gio: Add dummy win32 notification backend

This adds a null notification backend implementation for win32, purely
to avoid crashes due to a missing backend when applications use
GNotification. This backend does nothing except print a warning when a
notification is supposed to be emitted.

In future, it can be expanded to use win32 API to present toaster
notifications appropriately.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=776583

gio/Makefile.am

@@ -334,6 +334,7 @@ win32_actual_sources = \
 	gwin32networking.h \
 	gwin32networkmonitor.c \
 	gwin32networkmonitor.h \
+	gwin32notificationbackend.c \
 	$(NULL)
 
 win32_more_sources_for_vcproj = \

gio/giomodule.c

@@ -962,6 +962,7 @@ extern GType g_cocoa_notification_backend_get_type (void);
 #endif
 
 #ifdef G_PLATFORM_WIN32
+extern GType g_win32_notification_backend_get_type (void);
 
 #include <windows.h>
 extern GType _g_win32_network_monitor_get_type (void);
@@ -1166,6 +1167,7 @@ _g_io_modules_ensure_loaded (void)
       g_type_ensure (g_cocoa_notification_backend_get_type ());
 #endif
 #ifdef G_OS_WIN32
+      g_type_ensure (g_win32_notification_backend_get_type ());
       g_type_ensure (_g_winhttp_vfs_get_type ());
 #endif
       g_type_ensure (_g_local_vfs_get_type ());

gio/gwin32notificationbackend.c

@@ -0,0 +1,90 @@
+/* -*- mode: C; c-file-style: "gnu"; indent-tabs-mode: nil; -*- */
+/* GIO - GLib Input, Output and Streaming Library
+ *
+ * Copyright © 2018 Endless Mobile, Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General
+ * Public License along with this library; if not, see
+ * <http://www.gnu.org/licenses/>.
+ *
+ * Authors:
+ *  - Philip Withnall <withnall@endlessm.com>
+ */
+
+#include "config.h"
+
+#include "gnotificationbackend.h"
+
+#include "giomodule-priv.h"
+#include "gnotification-private.h"
+
+#define G_TYPE_WIN32_NOTIFICATION_BACKEND  (g_win32_notification_backend_get_type ())
+#define G_WIN32_NOTIFICATION_BACKEND(o)    (G_TYPE_CHECK_INSTANCE_CAST ((o), G_TYPE_WIN32_NOTIFICATION_BACKEND, GWin32NotificationBackend))
+
+typedef struct _GWin32NotificationBackend GWin32NotificationBackend;
+typedef GNotificationBackendClass         GWin32NotificationBackendClass;
+
+struct _GWin32NotificationBackend
+{
+  GNotificationBackend parent;
+};
+
+GType g_win32_notification_backend_get_type (void);
+
+G_DEFINE_TYPE_WITH_CODE (GWin32NotificationBackend, g_win32_notification_backend, G_TYPE_NOTIFICATION_BACKEND,
+  _g_io_modules_ensure_extension_points_registered ();
+  g_io_extension_point_implement (G_NOTIFICATION_BACKEND_EXTENSION_POINT_NAME,
+                                  g_define_type_id, "win32", 0))
+
+static gboolean
+g_win32_notification_backend_is_supported (void)
+{
+  /* This is the only backend supported on Windows, and always needs to be
+   * present to avoid no backend being selected. */
+  return TRUE;
+}
+
+static void
+g_win32_notification_backend_send_notification (GNotificationBackend *backend,
+                                                const gchar          *id,
+                                                GNotification        *notification)
+{
+  /* FIXME: See https://bugzilla.gnome.org/show_bug.cgi?id=776583. This backend
+   * exists purely to stop crashes when applications use g_notification*()
+   * on Windows, by providing a dummy backend implementation. (The alternative
+   * was to modify all of the backend call sites in g_notification*(), which
+   * seemed less scalable.) */
+  g_warning ("Notifications are not yet supported on Windows.");
+}
+
+static void
+g_win32_notification_backend_withdraw_notification (GNotificationBackend *backend,
+                                                    const gchar          *id)
+{
+  /* FIXME: Nothing needs doing here until send_notification() is implemented. */
+}
+
+static void
+g_win32_notification_backend_init (GWin32NotificationBackend *backend)
+{
+}
+
+static void
+g_win32_notification_backend_class_init (GWin32NotificationBackendClass *class)
+{
+  GNotificationBackendClass *backend_class = G_NOTIFICATION_BACKEND_CLASS (class);
+
+  backend_class->is_supported = g_win32_notification_backend_is_supported;
+  backend_class->send_notification = g_win32_notification_backend_send_notification;
+  backend_class->withdraw_notification = g_win32_notification_backend_withdraw_notification;
+}

gio/meson.build

@@ -440,6 +440,7 @@ else
     'gwin32outputstream.c',
     'gwin32networkmonitor.c',
     'gwin32networkmonitor.h',
+    'gwin32notificationbackend.c',
   )
 
   gio_win_rc = configure_file(