librexit -- remote exit for daemons

When writing a daemon providing some service, one usually wants to daemonize early, before setting up stuff which may not respond too well to a call to fork(2). However, this inhibits communicating whether and when the service actually becomes available. This information is obviously of interest, especially if we want to start services depending on the current one.

Solutions to this problem in rcs included praying and waiting, regular testing via a client, hard dependencies on pötterware and notifications over the infamous DBus. While all of these solutions may work one way or another, they complicate the calling site, e.g. by making classical rc scripts even more unwieldy. It would be nice if, instead, the process which we started originally would just communicate success and failure via its return status, like we'd expect from non-daemon setup one-shot commands. Thankfully, unlike Northern Ireland's customs borders, we can figure this one out easily with the help of some primitive IPC.

This library replaces the daemon(3) function and introduces a new function rexit. Like the usual daemon(3) function, our variant will fork(2) and call setsid(2) in the child process. However, the parent process will not exit(2) right away but wait for more information from the child process. It will ultimately exit in one of the following situations:

• The child process called rexit. In this case, it will exit(2) with the given status.
• The child terminated. In this case the child's exit status will be forwarded if it could be determined. Otherwise, it will terminate with EXIT_FAILURE.
• The child closed the pipe using for IPC. This is done automatically if the child calls one of the exec* system calls. In this case, the parent will terminate with EXIT_FAILURE.

Obviously, rexit should be called after all the important setup work is done right before/after the service effectively becomes available.