Details
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Bug
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Resolution: Done
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P1: Critical
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5.15.1
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Linux/X11
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dbd1c8b047700bb6d0adae848d6cbb89fa2fcfff (qt/qtbase/dev) fb61cd8df838eb7aa91adf9cc87d2ad2409e9c88 (qt/qtbase/5.12) 5519447b9db4023deca98e5b882845416a9c33f1 (qt/qtbase/5.15)
Description
This bug was found in KDE's kwin_x11. Bug 415008 - KWin-5.17.4 hangs inside loop in QXcbConnection::getTimestamp. The basic call stack looks like:
#2 0x00007fc46bb1dcc8 in QXcbConnection::getTimestamp() () at /lib64/libQt5XcbQpa.so.5 #3 0x00007fc46bb38e52 in QXcbNativeInterface::getTimestamp(QXcbScreen const*) () at /lib64/libQt5XcbQpa.so.5 #4 0x00007fc46bb39cb3 in QXcbNativeInterface::nativeResourceForScreen(QByteArray const&, QScreen*) () at /lib64/libQt5XcbQpa.so.5 #5 0x00007fc47fd2ac56 in QX11Info::getTimestamp() () at /lib64/libQt5X11Extras.so.5 #6 0x00007fc480100d18 in KWin::Platform::updateXTime() () at /lib64/libkwin.so.5
Kwin is running QX11Info::getTimestamp(), which is a blocking call and cannot throw exception. But X server disconnects (Xrdp terminates Xvnc). But the code block looks like:
xcb_timestamp_t QXcbConnection::getTimestamp()
{
...
xcb_generic_event_t *event = nullptr;
while (!event) {
connection()->sync();
event = eventQueue()->peek([window, dummyAtom](xcb_generic_event_t *event, int type) {
if (type != XCB_PROPERTY_NOTIFY)
return false;
auto propertyNotify = reinterpret_cast<xcb_property_notify_event_t *>(event);
return propertyNotify->window == window && propertyNotify->atom == dummyAtom;
});
}
QXcbConnection::getTimestamp() cannot step out of the while loop.
According to Qt's design, I cannot throw exception here.
So my proposal is use CurrentTime from X11 to mark an invalid timestamp. Because X11 server will never generates CurrenTime.