Why is my DBus call giving an UnknownMethod DBus error?

Question

I'm trying to wrap my head around managing a systemd service using DBus calls. However, I can't seem to figure out the correct way to make a method call, since whatever I do it throws a DBus error

// Create connection
auto connection = Gio::DBus::Connection::get_sync(Gio::DBus::BusType::BUS_TYPE_SESSION);
// Create the parameters vector
std::vector<std::string> param_vec;
param_vec.push_back("my_unit_name");
auto param_vec_variant = Glib::Variant<std::vector<std::string>>::create(param_vec);
auto params = Glib::Variant<std::vector<std::string>>::create_tuple(param_vec_variant);

try {
    // Make the "GetUnit" dbus call to systemd
    auto response = connection->call_sync(
        "/org/freedesktop/systemd1",
        "org.freedesktop.systemd1.Manager",
        "GetUnit",
        params
    );
}
catch (Gio::DBus::Error err) {
    // org.freedesktop.DBus.Error.UnknownMethod being thrown
    std::cerr << Gio::DBus::ErrorUtils::get_remote_error(err) << std::endl;
}

Can anyone point out where I'm going wrong?

Answer 1

As Philip pointed out, my problem was with the connection->call_sync method, which took the default parameter bus_name = "" (docs).

The bus_name for systemd is org.freedesktop.systemd1, so the correct solution to this problem would be to add this parameter:

auto response = connection->call_sync(
    "/org/freedesktop/systemd1",
    "org.freedesktop.systemd1.Manager",
    "GetUnit",
    params,
    "org.freedesktop.systemd1" // bus_name
);

However, after further digging I found that it's actually not recommended to use the Connection object for DBus calls, and that I should actually be using Proxy.

Eventually I generalised my code using variadic parameters and settled on this function (try-catch block removed for readability):

template <typename... Types>
Glib::VariantContainerBase dbus_call(const Glib::ustring& method, const Types&... params) {
    auto proxy = Gio::DBus::Proxy::create_for_bus_sync(Gio::DBus::BusType::BUS_TYPE_SESSION, "org.freedesktop.systemd1", "/org/freedesktop/systemd1", "org.freedesktop.systemd1.Manager");
    auto params_variant = Glib::Variant<std::tuple<Types...>>::create(std::tuple<Types...>(params...));

    return proxy->call_sync(method, params_variant);
}

This can be called like so:

auto response = dbus_call("GetUnit", "my_unit_name");