DBus client and server in the same process - process

Question

When I create a D-Bus server (via g_bus_own_name()) and the client to it (using g_dbus_proxy_new()) in the same process and then call g_dbus_proxy_call_sync(), it never returns. However, if server and client are in separate processes, everything is ok.
The following code illustrates my problem (I am using giomm C++ bindings here):
file main.cc:
#include <giomm.h>
#include <thread>
int server_main();
int client_main();
int main() {
Gio::init();
std::thread thr_server([](){ server_main(); });
sleep(1); // give some time to server to register
std::thread thr_client([](){ client_main(); });
sleep(10); // wait for the client to finish
}
file server.cc:
#include <giomm.h>
#include <iostream>
namespace {
static Glib::RefPtr<Gio::DBus::NodeInfo> introspection_data;
static Glib::ustring introspection_xml =
"<node name='/org/glibmm/DBusExample'>"
" <interface name='org.glibmm.DBusExample'>"
" <method name='Method'>"
" </method>"
" </interface>"
"</node>";
guint registered_id = 0;
}
static void on_method_call(const Glib::RefPtr<Gio::DBus::Connection>& /* connection */,
const Glib::ustring& /* sender */, const Glib::ustring& /* object_path */,
const Glib::ustring& /* interface_name */, const Glib::ustring& method_name,
const Glib::VariantContainerBase& parameters,
const Glib::RefPtr<Gio::DBus::MethodInvocation>& invocation)
{
if(method_name == "Method") {
std::cout << "Method was called\n";
}
}
const Gio::DBus::InterfaceVTable interface_vtable(sigc::ptr_fun(&on_method_call));
void on_bus_acquired(const Glib::RefPtr<Gio::DBus::Connection>& connection, const Glib::ustring& /* name */)
{
std::cout << "on_bus_acquired\n";
try {
registered_id = connection->register_object("/org/glibmm/DBusExample",
introspection_data->lookup_interface(),
interface_vtable);
}
catch(const Glib::Error& ex) {
std::cerr << "Registration of object failed." << std::endl;
}
return;
}
void on_name_acquired(const Glib::RefPtr<Gio::DBus::Connection>& /* connection */, const Glib::ustring& /* name */)
{}
void on_name_lost(const Glib::RefPtr<Gio::DBus::Connection>& connection, const Glib::ustring& /* name */) {
connection->unregister_object(registered_id);
}
int server_main()
{
try {
introspection_data = Gio::DBus::NodeInfo::create_for_xml(introspection_xml);
}
catch(const Glib::Error& ex) {
std::cerr << "Unable to create introspection data: " << ex.what() <<
"." << std::endl;
return 1;
}
const guint id = Gio::DBus::own_name(Gio::DBus::BUS_TYPE_SESSION,
"org.glibmm.DBusExample",
sigc::ptr_fun(&on_bus_acquired),
sigc::ptr_fun(&on_name_acquired),
sigc::ptr_fun(&on_name_lost));
//Keep the service running
auto loop = Glib::MainLoop::create();
loop->run();
Gio::DBus::unown_name(id);
return EXIT_SUCCESS;
}
file client.cc:
#include <giomm.h>
#include <iostream>
Glib::RefPtr<Glib::MainLoop> loop;
// A main loop idle callback to quit when the main loop is idle.
bool on_main_loop_idle() {
std::cout << "loop_idle\n";
loop->quit();
return false;
}
void on_dbus_proxy_available(Glib::RefPtr<Gio::AsyncResult>& result)
{
auto proxy = Gio::DBus::Proxy::create_finish(result);
if(!proxy) {
std::cerr << "The proxy to the user's session bus was not successfully "
"created." << std::endl;
loop->quit();
return;
}
try {
std::cout << "Calling...\n";
proxy->call_sync("Method");
std::cout << "It works!\n";
}
catch(const Glib::Error& error) {
std::cerr << "Got an error: '" << error.what() << "'." << std::endl;
}
// Connect an idle callback to the main loop to quit when the main loop is
// idle now that the method call is finished.
Glib::signal_idle().connect(sigc::ptr_fun(&on_main_loop_idle));
}
int client_main() {
loop = Glib::MainLoop::create();
auto connection =
Gio::DBus::Connection::get_sync(Gio::DBus::BUS_TYPE_SESSION);
if(!connection) {
std::cerr << "The user's session bus is not available." << std::endl;
return 1;
}
// Create the proxy to the bus asynchronously.
Gio::DBus::Proxy::create(connection, "org.glibmm.DBusExample",
"/org/glibmm/DBusExample", "org.glibmm.DBusExample",
sigc::ptr_fun(&on_dbus_proxy_available));
loop->run();
return EXIT_SUCCESS;
}
I compile the test with g++ -O2 -std=c++0x main.cc server.cc client.cc -o test $(pkg-config --cflags --libs giomm-2.4) and run:
./test
on_bus_acquired
Calling...
<it hangs>
However, when I change main.cc:
#include <giomm.h>
int server_main();
int client_main();
int main() {
Gio::init();
auto childid = fork();
if (childid == 0) {
server_main();
} else {
sleep(1);
client_main();
}
}
I get:
./test
on_bus_acquired
Calling...
Method was called
It works!
So call_sync() returns successfully.
I tried to exclude loops from server and client, and use a single-threaded main.cc:
#include <giomm.h>
#include <thread>
int server_main();
int client_main();
int main() {
Gio::init();
server_main();
client_main();
auto loop = Glib::MainLoop::create();
loop->run();
}
Nothing helps. The question is, what am I doing wrong? I want to use my d-bus server and client in one process.

Answer 1

I figured it out, the trick is to execute
Glib::VariantContainerBase result;
invocation->return_value(result);
in the end of on_method_call.