написание и упаковка системных демонов (Writing and packaging system daemons)
PORTING EXISTING DAEMONS
Since new-style init systems such as systemd are compatible with
traditional SysV init systems, it is not strictly necessary to
port existing daemons to the new style. However, doing so offers
additional functionality to the daemons as well as simplifying
integration into new-style init systems.
To port an existing SysV compatible daemon, the following steps
are recommended:
1. If not already implemented, add an optional command line
switch to the daemon to disable daemonization. This is useful
not only for using the daemon in new-style init systems, but
also to ease debugging.
2. If the daemon offers interfaces to other software running on
the local system via local AF_UNIX sockets, consider
implementing socket-based activation (see above). Usually, a
minimal patch is sufficient to implement this: Extend the
socket creation in the daemon code so that sd_listen_fds(3)
is checked for already passed sockets first. If sockets are
passed (i.e. when sd_listen_fds() returns a positive value),
skip the socket creation step and use the passed sockets.
Secondly, ensure that the file system socket nodes for local
AF_UNIX sockets used in the socket-based activation are not
removed when the daemon shuts down, if sockets have been
passed. Third, if the daemon normally closes all remaining
open file descriptors as part of its initialization, the
sockets passed from the init system must be spared. Since
new-style init systems guarantee that no left-over file
descriptors are passed to executed processes, it might be a
good choice to simply skip the closing of all remaining open
file descriptors if sockets are passed.
3. Write and install a systemd unit file for the service (and
the sockets if socket-based activation is used, as well as a
path unit file, if the daemon processes a spool directory),
see above for details.
4. If the daemon exposes interfaces via D-Bus, write and install
a D-Bus activation file for the service, see above for
details.
