plugin-manager: Don't try to starte effect when shutting down

Add internal state (starting, running, stopping), and use this instead
of MetaDisplay struct fields to determine whether to start animations.

This fixes issues when we try to animate things when shutting down.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2716>

src/compositor/meta-plugin-manager.c

@@ -37,10 +37,18 @@
 
 static GType plugin_type = G_TYPE_NONE;
 
+typedef enum _PluginManagerState
+{
+  PLUGIN_MANAGER_STATE_STARTING,
+  PLUGIN_MANAGER_STATE_RUNNING,
+  PLUGIN_MANAGER_STATE_STOPPING,
+} PluginManagerState;
+
 struct MetaPluginManager
 {
   MetaCompositor *compositor;
   MetaPlugin *plugin;
+  PluginManagerState state;
 };
 
 void
@@ -92,14 +100,31 @@ on_confirm_display_change (MetaMonitorManager *monitors,
   meta_plugin_manager_confirm_display_change (plugin_mgr);
 }
 
+static void
+on_started (MetaContext       *context,
+            MetaPluginManager *plugin_mgr)
+{
+  plugin_mgr->state = PLUGIN_MANAGER_STATE_RUNNING;
+}
+
+static void
+on_prepare_shutdown (MetaContext       *context,
+                     MetaPluginManager *plugin_mgr)
+{
+  plugin_mgr->state = PLUGIN_MANAGER_STATE_STOPPING;
+}
+
 MetaPluginManager *
 meta_plugin_manager_new (MetaCompositor *compositor)
 {
   MetaPluginManager *plugin_mgr;
   MetaPlugin *plugin;
   MetaMonitorManager *monitors;
+  MetaDisplay *display;
+  MetaContext *context;
 
   plugin_mgr = g_new0 (MetaPluginManager, 1);
+  plugin_mgr->state = PLUGIN_MANAGER_STATE_STARTING;
   plugin_mgr->compositor = compositor;
   plugin_mgr->plugin = plugin = g_object_new (plugin_type, NULL);
 
@@ -109,6 +134,13 @@ meta_plugin_manager_new (MetaCompositor *compositor)
   g_signal_connect (monitors, "confirm-display-change",
                     G_CALLBACK (on_confirm_display_change), plugin_mgr);
 
+  display = meta_compositor_get_display (compositor);
+  context = meta_display_get_context (display);
+  g_signal_connect (context, "started",
+                    G_CALLBACK (on_started), plugin_mgr);
+  g_signal_connect (context, "prepare-shutdown",
+                    G_CALLBACK (on_prepare_shutdown), plugin_mgr);
+
   return plugin_mgr;
 }
 
@@ -143,6 +175,21 @@ meta_plugin_manager_kill_switch_workspace (MetaPluginManager *plugin_mgr)
     klass->kill_switch_workspace (plugin);
 }
 
+static gboolean
+should_start_effect (MetaPluginManager *plugin_mgr)
+{
+  switch (plugin_mgr->state)
+    {
+    case PLUGIN_MANAGER_STATE_STARTING:
+    case PLUGIN_MANAGER_STATE_STOPPING:
+      return FALSE;
+    case PLUGIN_MANAGER_STATE_RUNNING:
+      return TRUE;
+    }
+
+  g_assert_not_reached ();
+}
+
 /*
  * Public method that the compositor hooks into for events that require
  * no additional parameters.
@@ -159,10 +206,9 @@ meta_plugin_manager_event_simple (MetaPluginManager *plugin_mgr,
 {
   MetaPlugin *plugin = plugin_mgr->plugin;
   MetaPluginClass *klass = META_PLUGIN_GET_CLASS (plugin);
-  MetaDisplay *display = meta_compositor_get_display (plugin_mgr->compositor);
   gboolean retval = FALSE;
 
-  if (display->display_opening)
+  if (!should_start_effect (plugin_mgr))
     return FALSE;
 
   switch (event)
@@ -230,9 +276,8 @@ meta_plugin_manager_event_size_change (MetaPluginManager *plugin_mgr,
 {
   MetaPlugin *plugin = plugin_mgr->plugin;
   MetaPluginClass *klass = META_PLUGIN_GET_CLASS (plugin);
-  MetaDisplay *display = meta_compositor_get_display (plugin_mgr->compositor);
 
-  if (display->display_opening)
+  if (!should_start_effect (plugin_mgr))
     return FALSE;
 
   if (!klass->size_change)
@@ -259,10 +304,9 @@ meta_plugin_manager_switch_workspace (MetaPluginManager   *plugin_mgr,
 {
   MetaPlugin *plugin = plugin_mgr->plugin;
   MetaPluginClass *klass = META_PLUGIN_GET_CLASS (plugin);
-  MetaDisplay *display = meta_compositor_get_display (plugin_mgr->compositor);
   gboolean retval = FALSE;
 
-  if (display->display_opening)
+  if (!should_start_effect (plugin_mgr))
     return FALSE;
 
   if (klass->switch_workspace)
@@ -317,9 +361,8 @@ meta_plugin_manager_show_tile_preview (MetaPluginManager *plugin_mgr,
 {
   MetaPlugin *plugin = plugin_mgr->plugin;
   MetaPluginClass *klass = META_PLUGIN_GET_CLASS (plugin);
-  MetaDisplay *display = meta_compositor_get_display (plugin_mgr->compositor);
 
-  if (display->display_opening)
+  if (!should_start_effect (plugin_mgr))
     return FALSE;
 
   if (klass->show_tile_preview)
@@ -336,9 +379,8 @@ meta_plugin_manager_hide_tile_preview (MetaPluginManager *plugin_mgr)
 {
   MetaPlugin *plugin = plugin_mgr->plugin;
   MetaPluginClass *klass = META_PLUGIN_GET_CLASS (plugin);
-  MetaDisplay *display = meta_compositor_get_display (plugin_mgr->compositor);
 
-  if (display->display_opening)
+  if (!should_start_effect (plugin_mgr))
     return FALSE;
 
   if (klass->hide_tile_preview)
@@ -359,9 +401,8 @@ meta_plugin_manager_show_window_menu (MetaPluginManager  *plugin_mgr,
 {
   MetaPlugin *plugin = plugin_mgr->plugin;
   MetaPluginClass *klass = META_PLUGIN_GET_CLASS (plugin);
-  MetaDisplay *display = meta_compositor_get_display (plugin_mgr->compositor);
 
-  if (display->display_opening)
+  if (!should_start_effect (plugin_mgr))
     return;
 
   if (klass->show_window_menu)
@@ -376,9 +417,8 @@ meta_plugin_manager_show_window_menu_for_rect (MetaPluginManager  *plugin_mgr,
 {
   MetaPlugin *plugin = plugin_mgr->plugin;
   MetaPluginClass *klass = META_PLUGIN_GET_CLASS (plugin);
-  MetaDisplay *display = meta_compositor_get_display (plugin_mgr->compositor);
 
-  if (display->display_opening)
+  if (!should_start_effect (plugin_mgr))
     return;
 
   if (klass->show_window_menu_for_rect)