Prevent O(n^2) behavior when calling QObject::deleteLater

When a deleteLater event is queued, a check if done if the same event
for the same receiver is queued before by scanning all pending events.
This leads to quadratic behavior, which is quite noticeable. By using
an unused bit in QObjectData, this can be prevented. Now the duplicate
event scanning in QCoreApplication is only done for the quit event.

Task-number: QTBUG-65712
Change-Id: Ie505acbbec802f91ebd0b94ac067e362c2476113
Reviewed-by: Erik Verbruggen <erik.verbruggen@qt.io>
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/corelib/kernel/qcoreapplication.cpp

@@ -1426,6 +1426,9 @@ void QCoreApplication::postEvent(QObject *receiver, QEvent *event, int priority)
         return;
     }
 
+    if (event->type() == QEvent::DeferredDelete)
+        receiver->d_ptr->deleteLaterCalled = true;
+
     if (event->type() == QEvent::DeferredDelete && data == QThreadData::current()) {
         // remember the current running eventloop for DeferredDelete
         // events posted in the receiver's thread.
@@ -1485,11 +1488,22 @@ bool QCoreApplication::compressEvent(QEvent *event, QObject *receiver, QPostEven
                 return true;
             }
         }
-    } else
+        return false;
+    }
 #endif
-        if ((event->type() == QEvent::DeferredDelete
-             || event->type() == QEvent::Quit)
-            && receiver->d_func()->postedEvents > 0) {
+
+    if (event->type() == QEvent::DeferredDelete) {
+        if (receiver->d_ptr->deleteLaterCalled) {
+            // there was a previous DeferredDelete event, so we can drop the new one
+            delete event;
+            return true;
+        }
+        // deleteLaterCalled is set to true in postedEvents when queueing the very first
+        // deferred deletion event.
+        return false;
+    }
+
+    if (event->type() == QEvent::Quit && receiver->d_func()->postedEvents > 0) {
         for (int i = 0; i < postedEvents->size(); ++i) {
             const QPostEvent &cur = postedEvents->at(i);
             if (cur.receiver != receiver
@@ -1501,6 +1515,7 @@ bool QCoreApplication::compressEvent(QEvent *event, QObject *receiver, QPostEven
             return true;
         }
     }
+
     return false;
 }
 

src/corelib/kernel/qobject.cpp

@@ -230,6 +230,7 @@ QObjectPrivate::QObjectPrivate(int version)
     connectedSignals[0] = connectedSignals[1] = 0;
     metaObject = 0;
     isWindow = false;
+    deleteLaterCalled = false;
 }
 
 QObjectPrivate::~QObjectPrivate()

src/corelib/kernel/qobject.h

@@ -106,7 +106,8 @@ public:
     uint sendChildEvents : 1;
     uint receiveChildEvents : 1;
     uint isWindow : 1; //for QWindow
-    uint unused : 25;
+    uint deleteLaterCalled : 1;
+    uint unused : 24;
     int postedEvents;
     QDynamicMetaObjectData *metaObject;
     QMetaObject *dynamicMetaObject() const;