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, 11 insertions, 4 deletions

diff --git a/src/plugins/platforms/xcb/qxcbcursor.cpp b/src/plugins/platforms/xcb/qxcbcursor.cpp
index 70138abede..8d151b760b 100644
--- a/src/plugins/platforms/xcb/qxcbcursor.cpp
+++ b/src/plugins/platforms/xcb/qxcbcursor.cpp
@@ -601,12 +601,19 @@ xcb_cursor_t QXcbCursor::createFontCursor(int cshape)
 
 xcb_cursor_t QXcbCursor::createBitmapCursor(QCursor *cursor)
 {
-    xcb_connection_t *conn = xcb_connection();
     QPoint spot = cursor->hotSpot();
     xcb_cursor_t c = XCB_NONE;
-    if (cursor->pixmap().depth() > 1)
-        c = qt_xcb_createCursorXRender(m_screen, cursor->pixmap().toImage(), spot);
-    if (!c) {
+    if (cursor->pixmap().depth() > 1) {
+#if QT_CONFIG(xcb_render)
+        if (connection()->hasXRender(0, 5))
+            c = qt_xcb_createCursorXRender(m_screen, cursor->pixmap().toImage(), spot);
+        else
+            qCWarning(lcQpaXcb, "xrender >= 0.5 required to create pixmap cursors");
+#else
+        qCWarning(lcQpaXcb, "This build of Qt does not support pixmap cursors");
+#endif
+    } else {
+        xcb_connection_t *conn = xcb_connection();
         xcb_pixmap_t cp = qt_xcb_XPixmapFromBitmap(m_screen, cursor->bitmap()->toImage());
         xcb_pixmap_t mp = qt_xcb_XPixmapFromBitmap(m_screen, cursor->mask()->toImage());
         c = xcb_generate_id(conn);