[dconf: 4/11] engine: add DCONF_READ_DEFAULT_VALUE flag

[Allison Ryan Lortie <desrt src gnome org>]

commit 4594876cd2b3c80a827cfe1526e0779b72777cc2
Author: Allison Ryan Lortie <desrt desrt ca>
Date:   Wed Dec 16 10:04:13 2015 -0500

    engine: add DCONF_READ_DEFAULT_VALUE flag
    
    Add a flag that allows checking the default value without constructing a
    read_through queue.  Make use of this new flag to simplify code in a
    couple of places.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=759128

 client/dconf-client.c            |   30 +-----------------------------
 common/dconf-enums.h             |    1 +
 engine/dconf-engine.c            |   16 +++++++++++++++-
 gsettings/dconfsettingsbackend.c |   19 +++----------------
 4 files changed, 20 insertions(+), 46 deletions(-)
---
diff --git a/client/dconf-client.c b/client/dconf-client.c
index e614256..0742748 100644
--- a/client/dconf-client.c
+++ b/client/dconf-client.c
@@ -245,34 +245,6 @@ dconf_client_read (DConfClient *client,
   return dconf_engine_read (client->engine, DCONF_READ_FLAGS_NONE, 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
@@ -296,7 +268,7 @@ dconf_client_read_default (DConfClient *client,
 {
   g_return_val_if_fail (DCONF_IS_CLIENT (client), NULL);
 
-  return dconf_engine_read (client->engine, DCONF_READ_FLAGS_NONE, dconf_client_get_reset_queue (), key);
+  return dconf_engine_read (client->engine, DCONF_READ_DEFAULT_VALUE, NULL, key);
 }
 
 /**
diff --git a/common/dconf-enums.h b/common/dconf-enums.h
index a2ac08a..2f10d1a 100644
--- a/common/dconf-enums.h
+++ b/common/dconf-enums.h
@@ -35,6 +35,7 @@ typedef enum
 typedef enum
 {
   DCONF_READ_FLAGS_NONE                 = 0,
+  DCONF_READ_DEFAULT_VALUE              = (1u << 0),
   DCONF_READ_USER_VALUE                 = (1u << 1)
 } DConfReadFlags;
 
diff --git a/engine/dconf-engine.c b/engine/dconf-engine.c
index ebd8ede..7a621c2 100644
--- a/engine/dconf-engine.c
+++ b/engine/dconf-engine.c
@@ -484,6 +484,12 @@ dconf_engine_read (DConfEngine    *engine,
    *   visible (because of a lock).  This includes any pending value
    *   that is in the read_through or pending queues.
    *
+   *   If DCONF_READ_DEFAULT_VALUE is given then we skip the writable
+   *   database and the queues (including read_through, which is
+   *   meaningless in this case) and skip directly to the non-writable
+   *   databases.  This is defined as the value that the user would see
+   *   if they were to have just done a reset for that key.
+   *
    * With respect to read_through and queued changed:
    *
    *   We only consider read_through and queued changes in the event
@@ -575,8 +581,16 @@ dconf_engine_read (DConfEngine    *engine,
     {
       gboolean found_key = FALSE;
 
+      /* If the user has requested the default value only, then ensure
+       * that we "find" a NULL value here.  This is equivalent to the
+       * user having reset the key, which is the definition of this
+       * flag.
+       */
+      if (flags & DCONF_READ_DEFAULT_VALUE)
+        found_key = TRUE;
+
       /* Step 2.  Check read_through. */
-      if (read_through)
+      if (!found_key && read_through)
         found_key = dconf_engine_find_key_in_queue (read_through, key, &value);
 
       /* Step 3.  Check queued changes if we didn't find it in read_through.
diff --git a/gsettings/dconfsettingsbackend.c b/gsettings/dconfsettingsbackend.c
index 0e96f5e..752e013 100644
--- a/gsettings/dconfsettingsbackend.c
+++ b/gsettings/dconfsettingsbackend.c
@@ -44,23 +44,10 @@ dconf_settings_backend_read (GSettingsBackend   *backend,
                              gboolean            default_value)
 {
   DConfSettingsBackend *dcsb = (DConfSettingsBackend *) backend;
-  GVariant *value;
 
-  if (default_value)
-    {
-      GQueue *read_through;
-
-      /* Mark the key as having been reset when trying to do the read... */
-      read_through = g_queue_new ();
-      g_queue_push_tail (read_through, dconf_changeset_new_write (key, NULL));
-      value = dconf_engine_read (dcsb->engine, DCONF_READ_FLAGS_NONE, read_through, key);
-      g_queue_free_full (read_through, (GDestroyNotify) dconf_changeset_unref);
-    }
-
-  else
-    value = dconf_engine_read (dcsb->engine, DCONF_READ_FLAGS_NONE, NULL, key);
-
-  return value;
+  return dconf_engine_read (dcsb->engine,
+                            default_value ? DCONF_READ_DEFAULT_VALUE : 0,
+                            NULL, key);
 }
 
 static GVariant *