[Unit]
Description=My Surviving Service
SurviveFinalKillSignal=yes
IgnoreOnIsolate=yes
DefaultDependencies=no
After=basic.target
Conflicts=reboot.target kexec.target poweroff.target halt.target rescue.target emergency.target
Before=shutdown.target rescue.target emergency.target
[Service]
Type=oneshot
ExecStart=sleep infinity

[Unit]
SurviveFinalKillSignal=yes
IgnoreOnIsolate=yes
DefaultDependencies=no

/* 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;
}