add QPropertyAlias functionality

As QPropertyAlias was public by accident in 6.0, we have to ensure that
it still works in 6.2.
This re-adds some tests for it, and reimplements the unlinking
functionality. To avoid performance regressions in hot-paths,
a new unlink_fast function is added, which behaves like the old unlink:
It ignores the special handling for QPropertyAlias, so that we can skip
the tag check. It is only used in QPropertyObserverNodeProtector and
clearDependencyObservers, where we already know the type of the
observer.

Fixes: QTBUG-95846
Change-Id: Ifb405b8327c4d61c673b1a912ed6e169d27c2d8f
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>
(cherry picked from commit e4d62651c278c468f71ce097979bc1183ffd0713)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

1 files changed, 34 insertions, 9 deletions

diff --git a/src/corelib/kernel/qproperty.cpp b/src/corelib/kernel/qproperty.cpp
index 3123858d83..942abfe6ba 100644
--- a/src/corelib/kernel/qproperty.cpp
+++ b/src/corelib/kernel/qproperty.cpp
@@ -558,9 +558,10 @@ QPropertyObserver::QPropertyObserver(ChangeHandler changeHandler)
     d.setChangeHandler(changeHandler);
 }
 
-QPropertyObserver::QPropertyObserver(QUntypedPropertyData *)
+QPropertyObserver::QPropertyObserver(QUntypedPropertyData *data)
 {
-    // ### Qt 7: Remove, currently left for binary compatibility
+    aliasData = data;
+    next.setTag(ObserverIsAlias);
 }
 
 /*! \internal
@@ -609,16 +610,38 @@ QPropertyObserver &QPropertyObserver::operator=(QPropertyObserver &&other) noexc
     return *this;
 }
 
+#define UNLINK_COMMON \
+    if (ptr->next) \
+        ptr->next->prev = ptr->prev; \
+    if (ptr->prev) \
+        ptr->prev.setPointer(ptr->next.data()); \
+    ptr->next = nullptr; \
+    ptr->prev.clear();
+
+/*!
+    \internal
+    Unlinks
+ */
 void QPropertyObserverPointer::unlink()
 {
-    if (ptr->next)
-        ptr->next->prev = ptr->prev;
-    if (ptr->prev)
-        ptr->prev.setPointer(ptr->next.data());
-    ptr->next = nullptr;
-    ptr->prev.clear();
+    UNLINK_COMMON
+    if (ptr->next.tag() == QPropertyObserver::ObserverIsAlias)
+        ptr->aliasData = nullptr;
 }
 
+/*!
+    \internal
+    Like unlink, but does not handle ObserverIsAlias.
+    Must only be called in places where we know that we are not dealing
+    with such an observer.
+ */
+void QPropertyObserverPointer::unlink_fast()
+{
+    Q_ASSERT(ptr->next.tag() != QPropertyObserver::ObserverIsAlias);
+    UNLINK_COMMON
+}
+#undef UNLINK_COMMON
+
 void QPropertyObserverPointer::setChangeHandler(QPropertyObserver::ChangeHandler changeHandler)
 {
     Q_ASSERT(ptr->next.tag() != QPropertyObserver::ObserverIsPlaceholder);
@@ -666,7 +689,7 @@ struct [[nodiscard]] QPropertyObserverNodeProtector {
 
     ~QPropertyObserverNodeProtector() {
         QPropertyObserverPointer d{static_cast<QPropertyObserver *>(&m_placeHolder)};
-        d.unlink();
+        d.unlink_fast();
     }
 };
 
@@ -721,6 +744,8 @@ void QPropertyObserverPointer::notify(QUntypedPropertyData *propertyDataPtr)
         case QPropertyObserver::ObserverIsPlaceholder:
             // recursion is already properly handled somewhere else
             break;
+        case QPropertyObserver::ObserverIsAlias:
+            break;
         default: Q_UNREACHABLE();
         }
         observer = next;