GDBus thread backend: fix obscure race condition

It was possible for outgoing messages to be delivered in the wrong order
due to an annoying race condition when dealing with delivery of "fast"
change messages.  This was hitting the in-order assertion on return of
those messages.

The race goes like this:

 - a write is requested from the main thread (1 in-flight, 0 pending)

 - the reply for this first change message comes back (but is not yet
   handled).

 - another write is requested from the main thread and immediately
   placed in-flight (with an idle on the worker thread for actually
   sending it).  2 in-flight, 0 pending.

 - a third write is requested from the main thread but goes to the
   pending queue (since the in-flight queue is full).  2 in-flight, 1
   pending.

 - the reply for the first change message is now handled in the worker
   thread, removing the first change from the in-flight queue.  The
   queue management sees the third write in the pending queue and
   promotes it to the in-flight queue (properly following the second
   write) but sends the message immediately since it's already in the
   worker thread (leapfrogging the second write which is still waiting
   in an idle).

 - the idle for the second write runs

We solve this problem by dispatching all writes via idles, even if we're
already in the worker thread.

1 files changed, 5 insertions, 1 deletions

diff --git a/gdbus/dconf-gdbus-thread.c b/gdbus/dconf-gdbus-thread.c
index e3d69d8..15cf70a 100644
--- a/gdbus/dconf-gdbus-thread.c
+++ b/gdbus/dconf-gdbus-thread.c
@@ -266,6 +266,7 @@ dconf_engine_dbus_call_async_func (GBusType                bus_type,
                                    GError                **error)
 {
   DConfGDBusCall *call;
+  GSource *source;
 
   call = g_slice_new (DConfGDBusCall);
   call->bus_type = bus_type;
@@ -276,7 +277,10 @@ dconf_engine_dbus_call_async_func (GBusType                bus_type,
   call->parameters = g_variant_ref_sink (parameters);
   call->handle = handle;
 
-  g_main_context_invoke (dconf_gdbus_get_worker_context (), dconf_gdbus_method_call, call);
+  source = g_idle_source_new ();
+  g_source_set_callback (source, dconf_gdbus_method_call, call, NULL);
+  g_source_attach (source, dconf_gdbus_get_worker_context ());
+  g_source_unref (source);
 
   return TRUE;
 }