man/sd_bus_service_reconnect.c

   1 /* SPDX-License-Identifier: MIT-0 */
   2
   3 /* A D-Bus service that automatically reconnects when the system bus is
   4  * restarted.
   5  *
   6  * Compile with 'cc sd_bus_service_reconnect.c $(pkg-config --libs --cflags libsystemd)'
   7  *
   8  * To allow the program to take ownership of the name 'org.freedesktop.ReconnectExample',
   9  * add the following as /etc/dbus-1/system.d/org.freedesktop.ReconnectExample.conf
  10  * and then reload the broker with 'systemctl reload dbus':
  11
  12 <?xml version="1.0"?> <!--*-nxml-*-->
  13 <!DOCTYPE busconfig PUBLIC "-//freedesktop//DTD D-BUS Bus Configuration 1.0//EN"
  14   "http://www.freedesktop.org/standards/dbus/1.0/busconfig.dtd">
  15 <busconfig>
  16   <policy user="root">
  17     <allow own="org.freedesktop.ReconnectExample"/>
  18     <allow send_destination="org.freedesktop.ReconnectExample"/>
  19     <allow receive_sender="org.freedesktop.ReconnectExample"/>
  20   </policy>
  21
  22   <policy context="default">
  23     <allow send_destination="org.freedesktop.ReconnectExample"/>
  24     <allow receive_sender="org.freedesktop.ReconnectExample"/>
  25   </policy>
  26 </busconfig>
  27
  28  *
  29  * To get the property via busctl:
  30  *
  31  * $ busctl --user get-property org.freedesktop.ReconnectExample \
  32  *                              /org/freedesktop/ReconnectExample \
  33  *                              org.freedesktop.ReconnectExample \
  34  *                              Example
  35  * s "example"
  36  */
  37
  38 #include <errno.h>
  39 #include <stdio.h>
  40 #include <stdlib.h>
  41 #include <systemd/sd-bus.h>
  42
  43 #define _cleanup_(f) __attribute__((cleanup(f)))
  44
  45 static int log_error(int r, const char *str) {
  46   fprintf(stderr, "%s failed: %s\n", str, strerror(-r));
  47   return r;
  48 }
  49
  50 typedef struct object {
  51   const char *example;
  52   sd_bus **bus;
  53   sd_event **event;
  54 } object;
  55
  56 static int property_get(
  57                 sd_bus *bus,
  58                 const char *path,
  59                 const char *interface,
  60                 const char *property,
  61                 sd_bus_message *reply,
  62                 void *userdata,
  63                 sd_bus_error *error) {
  64
  65   object *o = userdata;
  66
  67   if (strcmp(property, "Example") == 0)
  68     return sd_bus_message_append(reply, "s", o->example);
  69
  70   return sd_bus_error_setf(error,
  71                            SD_BUS_ERROR_UNKNOWN_PROPERTY,
  72                            "Unknown property '%s'",
  73                            property);
  74 }
  75
  76 /* https://www.freedesktop.org/software/systemd/man/sd_bus_add_object.html */
  77 static const sd_bus_vtable vtable[] = {
  78   SD_BUS_VTABLE_START(0),
  79   SD_BUS_PROPERTY(
  80     "Example", "s",
  81     property_get,
  82     0,
  83     SD_BUS_VTABLE_PROPERTY_CONST),
  84   SD_BUS_VTABLE_END
  85 };
  86
  87 static int setup(object *o);
  88
  89 static int on_disconnect(sd_bus_message *message, void *userdata, sd_bus_error *ret_error) {
  90   int r;
  91
  92   r = setup((object *)userdata);
  93   if (r < 0) {
  94     object *o = userdata;
  95     r = sd_event_exit(*o->event, r);
  96     if (r < 0)
  97       return log_error(r, "sd_event_exit()");
  98   }
  99
 100   return 1;
 101 }
 102
 103 /* Ensure the event loop exits with a clear error if acquiring the well-known
 104  * service name fails */
 105 static int request_name_callback(sd_bus_message *m, void *userdata, sd_bus_error *ret_error) {
 106   int r;
 107
 108   if (!sd_bus_message_is_method_error(m, NULL))
 109     return 1;
 110
 111   const sd_bus_error *error = sd_bus_message_get_error(m);
 112
 113   if (sd_bus_error_has_names(error, SD_BUS_ERROR_TIMEOUT, SD_BUS_ERROR_NO_REPLY))
 114     return 1; /* The bus is not available, try again later */
 115
 116   fprintf(stderr, "Failed to request name: %s\n", error->message);
 117   object *o = userdata;
 118   r = sd_event_exit(*o->event, -sd_bus_error_get_errno(error));
 119   if (r < 0)
 120     return log_error(r, "sd_event_exit()");
 121
 122   return 1;
 123 }
 124
 125 static int setup(object *o) {
 126   int r;
 127
 128   /* If we are reconnecting, then the bus object needs to be closed, detached
 129    * from the event loop and recreated.
 130    * https://www.freedesktop.org/software/systemd/man/sd_bus_detach_event.html
 131    * https://www.freedesktop.org/software/systemd/man/sd_bus_close_unref.html
 132    */
 133   if (*o->bus) {
 134     r = sd_bus_detach_event(*o->bus);
 135     if (r < 0)
 136       return log_error(r, "sd_bus_detach_event()");
 137     *o->bus = sd_bus_close_unref(*o->bus);
 138   }
 139
 140   /* Set up a new bus object for the system bus, configure it to wait for D-Bus
 141    * to be available instead of failing if it is not, and start it. All the
 142    * following operations are asynchronous and will not block waiting for D-Bus
 143    * to be available.
 144    * https://www.freedesktop.org/software/systemd/man/sd_bus_new.html
 145    * https://www.freedesktop.org/software/systemd/man/sd_bus_set_address.html
 146    * https://www.freedesktop.org/software/systemd/man/sd_bus_set_bus_client.html
 147    * https://www.freedesktop.org/software/systemd/man/sd_bus_negotiate_creds.html
 148    * https://www.freedesktop.org/software/systemd/man/sd_bus_set_watch_bind.html
 149    * https://www.freedesktop.org/software/systemd/man/sd_bus_set_connected_signal.html
 150    * https://www.freedesktop.org/software/systemd/man/sd_bus_start.html
 151    */
 152   r = sd_bus_new(o->bus);
 153   if (r < 0)
 154     return log_error(r, "sd_bus_new()");
 155   r = sd_bus_set_address(*o->bus, "unix:path=/run/dbus/system_bus_socket");
 156   if (r < 0)
 157     return log_error(r, "sd_bus_set_address()");
 158   r = sd_bus_set_bus_client(*o->bus, 1);
 159   if (r < 0)
 160     return log_error(r, "sd_bus_set_bus_client()");
 161   r = sd_bus_negotiate_creds(*o->bus, 1, SD_BUS_CREDS_UID|SD_BUS_CREDS_EUID|SD_BUS_CREDS_EFFECTIVE_CAPS);
 162   if (r < 0)
 163     return log_error(r, "sd_bus_negotiate_creds()");
 164   r = sd_bus_set_watch_bind(*o->bus, 1);
 165   if (r < 0)
 166     return log_error(r, "sd_bus_set_watch_bind()");
 167   r = sd_bus_start(*o->bus);
 168   if (r < 0)
 169     return log_error(r, "sd_bus_start()");
 170
 171   /* Publish an interface on the bus, specifying our well-known object access
 172    * path and public interface name.
 173    * https://www.freedesktop.org/software/systemd/man/sd_bus_add_object.html
 174    * https://dbus.freedesktop.org/doc/dbus-tutorial.html
 175    */
 176   r = sd_bus_add_object_vtable(*o->bus,
 177                                NULL,
 178                                "/org/freedesktop/ReconnectExample",
 179                                "org.freedesktop.ReconnectExample",
 180                                vtable,
 181                                o);
 182   if (r < 0)
 183     return log_error(r, "sd_bus_add_object_vtable()");
 184   /* By default the service is only assigned an ephemeral name. Also add a
 185    * well-known one, so that clients know whom to call. This needs to be
 186    * asynchronous, as D-Bus might not be yet available. The callback will check
 187    * whether the error is expected or not, in case it fails.
 188    * https://www.freedesktop.org/software/systemd/man/sd_bus_request_name.html
 189    */
 190   r = sd_bus_request_name_async(*o->bus,
 191                                 NULL,
 192                                 "org.freedesktop.ReconnectExample",
 193                                 0,
 194                                 request_name_callback,
 195                                 o);
 196   if (r < 0)
 197     return log_error(r, "sd_bus_request_name_async()");
 198   /* When D-Bus is disconnected this callback will be invoked, which will set up
 199    * the connection again. This needs to be asynchronous, as D-Bus might not yet
 200    * be available.
 201    * https://www.freedesktop.org/software/systemd/man/sd_bus_match_signal_async.html
 202    */
 203   r = sd_bus_match_signal_async(*o->bus,
 204                                 NULL,
 205                                 "org.freedesktop.DBus.Local",
 206                                 NULL,
 207                                 "org.freedesktop.DBus.Local",
 208                                 "Disconnected",
 209                                 on_disconnect,
 210                                 NULL,
 211                                 o);
 212   if (r < 0)
 213     return log_error(r, "sd_bus_match_signal_async()");
 214   /* Attach the bus object to the event loop so that calls and signals are
 215    * processed.
 216    * https://www.freedesktop.org/software/systemd/man/sd_bus_attach_event.html
 217    */
 218   r = sd_bus_attach_event(*o->bus, *o->event, 0);
 219   if (r < 0)
 220     return log_error(r, "sd_bus_attach_event()");
 221
 222   return 0;
 223 }
 224
 225 int main(int argc, char **argv) {
 226   /* The bus should be relinquished before the program terminates. The cleanup
 227    * attribute allows us to do it nicely and cleanly whenever we exit the block.
 228    */
 229   _cleanup_(sd_bus_flush_close_unrefp) sd_bus *bus = NULL;
 230   _cleanup_(sd_event_unrefp) sd_event *event = NULL;
 231   object o = {
 232     .example = "example",
 233     .bus = &bus,
 234     .event = &event,
 235   };
 236   int r;
 237
 238   /* Create an event loop data structure, with default parameters.
 239    * https://www.freedesktop.org/software/systemd/man/sd_event_default.html
 240    */
 241   r = sd_event_default(&event);
 242   if (r < 0)
 243     return log_error(r, "sd_event_default()");
 244
 245   /* By default the event loop will terminate when all sources have disappeared,
 246    * so we have to keep it 'occupied'. Register signal handling to do so.
 247    * https://www.freedesktop.org/software/systemd/man/sd_event_add_signal.html
 248    */
 249   r = sd_event_add_signal(event, NULL, SIGINT|SD_EVENT_SIGNAL_PROCMASK, NULL, NULL);
 250   if (r < 0)
 251     return log_error(r, "sd_event_add_signal(SIGINT)");
 252
 253   r = sd_event_add_signal(event, NULL, SIGTERM|SD_EVENT_SIGNAL_PROCMASK, NULL, NULL);
 254   if (r < 0)
 255     return log_error(r, "sd_event_add_signal(SIGTERM)");
 256
 257   r = setup(&o);
 258   if (r < 0)
 259     return EXIT_FAILURE;
 260
 261   /* Enter the main loop, it will exit only on sigint/sigterm.
 262    * https://www.freedesktop.org/software/systemd/man/sd_event_loop.html
 263    */
 264   r = sd_event_loop(event);
 265   if (r < 0)
 266     return log_error(r, "sd_event_loop()");
 267
 268   /* https://www.freedesktop.org/software/systemd/man/sd_bus_release_name.html */
 269   r = sd_bus_release_name(bus, "org.freedesktop.ReconnectExample");
 270   if (r < 0)
 271     return log_error(r, "sd_bus_release_name()");
 272
 273   return 0;
 274 }