xcb: prevent crash with pixmap cursors on XRender-less X servers

We were using xcb_render_* APIs without checking if the server even
supports this extension. Attempting to use an extension which is
not present will always result in a crash. This patch adds the required
guards and refactors how we detect presence of XRender extension.

Also instead of falling back to some odd-looking bitmapped version
just leave the current cursor unchanged. That is how we did it in
Qt4 AFAICT.

Task-number: QTBUG-66935
Change-Id: I4f27f1d65a77563ec34f3e0e94492c9236d7f9a6
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

1 files changed, 8 insertions, 3 deletions

diff --git a/src/plugins/platforms/xcb/qxcbconnection.cpp b/src/plugins/platforms/xcb/qxcbconnection.cpp
index 2431f99ab8..444e3a7669 100644
--- a/src/plugins/platforms/xcb/qxcbconnection.cpp
+++ b/src/plugins/platforms/xcb/qxcbconnection.cpp
@@ -2136,17 +2136,22 @@ void QXcbConnection::initializeXRender()
 {
 #if QT_CONFIG(xcb_render)
     const xcb_query_extension_reply_t *reply = xcb_get_extension_data(m_connection, &xcb_render_id);
-    if (!reply || !reply->present)
+    if (!reply || !reply->present) {
+        qCDebug(lcQpaXcb, "XRender extension not present on the X server");
         return;
+    }
 
     auto xrender_query = Q_XCB_REPLY(xcb_render_query_version, m_connection,
                                      XCB_RENDER_MAJOR_VERSION,
                                      XCB_RENDER_MINOR_VERSION);
-    if (!xrender_query || (xrender_query->major_version == 0 && xrender_query->minor_version < 5)) {
-        qWarning("QXcbConnection: Failed to initialize XRender");
+    if (!xrender_query) {
+        qCWarning(lcQpaXcb, "xcb_render_query_version failed");
         return;
     }
+
     has_render_extension = true;
+    m_xrenderVersion.first = xrender_query->major_version;
+    m_xrenderVersion.second = xrender_query->minor_version;
 #endif
 }