/* SPDX-License-Identifier: MIT-0 */
/* A D-Bus service that automatically reconnects when the system bus is


 * restarted.


 *


 * Compile with 'cc sd_bus_service_reconnect.c $(pkg-config --libs --cflags libsystemd)'


 *


 * To allow the program to take ownership of the name 'org.freedesktop.ReconnectExample',


 * add the following as /etc/dbus-1/system.d/org.freedesktop.ReconnectExample.conf


 * and then reload the broker with 'systemctl reload dbus':
<?xml version="1.0"?> <!--*-nxml-*-->
<!DOCTYPE busconfig PUBLIC "-//freedesktop//DTD D-BUS Bus Configuration 1.0//EN"


  "http://www.freedesktop.org/standards/dbus/1.0/busconfig.dtd">
<busconfig>


  <policy user="root">


    <allow own="org.freedesktop.ReconnectExample"/>


    <allow send_destination="org.freedesktop.ReconnectExample"/>


    <allow receive_sender="org.freedesktop.ReconnectExample"/>


  </policy>


  <policy context="default">


    <allow send_destination="org.freedesktop.ReconnectExample"/>


    <allow receive_sender="org.freedesktop.ReconnectExample"/>


  </policy>
</busconfig>


 *


 * To get the property via busctl:


 *


 * $ busctl --user get-property org.freedesktop.ReconnectExample \


 *                              /org/freedesktop/ReconnectExample \


 *                              org.freedesktop.ReconnectExample \


 *                              Example


 * s "example"


 */
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <systemd/sd-bus.h>
#define _cleanup_(f) __attribute__((cleanup(f)))
static int log_error(int r, const char *str) {


  fprintf(stderr, "%s failed: %s\n", str, strerror(-r));


  return r;
}
typedef struct object {


  const char *example;


  sd_bus **bus;


  sd_event **event;
} object;
static int property_get(


                sd_bus *bus,


                const char *path,


                const char *interface,


                const char *property,


                sd_bus_message *reply,


                void *userdata,


                sd_bus_error *error) {


  object *o = userdata;


  if (strcmp(property, "Example") == 0)


    return sd_bus_message_append(reply, "s", o->example);


  return sd_bus_error_setf(error,


                           SD_BUS_ERROR_UNKNOWN_PROPERTY,


                           "Unknown property '%s'",


                           property);
}
/* https://www.freedesktop.org/software/systemd/man/sd_bus_add_object.html */
static const sd_bus_vtable vtable[] = {


  SD_BUS_VTABLE_START(0),


  SD_BUS_PROPERTY(


    "Example", "s",


    property_get,


    0,


    SD_BUS_VTABLE_PROPERTY_CONST),


  SD_BUS_VTABLE_END
};
static int setup(object *o);
static int on_disconnect(sd_bus_message *message, void *userdata, sd_bus_error *ret_error) {


  int r;


  r = setup((object *)userdata);


  if (r < 0) {


    object *o = userdata;


    r = sd_event_exit(*o->event, r);


    if (r < 0)


      return log_error(r, "sd_event_exit()");


  }


  return 1;
}
/* Ensure the event loop exits with a clear error if acquiring the well-known


 * service name fails */
static int request_name_callback(sd_bus_message *m, void *userdata, sd_bus_error *ret_error) {


  int r;


  if (!sd_bus_message_is_method_error(m, NULL))


    return 1;


  const sd_bus_error *error = sd_bus_message_get_error(m);


  if (sd_bus_error_has_names(error, SD_BUS_ERROR_TIMEOUT, SD_BUS_ERROR_NO_REPLY))


    return 1; /* The bus is not available, try again later */


  fprintf(stderr, "Failed to request name: %s\n", error->message);


  object *o = userdata;


  r = sd_event_exit(*o->event, -sd_bus_error_get_errno(error));


  if (r < 0)


    return log_error(r, "sd_event_exit()");


  return 1;
}
static int setup(object *o) {


  int r;


  /* If we are reconnecting, then the bus object needs to be closed, detached


   * from the event loop and recreated.


   * https://www.freedesktop.org/software/systemd/man/sd_bus_detach_event.html


   * https://www.freedesktop.org/software/systemd/man/sd_bus_close_unref.html


   */


  if (*o->bus) {


    r = sd_bus_detach_event(*o->bus);


    if (r < 0)


      return log_error(r, "sd_bus_detach_event()");


    *o->bus = sd_bus_close_unref(*o->bus);


  }


  /* Set up a new bus object for the system bus, configure it to wait for D-Bus


   * to be available instead of failing if it is not, and start it. All the


   * following operations are asynchronous and will not block waiting for D-Bus


   * to be available.


   * https://www.freedesktop.org/software/systemd/man/sd_bus_new.html


   * https://www.freedesktop.org/software/systemd/man/sd_bus_set_address.html


   * https://www.freedesktop.org/software/systemd/man/sd_bus_set_bus_client.html


   * https://www.freedesktop.org/software/systemd/man/sd_bus_negotiate_creds.html


   * https://www.freedesktop.org/software/systemd/man/sd_bus_set_watch_bind.html


   * https://www.freedesktop.org/software/systemd/man/sd_bus_set_connected_signal.html


   * https://www.freedesktop.org/software/systemd/man/sd_bus_start.html


   */


  r = sd_bus_new(o->bus);


  if (r < 0)


    return log_error(r, "sd_bus_new()");


  r = sd_bus_set_address(*o->bus, "unix:path=/run/dbus/system_bus_socket");


  if (r < 0)


    return log_error(r, "sd_bus_set_address()");


  r = sd_bus_set_bus_client(*o->bus, 1);


  if (r < 0)


    return log_error(r, "sd_bus_set_bus_client()");


  r = sd_bus_negotiate_creds(*o->bus, 1, SD_BUS_CREDS_UID|SD_BUS_CREDS_EUID|SD_BUS_CREDS_EFFECTIVE_CAPS);


  if (r < 0)


    return log_error(r, "sd_bus_negotiate_creds()");


  r = sd_bus_set_watch_bind(*o->bus, 1);


  if (r < 0)


    return log_error(r, "sd_bus_set_watch_bind()");


  r = sd_bus_start(*o->bus);


  if (r < 0)


    return log_error(r, "sd_bus_start()");


  /* Publish an interface on the bus, specifying our well-known object access


   * path and public interface name.


   * https://www.freedesktop.org/software/systemd/man/sd_bus_add_object.html


   * https://dbus.freedesktop.org/doc/dbus-tutorial.html


   */


  r = sd_bus_add_object_vtable(*o->bus,


                               NULL,


                               "/org/freedesktop/ReconnectExample",


                               "org.freedesktop.ReconnectExample",


                               vtable,


                               o);


  if (r < 0)


    return log_error(r, "sd_bus_add_object_vtable()");


  /* By default the service is only assigned an ephemeral name. Also add a


   * well-known one, so that clients know whom to call. This needs to be


   * asynchronous, as D-Bus might not be yet available. The callback will check


   * whether the error is expected or not, in case it fails.


   * https://www.freedesktop.org/software/systemd/man/sd_bus_request_name.html


   */


  r = sd_bus_request_name_async(*o->bus,


                                NULL,


                                "org.freedesktop.ReconnectExample",


                                0,


                                request_name_callback,


                                o);


  if (r < 0)


    return log_error(r, "sd_bus_request_name_async()");


  /* When D-Bus is disconnected this callback will be invoked, which will set up


   * the connection again. This needs to be asynchronous, as D-Bus might not yet


   * be available.


   * https://www.freedesktop.org/software/systemd/man/sd_bus_match_signal_async.html


   */


  r = sd_bus_match_signal_async(*o->bus,


                                NULL,


                                "org.freedesktop.DBus.Local",


                                NULL,


                                "org.freedesktop.DBus.Local",


                                "Disconnected",


                                on_disconnect,


                                NULL,


                                o);


  if (r < 0)


    return log_error(r, "sd_bus_match_signal_async()");


  /* Attach the bus object to the event loop so that calls and signals are


   * processed.


   * https://www.freedesktop.org/software/systemd/man/sd_bus_attach_event.html


   */


  r = sd_bus_attach_event(*o->bus, *o->event, 0);


  if (r < 0)


    return log_error(r, "sd_bus_attach_event()");


  return 0;
}
int main(int argc, char **argv) {


  /* The bus should be relinquished before the program terminates. The cleanup


   * attribute allows us to do it nicely and cleanly whenever we exit the block.


   */


  _cleanup_(sd_bus_flush_close_unrefp) sd_bus *bus = NULL;


  _cleanup_(sd_event_unrefp) sd_event *event = NULL;


  object o = {


    .example = "example",


    .bus = &bus,


    .event = &event,


  };


  int r;


  /* Create an event loop data structure, with default parameters.


   * https://www.freedesktop.org/software/systemd/man/sd_event_default.html


   */


  r = sd_event_default(&event);


  if (r < 0)


    return log_error(r, "sd_event_default()");


  /* By default the event loop will terminate when all sources have disappeared,


   * so we have to keep it 'occupied'. Register signal handling to do so.


   * https://www.freedesktop.org/software/systemd/man/sd_event_add_signal.html


   */


  r = sd_event_add_signal(event, NULL, SIGINT|SD_EVENT_SIGNAL_PROCMASK, NULL, NULL);


  if (r < 0)


    return log_error(r, "sd_event_add_signal(SIGINT)");


  r = sd_event_add_signal(event, NULL, SIGTERM|SD_EVENT_SIGNAL_PROCMASK, NULL, NULL);


  if (r < 0)


    return log_error(r, "sd_event_add_signal(SIGTERM)");


  r = setup(&o);


  if (r < 0)


    return EXIT_FAILURE;


  /* Enter the main loop, it will exit only on sigint/sigterm.


   * https://www.freedesktop.org/software/systemd/man/sd_event_loop.html


   */


  r = sd_event_loop(event);


  if (r < 0)


    return log_error(r, "sd_event_loop()");


  /* https://www.freedesktop.org/software/systemd/man/sd_bus_release_name.html */


  r = sd_bus_release_name(bus, "org.freedesktop.ReconnectExample");


  if (r < 0)


    return log_error(r, "sd_bus_release_name()");


  return 0;
}