[mutter] wayland: No xdg-output events without a logical monitor

[Olivier Fourdan <ofourdan src gnome org>]

commit 68ec9ac017157def9b7c25dd8141dc0e93d9f918
Author: Olivier Fourdan <ofourdan redhat com>
Date:   Tue Sep 25 13:50:09 2018 +0200

    wayland: No xdg-output events without a logical monitor
    
    To avoid a known race condition in the wl_output protocol documented in
    https://phabricator.freedesktop.org/T7722, mutter delays the `wl_output`
    destruction but nullify the `logical_monitor` associated with the
    `wl_output` and the binding routine `bind_output()` makes sure not to
    send wl_output events if the `logical_monitor` is `NULL` (see commit
    1923db97).
    
    The binding routine for `xdg_output` however does not check for such a
    condition, hence if the output configuration changes while a client is
    binding to xdg-output (typically Xwayland at startup), mutter would
    crash while trying to access the `logical_monitor` which was nullified
    by the change in configuration.
    
    Just like `bind_output()` does for wl_output, do not send xdg-output
    events if there is no `logical_monitor` yet.
    
    Closes: https://gitlab.gnome.org/GNOME/mutter/issues/194

 src/wayland/meta-wayland-outputs.c | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/src/wayland/meta-wayland-outputs.c b/src/wayland/meta-wayland-outputs.c
index 544ce341d..bd0a3b1ff 100644
--- a/src/wayland/meta-wayland-outputs.c
+++ b/src/wayland/meta-wayland-outputs.c
@@ -591,6 +591,9 @@ meta_xdg_output_manager_get_xdg_output (struct wl_client   *client,
   wayland_output->xdg_output_resources =
     g_list_prepend (wayland_output->xdg_output_resources, xdg_output_resource);
 
+  if (!wayland_output->logical_monitor)
+    return;
+
   send_xdg_output_events (xdg_output_resource,
                           wayland_output,
                           wayland_output->logical_monitor,