diff options
authorIgor Kushnir <>2020-01-10 19:48:43 +0200
committerIgor Kushnir <>2021-03-05 22:19:14 +0200
commitb427721f58dce480519fe6f0d3f6442cde6c49c9 (patch)
parente23409738e2fff3e167754dd7ea75bd49c6820d0 (diff)
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 Change-Id: I4919c5b9b9227b3a8a29a11e7094f97960b3a121 Reviewed-by: Gatis Paeglis <> (cherry picked from commit f2d22d5a5126e7a73da620a60847fc124f724333) Reviewed-by: Liang Qi <>
3 files changed, 15 insertions, 5 deletions
diff --git a/src/plugins/platforms/xcb/qxcbclipboard.cpp b/src/plugins/platforms/xcb/qxcbclipboard.cpp
index ac8b029916..725c0e4d51 100644
--- a/src/plugins/platforms/xcb/qxcbclipboard.cpp
+++ b/src/plugins/platforms/xcb/qxcbclipboard.cpp
@@ -787,6 +787,12 @@ xcb_generic_event_t *QXcbClipboard::waitForClipboardEvent(xcb_window_t window, i
if (e) // found the waited for event
return e;
+ // It is safe to assume here that the pointed to node won't be re-used
+ // while we are holding the pointer to it. The nodes can be recycled
+ // only when they are dequeued, which is done only by
+ // QXcbConnection::processXcbEvents().
+ const QXcbEventNode *flushedTailNode = queue->flushedTail();
if (checkManager) {
auto reply = Q_XCB_REPLY(xcb_get_selection_owner, xcb_connection(), atom(QXcbAtom::CLIPBOARD_MANAGER));
if (!reply || reply->owner == XCB_NONE)
@@ -812,7 +818,7 @@ xcb_generic_event_t *QXcbClipboard::waitForClipboardEvent(xcb_window_t window, i
const auto elapsed = timer.elapsed();
if (elapsed < clipboard_timeout)
- queue->waitForNewEvents(clipboard_timeout - elapsed);
+ queue->waitForNewEvents(flushedTailNode, clipboard_timeout - elapsed);
} while (timer.elapsed() < clipboard_timeout);
return nullptr;
diff --git a/src/plugins/platforms/xcb/qxcbeventqueue.cpp b/src/plugins/platforms/xcb/qxcbeventqueue.cpp
index 4ca73e3048..b96bda3194 100644
--- a/src/plugins/platforms/xcb/qxcbeventqueue.cpp
+++ b/src/plugins/platforms/xcb/qxcbeventqueue.cpp
@@ -226,6 +226,8 @@ void QXcbEventQueue::run()
while (!m_closeConnectionDetected && (event = xcb_wait_for_event(connection))) {
+ // This lock can block only if there are users of waitForNewEvents().
+ // Currently only the clipboard implementation relies on it.
while (!m_closeConnectionDetected && (event = xcb_poll_for_queued_event(connection)))
@@ -350,12 +352,12 @@ bool QXcbEventQueue::peekEventQueue(PeekerCallback peeker, void *peekerData,
return result;
-void QXcbEventQueue::waitForNewEvents(unsigned long time)
+void QXcbEventQueue::waitForNewEvents(const QXcbEventNode *sinceFlushedTail,
+ unsigned long time)
QMutexLocker locker(&m_newEventsMutex);
- QXcbEventNode *tailBeforeFlush = m_flushedTail;
- if (tailBeforeFlush != m_flushedTail)
+ if (sinceFlushedTail != m_flushedTail)
m_newEventsCondition.wait(&m_newEventsMutex, time);
diff --git a/src/plugins/platforms/xcb/qxcbeventqueue.h b/src/plugins/platforms/xcb/qxcbeventqueue.h
index 11d0b8e963..c8f09ed9e3 100644
--- a/src/plugins/platforms/xcb/qxcbeventqueue.h
+++ b/src/plugins/platforms/xcb/qxcbeventqueue.h
@@ -106,7 +106,9 @@ public:
bool peekEventQueue(PeekerCallback peeker, void *peekerData = nullptr,
PeekOptions option = PeekDefault, qint32 peekerId = -1);
- void waitForNewEvents(unsigned long time = ULONG_MAX);
+ const QXcbEventNode *flushedTail() const { return m_flushedTail; }
+ void waitForNewEvents(const QXcbEventNode *sinceFlushedTail,
+ unsigned long time = ULONG_MAX);
QXcbEventNode *qXcbEventNodeFactory(xcb_generic_event_t *event);