DBus: Watch when name disappears from bus

Question

I am implimenting the StatusNotifierWatcher service using the low-level DBus C library. The StatusNotifierWatcher specification requires that the watcher can know when "A StatusNotifierItem instance has disappeared from the bus" So that it can send the StatusNotifierItemUnregistered signal.

An example implimentation:

#include <stdio.h>
#include <stdlib.h>
#include <dbus/dbus.h>

DBusConnection *conn = NULL;

void item_unregistered_signal(const char *name) {
    DBusMessage *signal = dbus_message_new_signal(
            "/org/freedesktop/StatusNotifierWatcher",
            "org.freedesktop.StatusNotifierWatcher",
            "StatusNotifierItemUnregistered");
    dbus_message_append_args(signal,
            DBUS_TYPE_STRING, &name,
            DBUS_TYPE_INVALID);
    dbus_connection_send(conn, signal, NULL);
    dbus_message_unref(signal);
}

void watch_name(const char *name, void(*cb)(const char *)) {
    // Not sure how to impliment
}

dbus_bool_t register_item(DBusConnection *connection, DBusMessage *message, void *_data) {
    DBusError error;
    char *name;

    if (!dbus_message_get_args(message, &error,
                DBUS_TYPE_STRING, &name,
                DBUS_TYPE_INVALID)) {
        fprintf(stderr, "Error parsing method args: %s\n", error.message);
        return FALSE;
    }

    watch_name(name, item_unregistered_signal);
    return TRUE;
}

static void check_and_abort(DBusError *error) {
    if (dbus_error_is_set(error)) {
        fprintf(stderr, "dbus_err: %s\n", error->message);
        exit(EXIT_FAILURE);
    }
}

int main() {
    DBusError error;
    dbus_error_init(&error);
    conn = dbus_bus_get(DBUS_BUS_SESSION, &error);
    check_and_abort(&error);

    dbus_bus_request_name(conn, "org.freedesktop.StatusNotifierWatcher",
            DBUS_NAME_FLAG_REPLACE_EXISTING,
            &error);
    check_and_abort(&error);

    dbus_connection_add_filter(conn, register_item, NULL, free);

    while(1) {
        dbus_connection_read_write_dispatch(conn, 1000);
    }
}

If I have the well-known name to a DBus service, how do I know when the name disappears from the bus?

Answer 1

Well, I figured this out and I'll post an answer for any future poor souls who need to work with libdbus.

org.freedesktop.DBus sends the NameOwnerChanged signal whenever any DBus name changes. One can use this signal to track if an item has disappeared because the NewOwner argument is a null string. This function can do that:

static DBusHandlerResult signal_handler(DBusConnection *connection,
        DBusMessage *message, void *_usr_data) {

    if (dbus_message_is_signal(message, "org.freedesktop.DBus",
            "NameOwnerChanged")) {
        const char *name;
        const char *old_owner;
        const char *new_owner;

        if (!dbus_message_get_args(message, NULL,
                DBUS_TYPE_STRING, &name,
                DBUS_TYPE_STRING, &old_owner,
                DBUS_TYPE_STRING, &new_owner,
                DBUS_TYPE_INVALID)) {
            fprintf(stderr, "Error getting OwnerChanged args");
            return DBUS_HANDLER_RESULT_NOT_YET_HANDLED;
        }
        if (strcmp(name, "") != 0) {
            // Name not lost, just swapped owners
            return DBUS_HANDLER_RESULT_NOT_YET_HANDLED;
        }
        name_lost(name);
        return DBUS_HANDLER_RESULT_HANDLED;
    }
    return DBUS_HANDLER_RESULT_NOT_YET_HANDLED;
}

One also needs to add a match so that your program will be called with this signal. I added this in main()

dbus_bus_add_match(conn, 
        "type='signal',\
        sender='org.freedesktop.DBus',\
        interface='org.freedesktop.DBus',\
        member='NameOwnerChanged'",
        &error);
check_and_abort(&error);

dbus_connection_add_filter(conn, signal_handler, NULL, free);