gdbusaddress: Disable proxy support for D-Bus addresses

See the discussion in the bug report: with proxy support enabled, a
proxy resolver is created. Doing that will load all the GIO modules, and
typically at least one of them will try to use GDBus during
initialisation, which will cause a deadlock.

Using a TCP address with GDBusAddress is still supported, but accessing
it over a proxy is not.

Document this.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=792499

gio/gdbusaddress.c

@@ -66,6 +66,9 @@
  * like `unix:tmpdir=/tmp/my-app-name`. The exact format of addresses
  * is explained in detail in the
  * [D-Bus specification](http://dbus.freedesktop.org/doc/dbus-specification.html#addresses).
+ *
+ * TCP D-Bus connections are supported, but accessing them via a proxy is
+ * currently not supported.
  */
 
 static gchar *get_session_address_platform_specific (GError **error);
@@ -675,6 +678,13 @@ g_dbus_address_connect (const gchar   *address_entry,
 
       g_assert (ret == NULL);
       client = g_socket_client_new ();
+
+      /* Disable proxy support to prevent a deadlock on startup, since loading a
+       * proxy resolver causes the GIO modules to be loaded, and there will
+       * almost certainly be one of them which then tries to use GDBus.
+       * See: https://bugzilla.gnome.org/show_bug.cgi?id=792499 */
+      g_socket_client_set_enable_proxy (client, FALSE);
+
       connection = g_socket_client_connect (client,
                                             connectable,
                                             cancellable,