WinRT: Timers can be blocked by other timers.

It is possible for timers to be blocked by other timers for an indefinite time due to the current implementation of the WaitForMultipleObjectsEx handling at qeventdispatcher_winrt.cpp:219.

This is most likely to occur if a high frequency recurring timer is scheduled along with other timers, and the high frequency timer handle is first in the timerHandles list at qeventdispatcher_winrt.cpp:216.

If more than one timer event handle is signalled when coming out of WaitForMultipleObjectsEx, only the first timer handle is processed - in this case the high frequency timer. Then the next time WaitForMultipleObjectsEx is called, it is possible that the high frequency timer handle is again only processed, ignoring the other potentially signalled timer event handlers.

In our experience the QML render timer is usually the high frequency timer that causes this issue.

A possible alternative WaitForMultipleObjectsEx implementation is as follows, where WaitForMultipleObjectsEx is called multiple times until WAIT_TIMEOUT is returned, which ensures that all signalled timer event handles are processed.

This is what we are currently using and seems to work as expected.

bool timerFired = false;
DWORD waitResult = WaitForMultipleObjectsEx(timerHandles.count(), timerHandles.constData(), FALSE, waitTime, TRUE);
while(waitResult >= WAIT_OBJECT_0 && waitResult < WAIT_OBJECT_0 + timerHandles.count()) {	
	timerFired = true;

	const HANDLE handle = timerHandles.value(waitResult - WAIT_OBJECT_0);
	ResetEvent(handle);
	const int timerId = d->timerHandleToId.value(handle);

	if (timerId == INTERRUPT_HANDLE)
		break;

	WinRTTimerInfo &info = d->timerInfos[timerId];
	Q_ASSERT(info.timerId != INVALID_TIMER_ID);

	QCoreApplication::postEvent(this, new QTimerEvent(timerId));

	// Update timer's targetTime
	const quint64 targetTime = qt_msectime() + info.interval;
	info.targetTime = targetTime;
				
	waitResult = WaitForMultipleObjectsEx(timerHandles.count(), timerHandles.constData(), FALSE, 0, TRUE);
}

if (timerFired) {
	emit awake();
	return true;
}