onscreen/native: Warn expecting next buffer on symoblic page flip

If we get a "ready" page flip feedback, it means the page flip was
symbolic, i.e. not real, e.g. as a result of an update that didn't
change the state of the primary plane. Warn if there is a "next fb"
meaning we expected to have a new buffer that we flipped to.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2854>

1 files changed, 3 insertions, 0 deletions

diff --git a/src/backends/native/meta-onscreen-native.c b/src/backends/native/meta-onscreen-native.c
index 034a41ea7..c55de436f 100644
--- a/src/backends/native/meta-onscreen-native.c
+++ b/src/backends/native/meta-onscreen-native.c
@@ -269,11 +269,14 @@ page_flip_feedback_ready (MetaKmsCrtc *kms_crtc,
   CoglFramebuffer *framebuffer =
     clutter_stage_view_get_onscreen (CLUTTER_STAGE_VIEW (view));
   CoglOnscreen *onscreen = COGL_ONSCREEN (framebuffer);
+  MetaOnscreenNative *onscreen_native = META_ONSCREEN_NATIVE (onscreen);
   CoglFrameInfo *frame_info;
 
   frame_info = cogl_onscreen_peek_head_frame_info (onscreen);
   frame_info->flags |= COGL_FRAME_INFO_FLAG_SYMBOLIC;
 
+  g_warn_if_fail (!onscreen_native->gbm.next_fb);
+
   meta_onscreen_native_notify_frame_complete (onscreen);
 }