"expect daemon" and daemons being ready to serve requests

[Steve Langasek]

Hi Robie,

On Tue, Oct 01, 2013 at 05:21:03PM +0100, Robie Basak wrote:
> Currently, when libvirtd is started (process A) with "-d" to daemonize,
> it:
>   1) Forks process B.
>   2) Forks process C (from B).
>   3) Binds a Unix socket (in C).
>   4) Listens on a Unix socket (in C).
>   5) Causes process A to exit (using an internal pipe to communicate
>      between processes C and A).

> libvirtd's packaging in Ubuntu (libvirt-bin) currently uses an upstart
> script that calls "libvirtd -d" and uses "expect daemon".

> Upstart detects that the daemon has started at step 2 above. This
> creates a race: libvirt-bin.postinst finishes before step 4, and so a
> subsequent uvtool.postinst that depends on libvirt-bin fails when it
> tries to set up a libvirt volume storage pool and cannot connect to
> libvirtd.

> I have other solutions in the bug: making uvtool.postinst explicitly
> wait for the Unix socket to appear, and switching to "expect stop".

> But libvirtd's behaviour seems reasonable to me. And in the Sys V world,
> an init.d script would only exit after step 5 above, so there would be
> no race. So it feels bad to introduce the SIGSTOP hack into libvirt in
> order to solve this problem.

> Could upstart wait() on process A before concluding that the service is
> started, in the case of "expect daemon" (and also, presumably, "expect
> fork")? Would changing upstart's behaviour to do this cause any problems or
> regressions that I haven't thought of?

> libvirt bug: https://launchpad.net/bugs/1228210

Indeed, as you may have gleaned from the IRC discussion, that's exactly the 
plan.  This is bug #530779 in upstart.

-- 
Steve Langasek                   Give me a lever long enough and a Free OS
Debian Developer                   to set it on, and I can move the world.
Ubuntu Developer                                    http://www.debian.org/
slangasek at ubuntu.com                                     vorlon at debian.org
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