In my post of yesterday I noted some things about the init scripts, small niceties that init scripts should do in Gentoo for them to work properly and to solve the issue of migrating pid files to /run. Today I’d like to add a few more notes of what I wish all daemons out there implemented at the very least.
First of all, while some people prefer for the daemon to not fork and background by itself, I honestly prefer it to — it makes so many things so much easier. But if you fork, wait till the forked process completed initialization before exiting! The reason why I’m saying this is that, unfortunately, it’s common for a daemon to start up, fork, then load its configuration file and find out there’s a mistake … leading to a script that thinks that the daemon started properly, while no process is left running. In init scripts, --wait allows you to tell the start-stop-daemon tool to wait for a moment to see if the daemon could start at all, but it’s not so nice, because you have to find the correct wait time empirically, and in almost every case you’re going to run longer than needed.
If you will background by yourself, please make sure that you create a pidfile to tell the init system which ID to signal to stop — and if you do have such a pidfile, please do not make it configurable on the configuration file, but set a compiled-in default and eventually allow an override at runtime. The runtime override is especially welcome if your software is supposed to have multiple instances configured on the same box — as then a single pidfile would conflict. Not having it configured on a file means that you no longer need to hack up a parser for the configuration file to be able to know what the user wants, but you can rely on either the default or your override.
Also if you do intend to support multiple instances of the same daemon make sure that you allow multiple configuration files to be passed in by he command-line. This simplifies a lot the whole handling of multiple-instances, and should be mandatory in that situation. Make sure you don’t re-use paths in that case either.
If you have messages you should also make sure that they are sent to syslog — please do not force, or even default, everything to log files! We have tons of syslog implementations, and at least the user does not have to guess which one of many files is going to be used for the messages from your last service start — at this point you probably guessed that there are a few things I hope to rectify in Munin 2.1.
I’m pretty sure that there are other concerns that could come up, but for now I guess this would be enough for me to have a much simpler life as an init script maintainer.