Re: What is the process group hack

[Wayne Marshall <wcm@b0llix.net>]

On Thu, 26 Apr 2012 20:45:28 +0530
harish badrinath <harishbadrinath@gmail.com> wrote:

> Hello,
> 
> I Could not figure out what "process group hack" is supposed
> to be utilized for ??
> Is it used to supervise daemons that stubbornly fork into the
> background. Could anyone please explain with an example, i
> would be really helpful. I have to the best of my abilities
> RTFM'ed and searched the internet.
> 
> Thank you,
> Harish
> 

This question refers to specific utilities in the daemontools
suite by Daniel Bernstein.

pgrphack is simply a wrapper around the setsid(2) system call.
It runs a process in a new session and process group.  See the
man page for setsid(2) for more information.  See also the
source file pgrphack.c in the daemontools distribution.

fghack is djb's "anti-backgrounding" utility.  It invokes
strangeness and mysteries to inhibit a process from forking into
the background.  Dunno about examples.  Normally it is not used
because: 1) nowadays most programs that daemonize will be
decently written and offer an option to run in the foreground;
or -- if such option is not provided -- 2) it is usually a
simple matter to delete a few lines of the offensive source code
to cut out the daemonizing behavior.

It should be mentioned that daemontools itself should be
considered deprecated by several alternatives currently
available and under active maintenance.  For example, runit,
daemontools-encore, and perp.  I believe most of these tools now
run each service in its own session and process group as a
matter of course, something that daemontools does not do, unless
you yourself explicitly run each service under the pgrphack
utility.

Why does this matter?  If the entire supervision tree runs under
a single process group, that entire process tree can be crashed
under certain conditions in some cases on some platforms.  Which
is a bad thing.

Internally segregating each service into its own process group is
more reliable, and generally eliminates any need for something
like pgrphack within the service definitions themselves.

Wayne
http://b0llix.net/perp/