backend: Add 'prepare-shutdown' signal

This will be used to avoid doing various work when we're shutting down,
e.g. react to changes triggering monitor reconfigurations.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1698>

src/backends/meta-backend-private.h

@@ -110,6 +110,8 @@ void meta_init_backend (GType         backend_gtype,
                         const GValue *values);
 void meta_release_backend (void);
 
+void meta_backend_prepare_shutdown (MetaBackend *backend);
+
 #ifdef HAVE_WAYLAND
 MetaWaylandCompositor * meta_backend_get_wayland_compositor (MetaBackend *backend);
 

src/backends/meta-backend.c

@@ -94,6 +94,7 @@ enum
   LAST_DEVICE_CHANGED,
   LID_IS_CLOSED_CHANGED,
   GPU_ADDED,
+  PREPARE_SHUTDOWN,
 
   N_SIGNALS
 };
@@ -869,6 +870,13 @@ meta_backend_class_init (MetaBackendClass *klass)
                   0,
                   NULL, NULL, NULL,
                   G_TYPE_NONE, 1, META_TYPE_GPU);
+  signals[PREPARE_SHUTDOWN] =
+    g_signal_new ("prepare-shutdown",
+                  G_TYPE_FROM_CLASS (klass),
+                  G_SIGNAL_RUN_LAST,
+                  0,
+                  NULL, NULL, NULL,
+                  G_TYPE_NONE, 0);
 
   mutter_stage_views = g_getenv ("MUTTER_STAGE_VIEWS");
   stage_views_disabled = g_strcmp0 (mutter_stage_views, "0") == 0;
@@ -1485,6 +1493,12 @@ meta_release_backend (void)
   g_clear_object (&_backend);
 }
 
+void
+meta_backend_prepare_shutdown (MetaBackend *backend)
+{
+  g_signal_emit (backend, signals[PREPARE_SHUTDOWN], 0);
+}
+
 /**
  * meta_is_stage_views_enabled:
  *

src/core/main.c

@@ -344,6 +344,10 @@ static void
 meta_finalize (void)
 {
   MetaDisplay *display = meta_get_display ();
+  MetaBackend *backend = meta_get_backend ();
+
+  if (backend)
+    meta_backend_prepare_shutdown (backend);
 
   if (display)
     meta_display_close (display,