[mutter] onscreen/native: Always free the next framebuffer on dispose

[Marge Bot <marge-bot src gnome org>]

commit bb6ae40a0feb93727dff8c51bf0560279d8d89db
Author: Jonas Ådahl <jadahl gmail com>
Date:   Sun Jan 9 23:37:45 2022 +0100

    onscreen/native: Always free the next framebuffer on dispose
    
    There was a sanity check that complained if there was still a "next
    framebuffer" when disposing an onscreen. This is correct to complain
    about under normal operation, as we always wait until receiving the page
    flip callback before cleaning up the onscreen and their state.
    
    However, when there are many hotplugs occurring, we might end up with
    race conditions when the above sanity check is not valid: when we have
    more than one monitor active, paint 1 one of them, but receive a hotplug
    event before we paint the other(s), we will discard the already painted
    onscreen before really issuing a page flip.
    
    In this situation, we will have the "next framebuffer", but having that
    is not a bug, it's a race condition, thus to not leak in this situation,
    make sure to clean up the next framebuffer here too.
    
    Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/2081
    Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2225>

 src/backends/native/meta-onscreen-native.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)
---
diff --git a/src/backends/native/meta-onscreen-native.c b/src/backends/native/meta-onscreen-native.c
index 372368ef4b..d6a64d0e0d 100644
--- a/src/backends/native/meta-onscreen-native.c
+++ b/src/backends/native/meta-onscreen-native.c
@@ -2146,10 +2146,7 @@ meta_onscreen_native_dispose (GObject *object)
   switch (renderer_gpu_data->mode)
     {
     case META_RENDERER_NATIVE_MODE_GBM:
-      /* flip state takes a reference on the onscreen so there should
-       * never be outstanding flips when we reach here. */
-      g_warn_if_fail (onscreen_native->gbm.next_fb == NULL);
-
+      g_clear_object (&onscreen_native->gbm.next_fb);
       free_current_bo (onscreen);
       break;
     case META_RENDERER_NATIVE_MODE_SURFACELESS: