Date: Tue, 20 Jul 2010 18:01:09 -0400 From: Greg Larkin <glarkin@FreeBSD.org> To: Ed Schouten <ed@80386.nl> Cc: ports@FreeBSD.org, rc@FreeBSD.org Subject: Re: General note on rc scripts and daemonizing Message-ID: <4C461CA5.2040307@FreeBSD.org> In-Reply-To: <20100717105658.GV1742@hoeg.nl> References: <20100717105658.GV1742@hoeg.nl>
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Ed Schouten wrote:
> Hello port maintainers,
>
> I think I'd better send an email about this to ports@, because I've seen
> it in various places and it is getting a bit tiresome to mail all port
> authors individually.
>
> I've seen various cases in the past where people write rc scripts that
> do the following:
>
> command="/usr/local/bin/dog"
> command_args="--bark > /dev/null 2>&1 &"
>
> So in this case `dog --bark' doesn't daemonize itself, so the & is
> sufficient here, right? Well, it is not. :-) The point is that we simply
> tell the kernel to redirect stdout/stderr and run it in the background.
> It doesn't tell the kernel that the process should run in a separate
> session (see getsid(2)/setsid(2)).
>
> This has various implications. The most important one I can think of, is
> that the daemon can still do open("/dev/tty", ...) if it wants and spam
> your TTY, even if the daemon is running as user `nobody'. This also
> means that if you run the rc script from within a pseudo-terminal, it
> can never actually destroy the pseudo-terminal for you, because maybe
> the daemon is interested in using it.
>
> Below is the output of `pstat -t' on one of my systems, where I decided
> to fire up MySQL:
>
> | LINE INQ CAN LIN LOW OUTQ USE LOW COL SESS PGID STATE
> | ...
> | pts/11 0 0 0 0 0 0 0 0 82711 0 G
>
> The kernel actually wants to clean up this pseudo-terminal (state = G),
> but it is prevented from doing so. It will only clean it up by the time
> MySQL is shut down.
>
> So how can this be solved? We already have a tool in base called
> daemon(8). It is simply a wrapper around daemon(3) (which calls
> setsid(2), which you can use to daemonize processes. So the next time
> you write an rc script and need to daemonize something which cannot do
> it by itself, please think of the kittens. ;-)
>
> [ CCing this to rc@. Maybe we should add some kind of built-in
> functionality to call daemon(8)? ]
>
Hi all,
Ed alerted me to this problem in the mail/nullmailer port some months
back, and I fixed it with his assistance. A user recently opened a PR
on another port I maintain (devel/viewvc), and I noticed that it had the
same problem with its standalone server. This was a little harder to
fix, since viewvc is a Python script.
Anyway, here are some examples for daemonizing processes that don't
already have support for doing it themselves:
Daemonizing an executable without internal daemon support:
http://www.freebsd.org/cgi/cvsweb.cgi/ports/mail/nullmailer/files/nullmailer.in?rev=1.3;content-type=text%2Fplain
Daemonizing a Python script:
http://www.freebsd.org/cgi/cvsweb.cgi/ports/devel/viewvc/files/viewvc.in?rev=1.4;content-type=text%2Fplain
I would love to see direct support for these use cases in /etc/rc.subr,
and am interested in working with someone to add it.
- -Greg
- --
Greg Larkin
http://www.FreeBSD.org/ - The Power To Serve
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