xcb: Check for SHM extension before using it

If we call any xcb_shm_*() function, libxcb will have to figure out
SHM's major number. However, if that fails because the SHM extension is
not available, xcb will disconnect from the server. Obviously, that's
not what we want and must be avoided.

Change-Id: I8eba5ff9e84e0e2017a5c0d88fda4efd0459bd1e
Signed-off-by: Uli Schlachter <psychon@znc.in>
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>

diff --git a/src/plugins/platforms/xcb/qxcbbackingstore.cpp b/src/plugins/platforms/xcb/qxcbbackingstore.cpp
index 76304be..b736952 100644 (file)
--- a/src/plugins/platforms/xcb/qxcbbackingstore.cpp
+++ b/src/plugins/platforms/xcb/qxcbbackingstore.cpp
@@ -115,8 +115,12 @@ QXcbShmImage::QXcbShmImage(QXcbScreen *screen, const QSize &size, uint depth, QI
     m_shm_info.shmaddr = m_xcb_image->data = (quint8 *)shmat (m_shm_info.shmid, 0, 0);
     m_shm_info.shmseg = xcb_generate_id(xcb_connection());
 
-    xcb_generic_error_t *error = xcb_request_check(xcb_connection(), xcb_shm_attach_checked(xcb_connection(), m_shm_info.shmseg, m_shm_info.shmid, false));
-    if (error) {
+    const xcb_query_extension_reply_t *shm_reply = xcb_get_extension_data(xcb_connection(), &xcb_shm_id);
+    bool shm_present = shm_reply != NULL && shm_reply->present;
+    xcb_generic_error_t *error = NULL;
+    if (shm_present)
+        error = xcb_request_check(xcb_connection(), xcb_shm_attach_checked(xcb_connection(), m_shm_info.shmseg, m_shm_info.shmid, false));
+    if (!shm_present || error) {
         free(error);
 
         shmdt(m_shm_info.shmaddr);