| Safe Haskell | None |
|---|---|
| Language | Haskell98 |
System.Posix.Daemonize
- daemonize :: IO () -> IO ()
- serviced :: CreateDaemon a -> IO ()
- data CreateDaemon a = CreateDaemon {}
- simpleDaemon :: CreateDaemon ()
- fatalError :: MonadIO m => String -> m a
- exitCleanly :: MonadIO m => m a
- syslog :: Priority -> ByteString -> IO ()
Simple daemonization
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.
The most trivial daemon would be
daemonize (forever $ return ())
which does nothing until killed.
Building system services
serviced :: CreateDaemon a -> IO () #
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.
data CreateDaemon a #
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.
Constructors
| CreateDaemon | |
Fields
| |
simpleDaemon :: CreateDaemon () #
The simplest possible instance of CreateDaemon is
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.
Intradaemon utilities
fatalError :: MonadIO m => String -> m a #
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.
exitCleanly :: MonadIO m => m a #
Use this function when the daemon should terminate normally. It logs a message, and exits with status 0.
Logging utilities
syslog :: Priority -> ByteString -> IO () #