Prevent posted event starvation by native Windows messages

Avoid live-lock if native messages are generated quickly enough by
allowing posted events to be sent if more than 10ms has passed since
we last sent them. This means that we can allow multiple calls to
sendPostedEvents() during a single call to processEvents(), but only
when the QEventLoop::EventLoopExec flag is passed to processEvents().

Task-number: QTBUG-1697

1 files changed, 11 insertions, 5 deletions

diff --git a/src/corelib/kernel/qeventdispatcher_win.cpp b/src/corelib/kernel/qeventdispatcher_win.cpp
index c6eef5e..105cf28 100644
--- a/src/corelib/kernel/qeventdispatcher_win.cpp
+++ b/src/corelib/kernel/qeventdispatcher_win.cpp
@@ -341,6 +341,7 @@ public:
     // for controlling when to send posted events
     QAtomicInt serialNumber;
     int lastSerialNumber;
+    int lastMessageTime;
     QAtomicInt wakeUps;
 
     // timers
@@ -364,7 +365,8 @@ public:
 };
 
 QEventDispatcherWin32Private::QEventDispatcherWin32Private()
-    : threadId(GetCurrentThreadId()), interrupt(false), internalHwnd(0), getMessageHook(0), serialNumber(0), lastSerialNumber(0), wakeUps(0)
+    : threadId(GetCurrentThreadId()), interrupt(false), internalHwnd(0), getMessageHook(0),
+      serialNumber(0), lastSerialNumber(0), lastMessageTime(0), wakeUps(0)
 {
     resolveTimerAPI();
 }
@@ -479,6 +481,7 @@ LRESULT CALLBACK qt_internal_proc(HWND hwnd, UINT message, WPARAM wp, LPARAM lp)
         int localSerialNumber = d->serialNumber;
         if (localSerialNumber != d->lastSerialNumber) {
             d->lastSerialNumber = localSerialNumber;
+            d->lastMessageTime = GetMessageTime();
             QCoreApplicationPrivate::sendPostedEvents(0, 0, d->threadData);
         }
         return 0;
@@ -495,9 +498,10 @@ LRESULT CALLBACK qt_GetMessageHook(int code, WPARAM wp, LPARAM lp)
         if (q) {
             QEventDispatcherWin32Private *d = q->d_func();
             int localSerialNumber = d->serialNumber;
-            if (HIWORD(GetQueueStatus(QS_INPUT | QS_RAWINPUT | QS_TIMER)) == 0) {
-                // no more input or timer events in the message queue, we can allow posted events to be
-                // sent now
+            if (HIWORD(GetQueueStatus(QS_INPUT | QS_RAWINPUT | QS_TIMER)) == 0
+                || GetMessageTime() - d->lastMessageTime >= 10) {
+                // no more input or timer events in the message queue or more than 10ms has elapsed since
+                // we send posted events, we can allow posted events to be sent now
                 (void) d->wakeUps.fetchAndStoreRelease(0);
                 MSG *msg = (MSG *) lp;
                 if (localSerialNumber != d->lastSerialNumber
@@ -732,7 +736,9 @@ bool QEventDispatcherWin32::processEvents(QEventLoop::ProcessEventsFlags flags)
             }
             if (haveMessage) {
                 if (d->internalHwnd == msg.hwnd && msg.message == WM_QT_SENDPOSTEDEVENTS) {
-                    if (seenWM_QT_SENDPOSTEDEVENTS) {
+                    if (seenWM_QT_SENDPOSTEDEVENTS && !(flags & QEventLoop::EventLoopExec)) {
+                        // when calling processEvents() "manually", we only want to send posted
+                        // events once
                         needWM_QT_SENDPOSTEDEVENTS = true;
                         continue;
                     }