Poettering: Revisiting how we put together Linux systems

The non-forking daemon approach as recommended for systemd is what basically every init system other than System-V init prefers (check out Upstart or, e.g., the s6 system mentioned here earlier). It allows the init system to notice when the daemon quits because it will receive a SIGCHLD, and the init system can then take appropriate steps like restart the daemon in question. In addition, it makes it reasonably easy to stop the daemon if that is necessary, because the init process always knows the daemon's PID (systemd uses cgroups to make this work even if the daemon forks other processes).

The »double-forking« strategy is needed with System-V init so that daemon processes will be adopted by init (PID 1). The problem with this is that in this case the init process does receive the notification if the daemon exits but has no idea what to do with it. The init process also has no idea which daemons are running on the system in the first place and where to find them, which is why many »proper Linux daemons« need to write their PID to a file just so the init script has a fighting chance of being able to perform a »stop« – but this is completely non-standardised, requires custom handling in every daemon's init script, and has a certain (if low) probability of being wrong.

In general it is a good idea to push this sort of thing down into the infrastructure rather than to force every daemon author to write it out themselves. That way we can be reasonably sure that it actually works consistently across different services and is well-debugged and well-documented. That this hasn't happened earlier is only too bad but that is not a reason not to start doing it now. People who would like to run their code on System-V init are free to include the required song and dance as an option, but few if any systems other than Linux actually use System-V init these days – and chances are that the simple style of daemon that suits systemd better is also more appropriate for the various init replacements that most other Unix-like systems have adopted in the meantime.