xcb: fix thread synchronization in QXcbEventQueue::waitForNewEvents() again

This patch amends a41701904e880f58e19b352ade1931d6cd1a7112

If peeking into the event queue looking for a clipboard event fails,
QXcbClipboard::waitForClipboardEvent() calls queue->peek for the second
time to "process other clipboard events, since someone is probably
requesting data from us". QXcbEventQueue::peek() in turn calls
QXcbEventQueue::flushBufferedEvents(). This second flushing can acquire
a waited-for clipboard event. The issue was that the code in
waitForNewEvents() ignored this possibility and assumed that there were
no clipboard events before or at its current m_flushedTail. If there
were no more events on the X11 connection after tailBeforeFlush,
the waitForNewEvents() in waitForClipboardEvent() blocked execution
for 5 seconds and eventually timed out.

The fix is to remember QXcbEventQueue::m_flushedTail just after looking
for and not finding a clipboard event in the queue. And then wait for
more events via QWaitCondition in waitForNewEvents() only if there were
no more events after the remembered m_flushedTail.

Fixes: QTBUG-75319
Pick-to: 5.15
Pick-to: 5.12
Change-Id: I4919c5b9b9227b3a8a29a11e7094f97960b3a121
Reviewed-by: Gatis Paeglis <gatis.paeglis@qt.io>

1 files changed, 3 insertions, 1 deletions

diff --git a/src/plugins/platforms/xcb/qxcbeventqueue.h b/src/plugins/platforms/xcb/qxcbeventqueue.h
index 4763b57ecb..67a1825ead 100644
--- a/src/plugins/platforms/xcb/qxcbeventqueue.h
+++ b/src/plugins/platforms/xcb/qxcbeventqueue.h
@@ -107,7 +107,9 @@ public:
     bool peekEventQueue(PeekerCallback peeker, void *peekerData = nullptr,
                         PeekOptions option = PeekDefault, qint32 peekerId = -1);
 
-    void waitForNewEvents(unsigned long time = std::numeric_limits<unsigned long>::max());
+    const QXcbEventNode *flushedTail() const { return m_flushedTail; }
+    void waitForNewEvents(const QXcbEventNode *sinceFlushedTail,
+                          unsigned long time = std::numeric_limits<unsigned long>::max());
 
 private:
     QXcbEventNode *qXcbEventNodeFactory(xcb_generic_event_t *event);