CTDB Segfault in a container-based env - Looking for pointers

[Amitay Isaacs]

On Fri, Jul 16, 2021 at 2:13 AM Michael Adam via samba-technical
<samba-technical at lists.samba.org> wrote:
>
>
>
> > On 15. Jul 2021, at 15:16, John Mulligan via samba-technical <samba-technical at lists.samba.org> wrote:
> >
> > On Wednesday, July 14, 2021 10:12:46 PM EDT Amitay Isaacs via samba-technical
> > wrote:
> >> Hi John,
> >>
> >> There are certain assumptions made in CTDB assuming it's running on a
> >> normal system. When running CTDB in a container, those assumptions are
> >> not valid any more and you might get unexpected behaviour.
> >>
> >
> > First, thanks for replying!
> >
> > Sure, I fully expect that. It was similar for smbd/winbind but in those cases
> > I was able to tune the environment sufficiently - for example they need to run
> > within the same pid namespace to function properly. The issue I'm having now
> > is that the segfault isn't mapping to anything obvious (yet) that I can change
> > in the environment.
> >
> >> One such assumption is that init (in some form) has the pid of 1 and
> >> CTDB daemon will never have PID of 1.  Obviously this is not true in
> >> your case.  From the logs you can see that the CTDB daemon is started
> >> as PID 1. In general, CTDB relies on init (in some form) to start/stop
> >> various file services (smb, nfs, etc.) via the event scripts.  So,
> >> working init is a requirement for normal operation of CTDB.
> >
> > Good point. I'll experiment with giving ctdb a parent process.
>
> Right, if we want to avoid systemd or other beefier systems that are not made for containers, we can consider “tini”: E.g. rook is using this.
>
> >> What are you trying to do exactly?  You cannot put CTDB in a container
> >> on its own without Samba daemons.
>
> Hmm, at least last I checked you can even run ctdb in a “traditional” non-containerized cluster without any samba daemons. :-)

Of course you can.  But that doesn't serve any useful purpose. :-)

> Maybe you are saying that if you want to run smbd/winbindd on top of ctdb, then they must run in the same container? I don’t think this is true either:
>
> We usually have multiple containers in one pod, and the containers within the pod can communicate just as normal. At least that’s what we did with the smbd and windbindd daemons: separate containers in one pod.
>

My understanding of containers is limited here, so I don't understand
how you can run ctdb and smbd in different containers.  Does mutex
locking on shared databases work across containers (or different
namespaces)?  How about unix datagram messaging using pids?
If mutex locking on shared databases works across containers, then
obviously you can run ctdb and smbd in different containers.
If unix datagram messaging works across containers, then obviously you
can run smbd and winbindd in different containers.

> > I'm not clear on what you mean by that. My longer term goal is to investigate
> > CTDB as part of the HA story for samba in containers (see our general effort
> > here [1]). Short term, I just want to run ctdb on its own with very few (or
> > none) event scripts just to get tdb replication working across multiple nodes
> > in a container based environment. Based on my reading of the docs and a tiny
> > bit of the code, bringing up smbd/etc is the responsibility of the event
> > scripts
>
> This is not quite true:
>
> Ctdb logically consists of two layers:
>
> (1) the mandatory core is the distributed tab database and messaging channel for smbd
> (2) the optional upper layer is about resource management (public IPs, services like smbd, winbindd, etc)
>
> Ctdb and samba can run together perfectly without #2 as long as someone takes care of the service management. E.g it has been done with pacemaker. In our case, Kubernetes / operators, etc, would provide this role and we would run ctdb without “CTDB_MANAGES_SAMBA=yes” etc...
>
> > so I think it should be possible to run ctdb on its own like that.
> >
> > Any thoughts on adding code to specifically handle the case where the callback
> > has already been called, but tevent calls it again?
>
> Right the crux here seems to be the question whether the tevent-using code in ctdb is not prepared for the situation that EPOLLHUP is issued, and if  it would be appropriate to just catch that condition (of being called again).

Well that's not really the crux here.  I know what the real issue is
(I did write that code), but I still don't understand the motivation
behind running ctdb and smbd in different containers.

>
> But it is of course good and correct to weed out any higher level config issues before diving into this.
>
> Cheers - Michael
>
> >

Amitay.