QEventDispatcherWin32: avoid livelock in a foreign event loop

According to Windows docs, GetMessage() function retrieves the messages
from the input queue in defined order, where posted messages are
processed ahead of input messages, even if they were posted later.
Therefore, if the application produces a posted event permanently, as
a result of processing that event, user input messages may be blocked
due to hard CPU usage by the application.

It's not a problem, if an internal Qt event loop is running. By calling
sendPostedEvents() on the beginning of processEvents(), we are sending
posted events only once per iteration. However, during execution of
the foreign loop, we should artificially lower the priority of the
WM_QT_SENDPOSTEDEVENTS message in order to enable delivery of other
input messages.

To solve the problem, it is proposed to postpone the
WM_QT_SENDPOSTEDEVENTS message until the message queue becomes empty,
as it works for the internal loop.

Task-number: QTBUG-77464
Change-Id: I8dedb6837c6fc41aa6f497e67ab2352c2b4f3772
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

1 files changed, 30 insertions, 1 deletions

diff --git a/tests/auto/gui/kernel/noqteventloop/tst_noqteventloop.cpp b/tests/auto/gui/kernel/noqteventloop/tst_noqteventloop.cpp
index 19c5c8a4a0..3d1876f00f 100644
--- a/tests/auto/gui/kernel/noqteventloop/tst_noqteventloop.cpp
+++ b/tests/auto/gui/kernel/noqteventloop/tst_noqteventloop.cpp
@@ -36,6 +36,7 @@
 #include <QtNetwork/qtcpserver.h>
 #include <QtNetwork/qtcpsocket.h>
 #include <QtCore/qelapsedtimer.h>
+#include <QtCore/qtimer.h>
 
 #include <QtCore/qt_windows.h>
 
@@ -49,7 +50,7 @@ class tst_NoQtEventLoop : public QObject
 private slots:
     void consumeMouseEvents();
     void consumeSocketEvents();
-
+    void deliverEventsInLivelock();
 };
 
 class Window : public QRasterWindow
@@ -312,6 +313,34 @@ void tst_NoQtEventLoop::consumeSocketEvents()
     QVERIFY(server.hasPendingConnections());
 }
 
+void tst_NoQtEventLoop::deliverEventsInLivelock()
+{
+    int argc = 1;
+    char *argv[] = { const_cast<char *>("test"), 0 };
+    QGuiApplication app(argc, argv);
+
+    QTimer livelockTimer;
+    livelockTimer.start(0);
+    QTimer::singleShot(100, Qt::CoarseTimer, &livelockTimer, &QTimer::stop);
+
+    QElapsedTimer elapsedTimer;
+    elapsedTimer.start();
+
+    // Exec own message loop
+    MSG msg;
+    forever {
+        if (elapsedTimer.hasExpired(3000) || !livelockTimer.isActive())
+            break;
+
+        if (::PeekMessage(&msg, NULL, 0, 0, PM_REMOVE)) {
+            ::TranslateMessage(&msg);
+            ::DispatchMessage(&msg);
+        }
+    }
+
+    QVERIFY(!livelockTimer.isActive());
+}
+
 #include <tst_noqteventloop.moc>
 
 QTEST_APPLESS_MAIN(tst_NoQtEventLoop)