Turning a process into a daemon involves a fixed set of
operations on unix systems, described in section 13.3 of Stevens
and Rago, "Advanced Programming in the Unix Environment." Since
they are fixed, they can be written as a single function,
daemonize taking an IO action which represents the daemon's
actual activity.
Briefly, daemonize sets the file creation mask to 0, forks twice,
changed the working directory to /, closes stdin, stdout, and
stderr, blocks sigHUP, and runs its argument. Strictly, it
should close all open file descriptors, but this is not possible in
a sensible way in Haskell.
serviced turns a program into a UNIX daemon (system service)
ready to be deployed to etcrc.d or similar startup folder. It
is meant to be used in the main function of a program, such as
serviced simpleDaemon
The resulting program takes one of three arguments: start,
stop, and restart. All control the status of a daemon by
looking for a file containing a text string holding the PID of
any running instance. Conventionally, this file is in
varrun/$name.pid, where $name is the executable's name. For
obvious reasons, this file is known as a PID file.
start makes the program write a PID file. If the file already
exists, it refuses to start, guaranteeing there is only one
instance of the daemon at any time.
stop read the PID file, and terminates the process whose pid is
written therein. First it does a soft kill, SIGTERM, giving the
daemon a chance to shut down cleanly, then three seconds later a
hard kill which the daemon cannot catch or escape.
restart is simple stop followed by start.
serviced also tries to drop privileges. If you don't specify a
user the daemon should run as, it will try to switch to a user
with the same name as the daemon, and otherwise to user daemon.
It goes through the same sequence for group. Just to complicate
matters, the name of the daemon is by default the name of the
executable file, but can again be set to something else in the
CreateDaemon record.
Finally, exceptions in the program are caught, logged to syslog,
and the program restarted.
The details of any given daemon are fixed by the CreateDaemon
record passed to serviced. You can also take a predefined form
of CreateDaemon, such as simpleDaemon below, and set what
options you want, rather than defining the whole record yourself.
The name of the daemon, which is used as
the name for the PID file, as the name that
appears in the system logs, and as the user
and group the daemon tries to run as if
none are explicitly specified. In general,
this should be Nothing, in which case the
system defaults to the name of the
executable file containing the daemon.
Most daemons are initially run as root,
and try to change to another user so they
have fewer privileges and represent less of
a security threat. This field specifies
which user it should try to run as. If it
is Nothing, or if the user does not exist
on the system, it next tries to become a
user with the same name as the daemon, and
if that fails, the user daemon.
The directory where the
daemon should write and look
for the PID file. Nothing
means varrun. Unless you
have a good reason to do
otherwise, leave this as
Nothing.
CreateDaemon {
privilegedAction = return ()
program = const $ forever $ return ()
name = Nothing,
user = Nothing,
group = Nothing,
syslogOptions = [],
pidfileDirectory = Nothing,
}
which does nothing forever with all default settings. We give it a
name, simpleDaemon, since you may want to use it as a template
and modify only the fields that you need.
When you encounter an error where the only sane way to handle it
is to write an error to the log and die messily, use fatalError.
This is a good candidate for things like not being able to find
configuration files on startup.