Use weston_compositor_add_destroy_listener_once() in plugins

This introduces a new convention of checking through the compositor destroy
listener if the plugin is already initialized. If the plugin is already
initialized, then the plugin entry function succeeds as a no-op. This makes it
safe to load the same plugin multiple times in a running compositor.

Currently module loading functions return failure if a plugin is already
loaded, but that will change in the future. Therefore we need this other method
of ensuring we do not double-initialize a plugin which would lead to list
corruptions the very least.

All plugins are converted to use the new helper, except:
- those that do not have a destroy listener already, and
- hmi-controller which does the same open-coded as the common code pattern
  did not fit there.

Plugins should always have a compositor destroy listener registered since they
very least allocate a struct to hold their data. Hence omissions are
highlighted in code.

Backends do not need this because weston_compositor_load_backend() already
protects against double-init. GL-renderer does not export a standard module
init function so cannot be initialized the usual way and therefore is not
vulnerable to double-init.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

ivi-shell/hmi-controller.c

@@ -1981,6 +1981,10 @@ wet_module_init(struct weston_compositor *ec,
 	struct hmi_controller *hmi_ctrl = NULL;
 	struct wl_event_loop *loop = NULL;
 
+	/* ad hoc weston_compositor_add_destroy_listener_once() */
+	if (wl_signal_get(&ec->destroy_signal, hmi_controller_destroy))
+		return 0;
+
 	hmi_ctrl = hmi_controller_create(ec);
 	if (hmi_ctrl == NULL)
 		return -1;

ivi-shell/ivi-shell.c

@@ -623,10 +623,14 @@ wet_shell_init(struct weston_compositor *compositor,
 	if (shell == NULL)
 		return -1;
 
-	init_ivi_shell(compositor, shell);
+	if (!weston_compositor_add_destroy_listener_once(compositor,
+							 &shell->destroy_listener,
+							 shell_destroy)) {
+		free(shell);
+		return 0;
+	}
 
-	shell->destroy_listener.notify = shell_destroy;
-	wl_signal_add(&compositor->destroy_signal, &shell->destroy_listener);
+	init_ivi_shell(compositor, shell);
 
 	shell->wake_listener.notify = wake_handler;
 	wl_signal_add(&compositor->wake_signal, &shell->wake_listener);