fixup probing for SHM support

window/src/os/x11/bitmap.rs

@@ -269,10 +269,7 @@ impl BufferImage {
     ) -> BufferImage {
         match ShmImage::new(conn, drawable, width, height) {
             Ok(shm) => BufferImage::Shared(shm),
-            Err(err) => {
+            Err(err) => BufferImage::Image(Image::new(width, height)),
-                eprintln!("X server doesn't support shm: {}", err);
-                BufferImage::Image(Image::new(width, height))
-            }
         }
     }
 }

window/src/os/x11/connection.rs

@@ -103,6 +103,39 @@ fn window_id_from_event(event: &xcb::GenericEvent) -> Option<xcb::xproto::Window
     }
 }
 
+/// Determine whether the server supports SHM.
+/// We can't simply run this on the main connection that we establish
+/// as lack of support is reported through the connection getting
+/// closed on us!  Instead we need to open a separate session to
+/// make this check.
+fn server_supports_shm() -> bool {
+    let display = unsafe { x11::xlib::XOpenDisplay(std::ptr::null()) };
+    if display.is_null() {
+        return false;
+    }
+    let screen_num = unsafe { x11::xlib::XDefaultScreen(display) };
+    let conn = unsafe { xcb::Connection::from_raw_conn(XGetXCBConnection(display)) };
+    unsafe { XSetEventQueueOwner(display, 1) };
+
+    // Take care here: xcb_shm_query_version can successfully return
+    // a nullptr, and a subsequent deref will segfault, so we need
+    // to check the ptr before accessing it!
+    match xcb::shm::query_version(&conn).get_reply() {
+        Ok(reply) => {
+            if let Err(err) = conn.has_error() {
+                eprintln!("While probing for X SHM support: {}", err);
+                return false;
+            }
+            let shm_available = !reply.ptr.is_null() && reply.shared_pixmaps();
+            shm_available
+        }
+        Err(err) => {
+            eprintln!("While probing for X SHM support: {}", err);
+            false
+        }
+    }
+}
+
 impl Connection {
     pub fn get() -> Option<Arc<Self>> {
         let mut res = None;
@@ -258,11 +291,7 @@ impl Connection {
 
         let keysyms = unsafe { xcb_key_symbols_alloc(conn.get_raw_conn()) };
 
-        // Take care here: xcb_shm_query_version can successfully return
+        let shm_available = server_supports_shm();
-        // a nullptr, and a subsequent deref will segfault, so we need
-        // to check the ptr before accessing it!
-        let reply = xcb::shm::query_version(&conn).get_reply()?;
-        let shm_available = !reply.ptr.is_null() && reply.shared_pixmaps();
         eprintln!("shm_available: {}", shm_available);
 
         let conn = Arc::new(Connection {