QProperty: Notify observers even when dependency is gone

Problem description:
--------------------
Assume we have two properties, P1 and P2. Assume further that we assign
a binding to P2, so that it depends on P1. Let the binding additionally
capture some (non-QProperty) boolean, and only create the dependency to
P1 if the boolean is true.

The state afterwards is
P1:[p1vaue|firstObserver]
                      |
                      |
                      v
                ---[p2binding]
	       /
P2:[p2value|binding]

If the boolean is set to false, and P1 changes its value, we still
correctly re-evaluate the binding and update P2's value. However, during
binding evaluation we will notice that there is no further dependency
from P2 on P1, and remove its observer.

The state afterwards is
P1:[p1vaue|firstObserver=nullptr]

                ---[p2binding]
	       /
P2:[p2value|binding]

Then, during the notify phase, we traverse the observer's again,
starting from P1's firstObserver. Given that it is nullptr now, we never
reach P2's binding, and thus won't send a notification from it.

Fix:
----

We store a list of all visited binding-observers (in a QVarLengthArray,
to avoid allocations as long as possible). After the binding evaluation
phase, we then use that list to send notifications from every binding
that we visited. As we already have a list of all bindings, we no longer
need to recurse on binding-observes during the notification process;
instead, we only need to deal with static callbacks and ChangeHandlers.

The pre-existing notification logic is still kept for the grouped update
case, where we already have a list of all delayed properties, and should
therefore not encounter the same issue. Unifying its codepath with the
existing logic is left as an exercise for a later patch.

Fixes: QTBUG-105204
Task-number: QTBUG-104982
Pick-to: 6.4 6.3 6.2
Change-Id: I2951f7d9597f4da0b8560a64dfb834f7ad86e757
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Sami Shalayel <sami.shalayel@qt.io>
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

1 files changed, 8 insertions, 2 deletions

diff --git a/src/corelib/kernel/qpropertyprivate.h b/src/corelib/kernel/qpropertyprivate.h
index ab69e966cf..1356a3702e 100644
--- a/src/corelib/kernel/qpropertyprivate.h
+++ b/src/corelib/kernel/qpropertyprivate.h
@@ -18,6 +18,7 @@
 #include <QtCore/qglobal.h>
 #include <QtCore/qtaggedpointer.h>
 #include <QtCore/qmetatype.h>
+#include <QtCore/qcontainerfwd.h>
 
 #include <functional>
 
@@ -28,6 +29,9 @@ class QBindingStorage;
 template<typename Class, typename T, auto Offset, auto Setter, auto Signal, auto Getter>
 class QObjectCompatProperty;
 
+struct QBindingObserverPtr;
+using PendingBindingObserverList = QVarLengthArray<QBindingObserverPtr>;
+
 namespace QtPrivate {
 // QPropertyBindingPrivatePtr operates on a RefCountingMixin solely so that we can inline
 // the constructor and copy constructor
@@ -115,6 +119,7 @@ private:
 class QUntypedPropertyBinding;
 class QPropertyBindingPrivate;
 struct QPropertyBindingDataPointer;
+class QPropertyObserver;
 struct QPropertyObserverPointer;
 
 class QUntypedPropertyData
@@ -308,8 +313,9 @@ private:
 
     enum NotificationResult { Delayed, Evaluated };
     NotificationResult notifyObserver_helper(
-            QUntypedPropertyData *propertyDataPtr, QPropertyObserverPointer observer,
-            QBindingStorage *storage) const;
+            QUntypedPropertyData *propertyDataPtr, QBindingStorage *storage,
+            QPropertyObserverPointer observer,
+            PendingBindingObserverList &bindingObservers) const;
 };
 
 template <typename T, typename Tag>