[dconf] engine: fix locking bug

[Ryan Lortie <desrt src gnome org>]

commit b7978d79b21340bb80725c6deb7e2aafba436175
Author: Ryan Lortie <desrt desrt ca>
Date:   Mon Nov 25 11:35:41 2013 -0500

    engine: fix locking bug
    
    Make sure we actually acquire the sources lock on entry to
    dconf_engine_read_user_value().
    
    Uncovered during testing.

 engine/dconf-engine.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/engine/dconf-engine.c b/engine/dconf-engine.c
index f9c3144..706a856 100644
--- a/engine/dconf-engine.c
+++ b/engine/dconf-engine.c
@@ -567,6 +567,8 @@ dconf_engine_read_user_value (DConfEngine *engine,
   if (engine->n_sources == 0 || !engine->sources[0]->writable)
     return NULL;
 
+  dconf_engine_acquire_sources (engine);
+
   /* First check read-through */
   if (read_through)
     found_key = dconf_engine_find_key_in_queue (read_through, key, &value);