Date: Sat, 17 Jul 2010 18:43:07 -0400 From: jhell <jhell@DataIX.net> To: Ed Schouten <ed@80386.nl> Cc: ports@freebsd.org, rc@freebsd.org Subject: Re: General note on rc scripts and daemonizing Message-ID: <alpine.BSF.2.00.1007171823210.26551@pragry.qngnvk.ybpny> In-Reply-To: <20100717105658.GV1742@hoeg.nl> References: <20100717105658.GV1742@hoeg.nl>
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On Sat, 17 Jul 2010 06:56, Ed Schouten wrote:
In Message-Id: <20100717105658.GV1742@hoeg.nl>
> 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 Ed,
Very nice note as well a very good practice. I have noticed this
for a while but never looked into it more so I could not really put a name
to it. Thanks.
Off topic of ports:
While this subject is hot, I have been doing the following on an
updated system, current version of xterm on two up-to-date stable/8
machines. I am having trouble narrowing down the cause of the controlling
pseudo terminal freezing until ^C is hit after using daemon(1) to spawn
ssh in the background to start a remote xterm.
# Open a pseudo terminal [pts/13]
xterm (the culprit)
# Mix up the terminal a little so its not so fresh. [pts/13]
ls -l
# Use daemon to start a remote xterm through ssh. [pts/13]
daemon ssh -M remotehost xterm
At this stage the remote x11 forwarded xterm opens and works properly
"set this terminal aside, its not the problem".
# On the originating pseudo terminal [pts/13]
su -
Password: **********
host# _
After that you should have to hit ^C to proceed to the next bang line or
enter anything for that matter.
Any clue at what might be going on or any more information that I could
provide to help deduce this ?.
Regards,
--
jhell,v
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