Saturday, January 29, 2011

How to testament... childs' death

Often Unix processes spawn childs. Contrary to the popular belief childs DO NOT automatically die when their parent exits.

Unix job control has a concept of sessions and session leaders. Session leader is the process that 'owns' terminal, and when this guy dies, each process of session gets SIGHUP. This usually leads to death. Every time you open new tab in your terminal emulator, you're creating new session. And bash (or other shell) is leader of this session. So your childs will likely die when terminal emulator tab will be closed, but they will easily outlive your process. And they will live forever when your main process is spawned from cron or your favorite continuous integration tool.

Of course the whole topic of Unix job control is slightly more complex. You can read more by reading GNU libc manual (or aptitude install glibc-doc && info libc). Posix man pages (aptitude install manpages-posix{,-dev}) contain lots of details, much more than usual Linux man pages.

So one of the ways to ensure your childs die is by using sessions. This is usually implemented by creating pseudo terminal pair (PTY) and giving slave side of that pair to child. Session leader (i.e. child) gets SIGHUP when master side of PTY pair is closed. Master side is closed when your main process exits (if you don't pass it's fd to child by mistake). When child dies, this causes delivery of SIGHUP to whole session. {spawn,fork}pty-like functions are used for that.

And there's another even less widely known way. There is a concept of orphaned process group that can be used to ensure SIGHUP delivery without relying on PTYs. I'll refer to posix man page for details. But basically you fork and deliver SIGSTOP to child. When your process group becomes orphaned (e.g. when main process dies), each member will be sent SIGCONT & SIGHUP. Added benefit of this approach is that if parent of your main process dies, your process and it's child will die too. This relies on main process or it's parent being process group leader, but that usually holds.

I'm using this to control a bunch of erlang VMs when running Membase in development mode. You can grab python code for doing that here. Ruby code for doing same is removed, but you can dig it's grave.

UPDATE: unfortunately this trick doesn't always work in OSX/Darwin due to different definition of orphaned process group in OSX: