[glib] g_test_dbus_down: Ensure next test does not use old connection

[Philip Withnall <pwithnall src gnome org>]

commit 3d50691a30270de8c880a5ad6b0be2f8ec36a673
Author: Simon McVittie <smcv debian org>
Date:   Wed Apr 4 08:35:39 2018 +0100

    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 | 16 ++++++++++++++++
 gio/gdbusprivate.h    |  1 +
 gio/gtestdbus.c       |  1 +
 3 files changed, 18 insertions(+)
---
diff --git a/gio/gdbusconnection.c b/gio/gdbusconnection.c
index 6f7e5fefc..2c74a73b3 100644
--- a/gio/gdbusconnection.c
+++ b/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
diff --git a/gio/gdbusprivate.h b/gio/gdbusprivate.h
index 21f2d5e5c..0d85c1d85 100644
--- a/gio/gdbusprivate.h
+++ b/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
 
diff --git a/gio/gtestdbus.c b/gio/gtestdbus.c
index 6eaf060d9..227f5b2ea 100644
--- a/gio/gtestdbus.c
+++ b/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;
 }