man/sd_bus_service_reconnect.c

   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',
   8  * add the following as /etc/dbus-1/system.d/org.freedesktop.ReconnectExample.conf
   9  * and then reload the broker with 'systemctl reload dbus':
  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
  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
 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
 125    * instead of failing if it is not, and start it. All the following operations are asyncronous
 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));
 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
 155    * D-Bus might not be yet available. The callback will check whether the error is
 156    * expected or not, in case it fails.
 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,
 163                                   request_name_callback,
 164                                   o));
 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 }