Various runtime OS resources can passed from a system runtime to the next, through the userspace reboot operation. Specifically:

·

File descriptors placed in the file descriptor store of services that remain active until the very end are passed to the next boot, where they are placed in the file descriptor store of the same unit. For this to work, units must declare DefaultDependencies=no (and avoid a manual Conflicts=shutdown.target or similar) to ensure they are not terminated as usual during the system shutdown operation. Alternatively, use FileDescriptorStorePreserve= to allow the file descriptor store to remain pinned even when the unit is down. See systemd.service(5) for details about the file descriptor store.

·

Similar to this, file descriptors associated with .socket units remain open (and connectible) if the units are not stopped during the transition. (Achieved by DefaultDependencies=no.)

·

The /run/ file system remains mounted and populated and may be used to pass state information between such userspace reboot cycles.

·

Service processes may continue to run over the transition, past soft-reboot and into the next session, if they are placed in services that remain active until the very end of shutdown (which again is achieved via DefaultDependencies=no). They must also be set up to avoid being killed by the aforementioned SIGTERM and SIGKILL via SurviveFinalKillSignal=yes, and also be configured to avoid being stopped on isolate via IgnoreOnIsolate=yes. They also have to be configured to be stopped on normal shutdown, reboot and maintenance mode. Finally, they have to be ordered after basic.target to ensure correct ordeering on boot. Note that in case any new or custom units are used to isolate to, or that implement an equivalent shutdown functionality, they will also have to be configured manually for correct ordering and conflicting. For example:

.RS 4

[Unit]
Description=My surviving service
SurviveFinalKillSignal=yes
IgnoreOnIsolate=yes
DefaultDependencies=no
After=basic.target
Conflicts=reboot.target
Before=reboot.target
Conflicts=kexec.target
Before=kexec.target
Conflicts=poweroff.target
Before=poweroff.target
Conflicts=halt.target
Before=halt.target
Conflicts=rescue.target
Before=rescue.target
Conflicts=emergency.target
Before=emergency.target

[Service]
Type=oneshot
ExecStart=sleep infinity
      

.RE

·

File system mounts may remain mounted during the transition, and complex storage attached, if configured to remain until the very end of the shutdown process. (Also achieved via DefaultDependencies=no, and by avoiding Conflicts=umount.target)

·

If the unit publishes a service over D-Bus, the connection needs to be re-established after soft-reboot as the D-Bus broker will be stopped and then started again. When using the sd-bus library this can be achieved by adapting the following example.

.RS 4

/* 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 asyncronous
   * 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;
}

.RE

Even though passing resources from one soft reboot cycle to the next is possible this way, we strongly suggest to use this functionality sparingly only, as it creates a more fragile system as resources from different versions of the OS and applications might be mixed with unforeseen consequences. In particular its recommended to avoid allowing processes to survive the soft reboot operation, as this means code updates will necessarily be incomplete, and processes typically pin various other resources (such as the file system they are backed by), thus increasing memory usage (as two versions of the OS/application/file system might be kept in memory). Leaving processes running during a soft-reboot operation requires disconnecting the service comprehensively from the rest of the OS, i.e. minimizing IPC and reducing sharing of resources with the rest of the OS. A possible mechanism to achieve this is the concept of Portable Services[1], but make sure no resource from the hosts OS filesystems is pinned via BindPaths= or similar unit settings, otherwise the old, originating filesystem will remain mounted as long as the unit is running.

1.

Portable Services

https://systemd.io/PORTABLE_SERVICES