[thirdparty/systemd.git] / man / sd_bus_service_reconnect.c

/* SPDX-License-Identifier: MIT-0 */

/* Implements 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)))

#define check(x) ({                             \
  int _r = (x);                                 \
  errno = _r < 0 ? -_r : 0;                     \
  printf(#x ": %m\n");                          \
  if (_r < 0)                                   \
    return EXIT_FAILURE;                        \
  })

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) {
  check(setup((object *)userdata));
  return 0;
}

/* 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) {
  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 */

  printf("Failed to request name: %s\n", error->message);
  object *o = userdata;
  check(sd_event_exit(*o->event, -sd_bus_error_get_errno(error)));

  return 1;
}

static int setup(object *o) {
  /* 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) {
    check(sd_bus_detach_event(*o->bus));
    *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
   */
  check(sd_bus_new(o->bus));
  check(sd_bus_set_address(*o->bus, "unix:path=/run/dbus/system_bus_socket"));
  check(sd_bus_set_bus_client(*o->bus, 1));
  check(sd_bus_negotiate_creds(*o->bus, 1, SD_BUS_CREDS_UID|SD_BUS_CREDS_EUID|SD_BUS_CREDS_EFFECTIVE_CAPS));
  check(sd_bus_set_watch_bind(*o->bus, 1));
  check(sd_bus_start(*o->bus));

  /* 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
   */
  check(sd_bus_add_object_vtable(*o->bus,
                                 NULL,
                                 "/org/freedesktop/ReconnectExample",
                                 "org.freedesktop.ReconnectExample",
                                 vtable,
                                 o));
  /* 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
   */
  check(sd_bus_request_name_async(*o->bus,
                                  NULL,
                                  "org.freedesktop.ReconnectExample",
                                  0,
                                  request_name_callback,
                                  o));
  /* 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
   */
  check(sd_bus_match_signal_async(*o->bus,
                                  NULL,
                                  "org.freedesktop.DBus.Local",
                                  NULL,
                                  "org.freedesktop.DBus.Local",
                                  "Disconnected",
                                  on_disconnect,
                                  NULL,
                                  o));
  /* 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
   */
  check(sd_bus_attach_event(*o->bus, *o->event, 0));

  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,
  };

  /* Create an event loop data structure, with default parameters.
   * https://www.freedesktop.org/software/systemd/man/sd_event_default.html
   */
  check(sd_event_default(&event));

  /* 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
   */
  check(sd_event_add_signal(event, NULL, SIGINT|SD_EVENT_SIGNAL_PROCMASK, NULL, NULL));
  check(sd_event_add_signal(event, NULL, SIGTERM|SD_EVENT_SIGNAL_PROCMASK, NULL, NULL));

  check(setup(&o));

  /* Enter the main loop, it will exit only on sigint/sigterm.
   * https://www.freedesktop.org/software/systemd/man/sd_event_loop.html
   */
  check(sd_event_loop(event));

  /* https://www.freedesktop.org/software/systemd/man/sd_bus_release_name.html */
  check(sd_bus_release_name(bus, "org.freedesktop.ReconnectExample"));

  return 0;
}
Commit	Line	Data
34bbda18 LB	1	/* SPDX-License-Identifier: MIT-0 */
	2
	3	/* Implements a D-Bus service that automatically reconnects when the system bus is restarted.
	4	*
	5	* Compile with 'cc sd_bus_service_reconnect.c $(pkg-config --libs --cflags libsystemd)'
	6	*
	7	* To allow the program to take ownership of the name 'org.freedesktop.ReconnectExample',
e07e7017 LB	8	* add the following as /etc/dbus-1/system.d/org.freedesktop.ReconnectExample.conf
e07e7017 LB	9	* and then reload the broker with 'systemctl reload dbus':
34bbda18 LB	10
	11	<?xml version="1.0"?> <!---nxml--->
	12	<!DOCTYPE busconfig PUBLIC "-//freedesktop//DTD D-BUS Bus Configuration 1.0//EN"
	13	"http://www.freedesktop.org/standards/dbus/1.0/busconfig.dtd">
	14	<busconfig>
	15	<policy user="root">
	16	<allow own="org.freedesktop.ReconnectExample"/>
	17	<allow send_destination="org.freedesktop.ReconnectExample"/>
	18	<allow receive_sender="org.freedesktop.ReconnectExample"/>
	19	</policy>
	20
	21	<policy context="default">
	22	<allow send_destination="org.freedesktop.ReconnectExample"/>
	23	<allow receive_sender="org.freedesktop.ReconnectExample"/>
	24	</policy>
	25	</busconfig>
	26
	27	*
	28	* To get the property via busctl:
	29	*
	30	* $ busctl --user get-property org.freedesktop.ReconnectExample \
	31	* /org/freedesktop/ReconnectExample \
	32	* org.freedesktop.ReconnectExample \
	33	* Example
	34	* s "example"
	35	*/
	36
	37	#include <errno.h>
	38	#include <stdio.h>
	39	#include <stdlib.h>
	40	#include <systemd/sd-bus.h>
	41
	42	#define _cleanup_(f) __attribute__((cleanup(f)))
	43
	44	#define check(x) ({ \
	45	int _r = (x); \
	46	errno = _r < 0 ? -_r : 0; \
	47	printf(#x ": %m\n"); \
	48	if (_r < 0) \
	49	return EXIT_FAILURE; \
	50	})
	51
	52	typedef struct object {
	53	const char *example;
	54	sd_bus **bus;
	55	sd_event **event;
	56	} object;
	57
	58	static int property_get(
	59	sd_bus *bus,
	60	const char *path,
	61	const char *interface,
	62	const char *property,
	63	sd_bus_message *reply,
	64	void *userdata,
	65	sd_bus_error *error) {
	66
	67	object *o = userdata;
	68
	69	if (strcmp(property, "Example") == 0)
	70	return sd_bus_message_append(reply, "s", o->example);
	71
	72	return sd_bus_error_setf(error,
	73	SD_BUS_ERROR_UNKNOWN_PROPERTY,
74	"Unknown property '%s'",
75	property);
76	}
77
78	/* https://www.freedesktop.org/software/systemd/man/sd_bus_add_object.html */
79	static const sd_bus_vtable vtable[] = {
80	SD_BUS_VTABLE_START(0),
81	SD_BUS_PROPERTY(
82	"Example", "s",
83	property_get,
84	0,
85	SD_BUS_VTABLE_PROPERTY_CONST),
86	SD_BUS_VTABLE_END
87	};
88
89	static int setup(object *o);
90
91	static int on_disconnect(sd_bus_message message, void userdata, sd_bus_error *ret_error) {
92	check(setup((object *)userdata));
93	return 0;
94	}
95
e07e7017 LB	96	/* Ensure the event loop exits with a clear error if acquiring the well-known service name fails */
	97	static int request_name_callback(sd_bus_message m, void userdata, sd_bus_error *ret_error) {
	98	if (!sd_bus_message_is_method_error(m, NULL))
	99	return 1;
	100
	101	const sd_bus_error *error = sd_bus_message_get_error(m);
	102
	103	if (sd_bus_error_has_names(error, SD_BUS_ERROR_TIMEOUT, SD_BUS_ERROR_NO_REPLY))
	104	return 1; /* The bus is not available, try again later */
	105
	106	printf("Failed to request name: %s\n", error->message);
	107	object *o = userdata;
	108	check(sd_event_exit(*o->event, -sd_bus_error_get_errno(error)));
	109
	110	return 1;
	111	}
	112
34bbda18 LB	113	static int setup(object *o) {
	114	/* If we are reconnecting, then the bus object needs to be closed, detached from
	115	* the event loop and recreated.
	116	* https://www.freedesktop.org/software/systemd/man/sd_bus_detach_event.html
	117	* https://www.freedesktop.org/software/systemd/man/sd_bus_close_unref.html
	118	*/
	119	if (*o->bus) {
	120	check(sd_bus_detach_event(*o->bus));
	121	o->bus = sd_bus_close_unref(o->bus);
	122	}
	123
	124	/* Set up a new bus object for the system bus, configure it to wait for D-Bus to be available
49d6e3c8	125	* instead of failing if it is not, and start it. All the following operations are asynchronous
34bbda18 LB	126	* and will not block waiting for D-Bus to be available.
	127	* https://www.freedesktop.org/software/systemd/man/sd_bus_new.html
	128	* https://www.freedesktop.org/software/systemd/man/sd_bus_set_address.html
	129	* https://www.freedesktop.org/software/systemd/man/sd_bus_set_bus_client.html
	130	* https://www.freedesktop.org/software/systemd/man/sd_bus_negotiate_creds.html
	131	* https://www.freedesktop.org/software/systemd/man/sd_bus_set_watch_bind.html
	132	* https://www.freedesktop.org/software/systemd/man/sd_bus_set_connected_signal.html
	133	* https://www.freedesktop.org/software/systemd/man/sd_bus_start.html
	134	*/
	135	check(sd_bus_new(o->bus));
	136	check(sd_bus_set_address(*o->bus, "unix:path=/run/dbus/system_bus_socket"));
	137	check(sd_bus_set_bus_client(*o->bus, 1));
	138	check(sd_bus_negotiate_creds(*o->bus, 1, SD_BUS_CREDS_UID\|SD_BUS_CREDS_EUID\|SD_BUS_CREDS_EFFECTIVE_CAPS));
	139	check(sd_bus_set_watch_bind(*o->bus, 1));
34bbda18 LB	140	check(sd_bus_start(*o->bus));
	141
	142	/* Publish an interface on the bus, specifying our well-known object access
	143	* path and public interface name.
	144	* https://www.freedesktop.org/software/systemd/man/sd_bus_add_object.html
	145	* https://dbus.freedesktop.org/doc/dbus-tutorial.html
	146	*/
	147	check(sd_bus_add_object_vtable(*o->bus,
	148	NULL,
	149	"/org/freedesktop/ReconnectExample",
	150	"org.freedesktop.ReconnectExample",
	151	vtable,
	152	o));
	153	/* By default the service is only assigned an ephemeral name. Also add a well-known
	154	* one, so that clients know whom to call. This needs to be asynchronous, as
e07e7017 LB	155	* D-Bus might not be yet available. The callback will check whether the error is
e07e7017 LB	156	* expected or not, in case it fails.
34bbda18 LB	157	* https://www.freedesktop.org/software/systemd/man/sd_bus_request_name.html
	158	*/
	159	check(sd_bus_request_name_async(*o->bus,
	160	NULL,
	161	"org.freedesktop.ReconnectExample",
	162	0,
e07e7017 LB	163	request_name_callback,
e07e7017 LB	164	o));
34bbda18 LB	165	/* When D-Bus is disconnected this callback will be invoked, which will
	166	* set up the connection again. This needs to be asynchronous, as D-Bus might not
	167	* yet be available.
	168	* https://www.freedesktop.org/software/systemd/man/sd_bus_match_signal_async.html
	169	*/
	170	check(sd_bus_match_signal_async(*o->bus,
	171	NULL,
	172	"org.freedesktop.DBus.Local",
	173	NULL,
	174	"org.freedesktop.DBus.Local",
	175	"Disconnected",
	176	on_disconnect,
	177	NULL,
	178	o));
	179	/* Attach the bus object to the event loop so that calls and signals are processed.
	180	* https://www.freedesktop.org/software/systemd/man/sd_bus_attach_event.html
	181	*/
	182	check(sd_bus_attach_event(o->bus, o->event, 0));
	183
	184	return 0;
	185	}
	186
	187	int main(int argc, char **argv) {
	188	/* The bus should be relinquished before the program terminates. The cleanup
	189	* attribute allows us to do it nicely and cleanly whenever we exit the
	190	* block.
	191	*/
	192	_cleanup_(sd_bus_flush_close_unrefp) sd_bus *bus = NULL;
	193	_cleanup_(sd_event_unrefp) sd_event *event = NULL;
	194	object o = {
	195	.example = "example",
	196	.bus = &bus,
	197	.event = &event,
	198	};
	199
	200	/* Create an event loop data structure, with default parameters.
	201	* https://www.freedesktop.org/software/systemd/man/sd_event_default.html
	202	*/
	203	check(sd_event_default(&event));
	204
	205	/* By default the event loop will terminate when all sources have disappeared, so
	206	* we have to keep it 'occupied'. Register signal handling to do so.
	207	* https://www.freedesktop.org/software/systemd/man/sd_event_add_signal.html
	208	*/
	209	check(sd_event_add_signal(event, NULL, SIGINT\|SD_EVENT_SIGNAL_PROCMASK, NULL, NULL));
	210	check(sd_event_add_signal(event, NULL, SIGTERM\|SD_EVENT_SIGNAL_PROCMASK, NULL, NULL));
	211
	212	check(setup(&o));
	213
	214	/* Enter the main loop, it will exit only on sigint/sigterm.
	215	* https://www.freedesktop.org/software/systemd/man/sd_event_loop.html
	216	*/
	217	check(sd_event_loop(event));
	218
	219	/* https://www.freedesktop.org/software/systemd/man/sd_bus_release_name.html */
	220	check(sd_bus_release_name(bus, "org.freedesktop.ReconnectExample"));
	221
	222	return 0;
	223	}