[dconf] writer: avoid spurious rewrites on Init

[Ryan Lortie <ryanl src gnome org>]

commit dab1eaf9af416a2b32a4ba9a0b23cd787bf7eb0e
Author: Ryan Lortie <desrt desrt ca>
Date:   Fri Jan 11 14:17:22 2013 -0500

    writer: avoid spurious rewrites on Init
    
    Don't rewrite the gvdb file unless an actual change has been applied to
    the database (unless creating a non-native database for the first time).

 service/dconf-gvdb-utils.c |    4 ++++
 service/dconf-gvdb-utils.h |    1 +
 service/dconf-writer.c     |   23 ++++++++++++++++++++++-
 3 files changed, 27 insertions(+), 1 deletions(-)
---
diff --git a/service/dconf-gvdb-utils.c b/service/dconf-gvdb-utils.c
index 0fc3478..3052edc 100644
--- a/service/dconf-gvdb-utils.c
+++ b/service/dconf-gvdb-utils.c
@@ -30,6 +30,7 @@
 
 DConfChangeset *
 dconf_gvdb_utils_read_file (const gchar  *filename,
+                            gboolean     *file_missing,
                             GError      **error)
 {
   DConfChangeset *database;
@@ -95,6 +96,9 @@ dconf_gvdb_utils_read_file (const gchar  *filename,
       g_free (names);
     }
 
+  if (file_missing)
+    *file_missing = (table == NULL);
+
   return database;
 }
 
diff --git a/service/dconf-gvdb-utils.h b/service/dconf-gvdb-utils.h
index ff5d9fe..7c3259b 100644
--- a/service/dconf-gvdb-utils.h
+++ b/service/dconf-gvdb-utils.h
@@ -26,6 +26,7 @@
 #include "../common/dconf-changeset.h"
 
 DConfChangeset *                dconf_gvdb_utils_read_file              (const gchar     *filename,
+                                                                         gboolean        *file_missing,
                                                                          GError         **error);
 gboolean                        dconf_gvdb_utils_write_file             (const gchar     *filename,
                                                                          DConfChangeset  *database,
diff --git a/service/dconf-writer.c b/service/dconf-writer.c
index a907b55..7f9f2a5 100644
--- a/service/dconf-writer.c
+++ b/service/dconf-writer.c
@@ -41,6 +41,7 @@ struct _DConfWriterPrivate
   gchar *basepath;
   gchar *name;
   guint64 tag;
+  gboolean need_write;
 
   DConfChangeset *uncommited_values;
   DConfChangeset *commited_values;
@@ -100,10 +101,18 @@ dconf_writer_real_begin (DConfWriter  *writer,
    */
   if (writer->priv->commited_values == NULL)
     {
-      writer->priv->commited_values = dconf_gvdb_utils_read_file (writer->priv->filename, error);
+      gboolean missing;
+
+      writer->priv->commited_values = dconf_gvdb_utils_read_file (writer->priv->filename, &missing, error);
 
       if (!writer->priv->commited_values)
         return FALSE;
+
+      /* If this is a non-native writer and the file doesn't exist, we
+       * will need to write it on commit so that the client can open it.
+       */
+      if (missing && !writer->priv->native)
+        writer->priv->need_write = TRUE;
     }
 
   writer->priv->uncommited_values = dconf_changeset_new_database (writer->priv->commited_values);
@@ -130,6 +139,8 @@ dconf_writer_real_change (DConfWriter    *writer,
 
       g_queue_push_tail (&writer->priv->uncommited_changes, change);
     }
+
+  writer->priv->need_write = TRUE;
 }
 
 static gboolean
@@ -138,6 +149,16 @@ dconf_writer_real_commit (DConfWriter  *writer,
 {
   gint invalidate_fd = -1;
 
+  if (!writer->priv->need_write)
+    {
+      g_assert (g_queue_is_empty (&writer->priv->uncommited_changes));
+      g_assert (g_queue_is_empty (&writer->priv->commited_changes));
+      dconf_changeset_unref (writer->priv->uncommited_values);
+      writer->priv->uncommited_values = NULL;
+
+      return TRUE;
+    }
+
   if (!writer->priv->native)
     /* If it fails, it doesn't matter... */
     invalidate_fd = open (writer->priv->filename, O_WRONLY);