gfdonotificationbackend: Fix possible invalid pointer in dbus callback

The way things were before: a FreedesktopNotification struct is
allocated before the dbus call, and this same struct is possibly re-used
for other dbus calls. If the server becomes unavailable, the callback
will be invoked after the call times out, which leaves a long time where
other dbus calls can happen, re-using the same FreedesktopNotification
as user data. When the first call times out, the callback is invoked,
and the user data is freed. Subsequent calls that used the same user
data will time out later on, and try to free a pointer that was already
freed, hence segfaults.

This bug can be reproduced in Cinnamon 3.6.7, as mentioned in:
<linuxmint/cinnamon#7491>

This commit fixes that by always allocating a new
FreedesktopNotification before invoking dbus_call(), ensuring that the
callback always have a valid user data.

Signed-off-by: Arnaud Rebillout <elboulangero@gmail.com>

gio/gfdonotificationbackend.c

@@ -62,7 +62,6 @@ typedef struct
   GVariant *default_action_target;
 } FreedesktopNotification;
 
-
 static void
 freedesktop_notification_free (gpointer data)
 {
@@ -76,6 +75,24 @@ freedesktop_notification_free (gpointer data)
   g_slice_free (FreedesktopNotification, n);
 }
 
+static FreedesktopNotification *
+freedesktop_notification_new (GFdoNotificationBackend *backend,
+                              const gchar             *id,
+                              GNotification           *notification)
+{
+  FreedesktopNotification *n;
+
+  n = g_slice_new0 (FreedesktopNotification);
+  n->backend = backend;
+  n->id = g_strdup (id);
+  n->notify_id = 0;
+  g_notification_get_default_action (notification,
+                                     &n->default_action,
+                                     &n->default_action_target);
+
+  return n;
+}
+
 static FreedesktopNotification *
 g_fdo_notification_backend_find_notification (GFdoNotificationBackend *backend,
                                               const gchar             *id)
@@ -319,8 +336,19 @@ notification_sent (GObject      *source_object,
   val = g_dbus_connection_call_finish (G_DBUS_CONNECTION (source_object), result, &error);
   if (val)
     {
+      GFdoNotificationBackend *backend = n->backend;
+      FreedesktopNotification *match;
+
       g_variant_get (val, "(u)", &n->notify_id);
       g_variant_unref (val);
+
+      match = g_fdo_notification_backend_find_notification_by_notify_id (backend, n->notify_id);
+      if (match != NULL)
+        {
+          backend->notifications = g_slist_remove (backend->notifications, match);
+          freedesktop_notification_free (match);
+        }
+      backend->notifications = g_slist_prepend (backend->notifications, n);
     }
   else
     {
@@ -331,9 +359,7 @@ notification_sent (GObject      *source_object,
           warning_printed = TRUE;
         }
 
-      n->backend->notifications = g_slist_remove (n->backend->notifications, n);
       freedesktop_notification_free (n);
-
       g_error_free (error);
     }
 }
@@ -378,7 +404,7 @@ g_fdo_notification_backend_send_notification (GNotificationBackend *backend,
                                               GNotification        *notification)
 {
   GFdoNotificationBackend *self = G_FDO_NOTIFICATION_BACKEND (backend);
-  FreedesktopNotification *n;
+  FreedesktopNotification *n, *tmp;
 
   if (self->notify_subscription == 0)
     {
@@ -391,24 +417,11 @@ g_fdo_notification_backend_send_notification (GNotificationBackend *backend,
                                             notify_signal, backend, NULL);
     }
 
-  n = g_fdo_notification_backend_find_notification (self, id);
-  if (n == NULL)
-    {
-      n = g_slice_new0 (FreedesktopNotification);
-      n->backend = self;
-      n->id = g_strdup (id);
-      n->notify_id = 0;
-
-      n->backend->notifications = g_slist_prepend (n->backend->notifications, n);
-    }
-  else
-    {
-      /* Only clear default action. All other fields are still valid */
-      g_clear_pointer (&n->default_action, g_free);
-      g_clear_pointer (&n->default_action_target, g_variant_unref);
-    }
+  n = freedesktop_notification_new (self, id, notification);
 
-  g_notification_get_default_action (notification, &n->default_action, &n->default_action_target);
+  tmp = g_fdo_notification_backend_find_notification (self, id);
+  if (tmp)
+    n->notify_id = tmp->notify_id;
 
   call_notify (backend->dbus_connection, backend->application, n->notify_id, notification, notification_sent, n);
 }