GApplication: allow null application_id

GApplication application ID is now permitted to be NULL, in which case
G_APPLICATION_NON_UNIQUE will be implicitly enabled.

https://bugzilla.gnome.org/show_bug.cgi?id=671249

gio/gapplication.c

@@ -46,22 +46,12 @@
  * @title: GApplication
  * @short_description: Core application class
  *
- * A #GApplication is the foundation of an application, unique for a
+ * A #GApplication is the foundation of an application.  It wraps some
- * given application identifier.  The GApplication class wraps some
  * low-level platform-specific services and is intended to act as the
  * foundation for higher-level application classes such as
  * #GtkApplication or #MxApplication.  In general, you should not use
  * this class outside of a higher level framework.
  *
- * One of the core features that GApplication provides is process
- * uniqueness, in the context of a "session".  The session concept is
- * platform-dependent, but corresponds roughly to a graphical desktop
- * login.  When your application is launched again, its arguments
- * are passed through platform communication to the already running
- * program. The already running instance of the program is called the
- * <firstterm>primary instance</firstterm>. On Linux, the D-Bus session
- * bus is used for communication.
- *
  * GApplication provides convenient life cycle management by maintaining
  * a <firstterm>use count</firstterm> for the primary application instance.
  * The use count can be changed using g_application_hold() and
@@ -70,9 +60,20 @@
  * ensure that the application stays alive as long as it has any opened
  * windows.
  *
- * Before using GApplication, you must choose an "application identifier".
+ * Another feature that GApplication (optionally) provides is process
- * The expected form of an application identifier is very close to that of
+ * uniqueness.  Applications can make use of this functionality by
- * of a <ulink url="http://dbus.freedesktop.org/doc/dbus-specification.html#message-protocol-names-interface">DBus bus name</ulink>.
+ * providing a unique application ID.  If given, only one application
+ * with this ID can be running at a time per session.  The session
+ * concept is platform-dependent, but corresponds roughly to a graphical
+ * desktop login.  When your application is launched again, its
+ * arguments are passed through platform communication to the already
+ * running program.  The already running instance of the program is
+ * called the <firstterm>primary instance</firstterm>. On Linux, the
+ * D-Bus session bus is used for communication.
+ *
+ * If used, the expected form of an application identifier is very close
+ * to that of of a
+ * <ulink url="http://dbus.freedesktop.org/doc/dbus-specification.html#message-protocol-names-interface">DBus bus name</ulink>.
  * Examples include: "com.example.MyApp", "org.example.internal-apps.Calculator".
  * For details on valid application identifiers, see g_application_id_is_valid().
  *
@@ -616,8 +617,6 @@ g_application_constructed (GObject *object)
 {
   GApplication *application = G_APPLICATION (object);
 
-  g_assert (application->priv->id != NULL);
-
   if (g_application_get_default () == NULL)
     g_application_set_default (application);
 }
@@ -905,14 +904,18 @@ g_application_id_is_valid (const gchar *application_id)
 /* Public Constructor {{{1 */
 /**
  * g_application_new:
- * @application_id: the application id
+ * @application_id: (allow-none): the application id
  * @flags: the application flags
  *
  * Creates a new #GApplication instance.
  *
  * This function calls g_type_init() for you.
  *
- * The application id must be valid.  See g_application_id_is_valid().
+ * If non-%NULL, the application id must be valid.  See
+ * g_application_id_is_valid().
+ *
+ * If no application ID is given then some features of #GApplication
+ * (most notably application uniqueness) will be disabled.
  *
  * Returns: a new #GApplication instance
  **/
@@ -920,7 +923,7 @@ GApplication *
 g_application_new (const gchar       *application_id,
                    GApplicationFlags  flags)
 {
-  g_return_val_if_fail (g_application_id_is_valid (application_id), NULL);
+  g_return_val_if_fail (application_id == NULL || g_application_id_is_valid (application_id), NULL);
 
   g_type_init ();
 
@@ -952,14 +955,15 @@ g_application_get_application_id (GApplication *application)
 /**
  * g_application_set_application_id:
  * @application: a #GApplication
- * @application_id: the identifier for @application
+ * @application_id: (allow-none): the identifier for @application
  *
  * Sets the unique identifier for @application.
  *
  * The application id can only be modified if @application has not yet
  * been registered.
  *
- * The application id must be valid.  See g_application_id_is_valid().
+ * If non-%NULL, the application id must be valid.  See
+ * g_application_id_is_valid().
  *
  * Since: 2.28
  **/
@@ -971,7 +975,7 @@ g_application_set_application_id (GApplication *application,
 
   if (g_strcmp0 (application->priv->id, application_id) != 0)
     {
-      g_return_if_fail (g_application_id_is_valid (application_id));
+      g_return_if_fail (application_id == NULL || g_application_id_is_valid (application_id));
       g_return_if_fail (!application->priv->is_registered);
 
       g_free (application->priv->id);
@@ -1146,6 +1150,9 @@ g_application_get_is_remote (GApplication *application)
  * application identifier as a unique bus name on the session bus using
  * GDBus.
  *
+ * If there is no application ID or if %G_APPLICATION_NON_UNIQUE was
+ * given, then this process will always become the primary instance.
+ *
  * Due to the internal architecture of GDBus, method calls can be
  * dispatched at any time (even if a main loop is not running).  For
  * this reason, you must ensure that any object paths that you wish to
@@ -1155,7 +1162,8 @@ g_application_get_is_remote (GApplication *application)
  * returned with no work performed.
  *
  * The #GApplication::startup signal is emitted if registration succeeds
- * and @application is the primary instance.
+ * and @application is the primary instance (including the non-unique
+ * case).
  *
  * In the event of an error (such as @cancellable being cancelled, or a
  * failure to connect to the session bus), %FALSE is returned and @error
@@ -1178,6 +1186,9 @@ g_application_register (GApplication  *application,
 
   if (!application->priv->is_registered)
     {
+      if (application->priv->id == NULL)
+        application->priv->flags |= G_APPLICATION_NON_UNIQUE;
+
       application->priv->impl =
         g_application_impl_register (application, application->priv->id,
                                      application->priv->flags,

gio/gapplicationimpl-dbus.c

@@ -194,6 +194,10 @@ application_path_from_appid (const gchar *appid)
 {
   gchar *appid_path, *iter;
 
+  if (appid == NULL)
+    /* this is a private implementation detail */
+    return g_strdup ("/org/gtk/Application/anonymous");
+
   appid_path = g_strconcat ("/", appid, NULL);
   for (iter = appid_path; *iter; iter++)
     {
@@ -358,6 +362,8 @@ g_application_impl_register (GApplication        *application,
   GDBusActionGroup *actions;
   GApplicationImpl *impl;
 
+  g_assert ((flags & G_APPLICATION_NON_UNIQUE) || appid != NULL);
+
   impl = g_slice_new0 (GApplicationImpl);
 
   impl->app = application;

gio/gioenums.h

@@ -1355,10 +1355,11 @@ typedef enum
  *     to the #GApplication::command-line signal handler, via
  *     g_application_command_line_getenv().
  * @G_APPLICATION_NON_UNIQUE: Make no attempts to do any of the typical
- *     single-instance application negotiation.  The application neither
+ *     single-instance application negotiation, even if the application
- *     attempts to become the owner of the application ID nor does it
+ *     ID is given.  The application neither attempts to become the
- *     check if an existing owner already exists.  Everything occurs in
+ *     owner of the application ID nor does it check if an existing
- *     the local process.  Since: 2.30.
+ *     owner already exists.  Everything occurs in the local process.
+ *     Since: 2.30.
  *
  * Flags used to define the behaviour of a #GApplication.
  *