g_test_dbus_down: Ensure next test does not use old connection

There's a race condition somewhere in GTestDBus that can result in
the next test being started at a time when g_bus_get() would still
return the connection that is in the process of closing. This can
be reproduced reasonably reliably by running the gapplication test
10K times in a loop.

Instead of relying on waiting for the weak reference to be released,
we can force the issue by clearing it.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=768996
Bug-Debian: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=894677

gio/gdbusconnection.c

@@ -7233,6 +7233,22 @@ _g_bus_get_singleton_if_exists (GBusType bus_type)
   return ret;
 }
 
+/* May be called from any thread. Must not hold message_bus_lock. */
+void
+_g_bus_forget_singleton (GBusType bus_type)
+{
+  GWeakRef *singleton;
+
+  G_LOCK (message_bus_lock);
+
+  singleton = message_bus_get_singleton (bus_type, NULL);
+
+  if (singleton != NULL)
+    g_weak_ref_set (singleton, NULL);
+
+  G_UNLOCK (message_bus_lock);
+}
+
 /**
  * g_bus_get_sync:
  * @bus_type: a #GBusType

gio/gdbusprivate.h

@@ -146,6 +146,7 @@ gchar *_g_dbus_hexencode (const gchar *str,
 
 /* Implemented in gdbusconnection.c */
 GDBusConnection *_g_bus_get_singleton_if_exists (GBusType bus_type);
+void             _g_bus_forget_singleton        (GBusType bus_type);
 
 G_END_DECLS
 

gio/gtestdbus.c

@@ -823,6 +823,7 @@ g_test_dbus_down (GTestDBus *self)
     _g_object_dispose_and_wait_weak_notify (connection);
 
   g_test_dbus_unset ();
+  _g_bus_forget_singleton (G_BUS_TYPE_SESSION);
   self->priv->up = FALSE;
 }