Let QXcbConnection::getTimestamp properly exit when X server quits

QXcbConnection::getTimestamp uses dummy events to get timestamp from
X server. However, in some cases, X server shuts down while client tries
to get timestamp. In this case, QXcbConnection::getTimestamp keeps
getting null event and thus falls into indefinite loop.

This fix checks if xcb connection is still valid and use a special
xcb_timestamp_t value, CurrentTime (0L), as returned value.
CurrentTime should not be generated by X server and if getTimestamp
returns this value, it means an "exception" case is triggered.

This fix is introduced because in kwin_x11 (KDE project), X server can
exit on logout. kwin_x11 should handle disconnection from X server.
But the indefinite loop prevents kwin_x11 to process disconnection
event and therefore kwin_x11 cannot quit properly.

Fixes: QTBUG-88435
Pick-to: 5.12 5.15 6.0
Change-Id: Iaf7ef3f8a35fa8389d22a608e3c49041bf90e1b9
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Liang Qi <liang.qi@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/plugins/platforms/xcb/qxcbconnection.cpp

@@ -767,7 +767,10 @@ xcb_timestamp_t QXcbConnection::getTimestamp()
 
     xcb_generic_event_t *event = nullptr;
 
-    while (!event) {
+    // When disconnection is caused by X server, event will never be able to hold
+    // a valid pointer. isConnected(), which calls xcb_connection_has_error(),
+    // can handle this type of disconnection and properly quits the loop.
+    while (isConnected() && !event) {
         connection()->sync();
         event = eventQueue()->peek([window, dummyAtom](xcb_generic_event_t *event, int type) {
             if (type != XCB_PROPERTY_NOTIFY)
@@ -777,6 +780,14 @@ xcb_timestamp_t QXcbConnection::getTimestamp()
         });
     }
 
+    if (!event) {
+        // https://www.x.org/releases/X11R7.7/doc/xproto/x11protocol.html#glossary
+        // > One timestamp value (named CurrentTime) is never generated by the
+        // > server. This value is reserved for use in requests to represent the
+        // > current server time.
+        return XCB_CURRENT_TIME;
+    }
+
     xcb_property_notify_event_t *pn = reinterpret_cast<xcb_property_notify_event_t *>(event);
     xcb_timestamp_t timestamp = pn->time;
     free(event);