Re: [PATCH v2 7/7] systemd: add support initial xen systemd service files

From: "Luis R. Rodriguez" <mcgrof@suse.com>
To: Ian Campbell <Ian.Campbell@citrix.com>
Cc: "Stefano Stabellini" <stefano.stabellini@eu.citrix.com>,
	"Jan Rękorajski" <baggins@pld-linux.org>,
	"Ian Jackson" <ian.jackson@eu.citrix.com>,
	"Jacek Konieczny" <jajcus@jajcus.net>,
	"M A Young" <m.a.young@durham.ac.uk>,
	xen-devel@lists.xenproject.org,
	"Luis R. Rodriguez" <mcgrof@do-not-panic.com>
Subject: Re: [PATCH v2 7/7] systemd: add support initial xen systemd	service files
Date: Mon, 24 Mar 2014 17:36:57 +0100	[thread overview]
Message-ID: <20140324163657.GA3463@wotan.suse.de> (raw)
In-Reply-To: <1395655873.19365.17.camel@kazak.uk.xensource.com>

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On Mon, Mar 24, 2014 at 10:11:13AM +0000, Ian Campbell wrote:
> On Sat, 2014-03-22 at 03:26 +0100, Luis R. Rodriguez wrote:
> > On Fri, Mar 21, 2014 at 10:08:14AM +0000, Ian Campbell wrote:
> > > On Wed, 2014-03-19 at 13:58 -0700, Luis R. Rodriguez wrote:
> > > > [...]
> > > > diff --git a/tools/hotplug/Linux/systemd/oxenstored.service.in b/tools/hotplug/Linux/systemd/oxenstored.service.in
> > > > [...]
> > > > +ExecStartPost=-@BINDIR@/xenstore-write "/local/domain/0/name" "Domain-0"
> > > > diff --git a/tools/hotplug/Linux/systemd/xenstored.service.in b/tools/hotplug/Linux/systemd/xenstored.service.in
> > > [...]
> > > > +ExecStartPost=@BINDIR@/xenstore-write "/local/domain/0/name" "Domain-0"
> > > 
> > > I accidentally deleted the subthread about writing domid here too, but I
> > > was wondering if it might be better to have a common xenstore service
> > > which depends on oxenstore.service || cxenstore.service and then does
> > > this kind of common implementation agnostic setup in one place where it
> > > can't get out of sync easily?
> > 
> > That's what I was hoping for to achieve with the socket file but that
> > seems to not work as expected, even if you claim the socket explicitly
> > as part of both oxenstored and xenstored. To me this could likely be
> > an enhancement to systemd but not sure.
> > 
> > systemd does not allow one to use || as part of the language for requirements,
> 
> Does it not have some sort of "Provides: some-virtual-facility" which
> two things can provide and other things can depend on?

Not that I have seen.

> > This means we either do some sort of meta @VARIABLE@ substitution or a common
> > init routine which will do the or checking for us. The only problem with this
> > is systemd will treat the Forking type service ExecStart as the process to care
> > for, and if we add a wrapper that'd be dead. I haven't tried to implement one
> > but I think this could confuse systemd or administrators.
> 
> Yes, lets not go that route.

I studied this approach a bit more and it turns out that execve() does just what
we want, it will carry the same PID for the spawned process, it won't return, it'll
just go onto the other process, if it fails it returns back, in which case we can
try the other cxenstored. We'd just have to rename the C xentstored to censtored.
This would simplify things for both init systems and systemd. In the init system
the check for oxenstored can be removed, in systemd we'd just have one systemd 
service file.

In light of this I'd like for us to consider this approach now.

Thoughts?

  Luis

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