[dconf] DConfClient: add read_default() API

[Allison Ryan Lortie <desrt src gnome org>]

commit 014d634529f2a88ff638834dbbe827f6bb82aa16
Author: Allison Ryan Lortie <desrt desrt ca>
Date:   Mon Nov 30 16:39:59 2015 +0000

    DConfClient: add read_default() API
    
    Add an API to read the default value of a key.
    
    Add a testcase.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=758860

 client/dconf-client.c |   54 +++++++++++++++++++++++++++++++++++++++++++++++++
 client/dconf-client.h |    3 ++
 tests/client.c        |    3 ++
 3 files changed, 60 insertions(+), 0 deletions(-)
---
diff --git a/client/dconf-client.c b/client/dconf-client.c
index 105ca88..ea2e07c 100644
--- a/client/dconf-client.c
+++ b/client/dconf-client.c
@@ -242,6 +242,60 @@ dconf_client_read (DConfClient *client,
   return dconf_engine_read (client->engine, NULL, key);
 }
 
+/* This provides a "read through" queue that resets all of the keys.
+ * This is a good way to get the default value for a key.
+ *
+ * We cache the value of this queue in a static instead of generating
+ * and freeing it each time.
+ */
+static GQueue *
+dconf_client_get_reset_queue (void)
+{
+  static GQueue *reset_queue;
+
+  if (g_once_init_enter (&reset_queue))
+    {
+      DConfChangeset *reset_all;
+      GQueue *tmp;
+
+      reset_all = dconf_changeset_new ();
+      dconf_changeset_set (reset_all, "/", NULL);
+      dconf_changeset_seal (reset_all);
+
+      tmp = g_queue_new ();
+      g_queue_push_tail (tmp, reset_all);
+      g_once_init_leave (&reset_queue, tmp);
+    }
+
+  return reset_queue;
+}
+
+/**
+ * dconf_client_read_default:
+ * @client: a #DConfClient
+ * @key: the key to read the default value of
+ *
+ * Reads the current default value of @key.
+ *
+ * The default value of the key is the value that the key would have if
+ * were to be reset.  This is usually %NULL, but it may be something
+ * else in the case that the system administrator has defined a default
+ * value for a key.
+ *
+ * If there are outstanding "fast" changes in progress they may affect
+ * the result of this call.
+ *
+ * Returns: a #GVariant, or %NULL
+ */
+GVariant *
+dconf_client_read_default (DConfClient *client,
+                           const gchar *key)
+{
+  g_return_val_if_fail (DCONF_IS_CLIENT (client), NULL);
+
+  return dconf_engine_read (client->engine, dconf_client_get_reset_queue (), key);
+}
+
 /**
  * dconf_client_list:
  * @client: a #DConfClient
diff --git a/client/dconf-client.h b/client/dconf-client.h
index 1e07f80..e50792b 100644
--- a/client/dconf-client.h
+++ b/client/dconf-client.h
@@ -39,6 +39,9 @@ DConfClient *           dconf_client_new                                (void);
 GVariant *              dconf_client_read                               (DConfClient          *client,
                                                                          const gchar          *key);
 
+GVariant *              dconf_client_read_default                       (DConfClient          *client,
+                                                                         const gchar          *key);
+
 gchar **                dconf_client_list                               (DConfClient          *client,
                                                                          const gchar          *dir,
                                                                          gint                 *length);
diff --git a/tests/client.c b/tests/client.c
index 7d0f95d..4a3505c 100644
--- a/tests/client.c
+++ b/tests/client.c
@@ -68,6 +68,7 @@ queue_up_100_writes (DConfClient *client)
 
       /* We should always see the most recently written value. */
       check_and_free (dconf_client_read (client, "/test/value"), g_variant_new_int32 (i));
+      check_and_free (dconf_client_read_default (client, "/test/value"), NULL);
     }
 
   g_assert_cmpint (g_queue_get_length (&dconf_mock_dbus_outstanding_call_handles), ==, 2);
@@ -140,6 +141,7 @@ test_fast (void)
       g_assert (changed_was_called);
 
       check_and_free (dconf_client_read (client, "/test/value"), g_variant_new_int32 (99));
+      check_and_free (dconf_client_read_default (client, "/test/value"), NULL);
     }
 
   /* Because of the pending-merging logic, we should only have had to
@@ -154,6 +156,7 @@ test_fast (void)
 
   /* Should read back now as NULL */
   check_and_free (dconf_client_read (client, "/test/value"), NULL);
+  check_and_free (dconf_client_read_default (client, "/test/value"), NULL);
 
   /* Cleanup */
   g_signal_handlers_disconnect_by_func (client, changed, NULL);