[glibmm] Gio::DBus::Connection: Make the wrap() function thread-safe

[Kjell Ahlstedt <kjellahl src gnome org>]

commit c95ead41a626984716171e3b49b77db7133a4fac
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Tue Oct 22 12:33:52 2019 +0200

    Gio::DBus::Connection: Make the wrap() function thread-safe
    
    Add a std::mutex that guarantees that two threads don't create C++
    wrappers for the same GDBusConnection instance.
    
    Fixes #56

 gio/src/dbusconnection.ccg | 34 +++++++++++++++++++++++++++++++++-
 gio/src/dbusconnection.hg  |  1 +
 2 files changed, 34 insertions(+), 1 deletion(-)
---
diff --git a/gio/src/dbusconnection.ccg b/gio/src/dbusconnection.ccg
index a7464c24..4491a0cc 100644
--- a/gio/src/dbusconnection.ccg
+++ b/gio/src/dbusconnection.ccg
@@ -24,6 +24,7 @@
 #include <giomm/menumodel.h>
 #include <giomm/unixfdlist.h>
 #include "slot_async.h"
+#include <mutex>
 
 namespace
 {
@@ -84,7 +85,16 @@ DBusConnection_Message_Filter_giomm_callback_destroy(void* data)
 }
 
 } // extern "C"
-}
+
+// Glib::wrap(GDBusConnection* object, bool take_copy) must be protected by a
+// mutex while a C++ wrapper is being constructed. The same GDBusConnection
+// instance can be used in more than one thread. The wrap() function can be
+// called simultaneously in different threads for the same GDBusConnection
+// instance before it has been given a C++ wrapper.
+// https://gitlab.gnome.org/GNOME/glibmm/issues/56
+std::mutex wrap_mutex;
+
+} // anonymous namespace
 
 namespace Gio
 {
@@ -722,3 +732,25 @@ Connection::register_subtree(
 } // namespace DBus
 
 } // namespace Gio
+
+namespace Glib
+{
+
+Glib::RefPtr<Gio::DBus::Connection> wrap(GDBusConnection* object, bool take_copy)
+{
+  Glib::ObjectBase* pCppObject = nullptr;
+  if (!ObjectBase::_get_current_wrapper((GObject*)object))
+  {
+    // 'object' does not yet have a C++ wrapper. Construction of the C++ wrapper
+    // must be thread-safe. See comments at the definition of wrap_mutex.
+    std::lock_guard<std::mutex> lock(wrap_mutex);
+    pCppObject = Glib::wrap_auto((GObject*)object, take_copy);
+  }
+  else
+    pCppObject = Glib::wrap_auto((GObject*)object, take_copy);
+
+  return 
Glib::make_refptr_for_instance<Gio::DBus::Connection>(dynamic_cast<Gio::DBus::Connection*>(pCppObject));
+  //We use dynamic_cast<> in case of multiple inheritance.
+}
+
+} // namespace Glib
diff --git a/gio/src/dbusconnection.hg b/gio/src/dbusconnection.hg
index 306f8705..6f97953d 100644
--- a/gio/src/dbusconnection.hg
+++ b/gio/src/dbusconnection.hg
@@ -78,6 +78,7 @@ protected:
   _CLASS_GOBJECT(Connection, GDBusConnection, G_DBUS_CONNECTION, Glib::Object, GObject)
   _IMPLEMENTS_INTERFACE(Initable)
   _IMPLEMENTS_INTERFACE(AsyncInitable)
+  _CUSTOM_WRAP_FUNCTION
 
 protected: