engine: accept out-of-order error reply messages

Due to a quirk of the implementation of the D-Bus daemon (see upstream
bug https://bugs.freedesktop.org/show_bug.cgi?id=59780 for details)
error messages can arrive out of sequence with repsect to the order of
the method calls that they are in reply to.

This may or may not be a violation of the strict-ordering principle of
D-Bus (which does not actually appear to be written into the spec) but
either way, we should dial down our assumptions about ordering since the
D-Bus implementation that everyone is using does not currently work this
way.

https://bugzilla.gnome.org/show_bug.cgi?id=687120

1 files changed, 20 insertions, 3 deletions

diff --git a/engine/dconf-engine.c b/engine/dconf-engine.c
index 30c4881..5a7b6ac 100644
--- a/engine/dconf-engine.c
+++ b/engine/dconf-engine.c
@@ -878,7 +878,6 @@ dconf_engine_change_completed (DConfEngine  *engine,
                                const GError *error)
 {
   OutstandingChange *oc = handle;
-  DConfChangeset *expected;
 
   dconf_engine_lock_queues (engine);
 
@@ -888,9 +887,27 @@ dconf_engine_change_completed (DConfEngine  *engine,
    *
    * The reply we just received should therefore be at the head of
    * our 'in flight' queue.
+   *
+   * Due to https://bugs.freedesktop.org/show_bug.cgi?id=59780 it is
+   * possible that we receive an out-of-sequence error message, however,
+   * so only assume that messages are in-order for positive replies.
    */
-  expected = g_queue_pop_head (&engine->in_flight);
-  g_assert (expected && oc->change == expected);
+  if (reply)
+    {
+      DConfChangeset *expected;
+
+      expected = g_queue_pop_head (&engine->in_flight);
+      g_assert (expected && oc->change == expected);
+    }
+  else
+    {
+      gboolean found;
+
+      g_assert (error != NULL);
+
+      found = g_queue_remove (&engine->in_flight, oc->change);
+      g_assert (found);
+    }
 
   /* We just popped a change from the in-flight queue, possibly
    * making room for another to be added.  Check that.