print lxc-start-ephemeral container name in a machine-readable way

[Robie Basak]

A script cannot call lxc-start-ephemeral and get a named container in a reliable, race-free way.

Having the caller specify a name directly is racy, since the name could have been taken in between checking that it doesn't exist already and calling lxc-start-ephemeral.

Allowing lxc-start-ephemeral allows it to take care of the mkdir (and retries) to generate an LXC container name in a race free manner, but this needs -d to return the name in a machine-readable way, so that we can create ephemeral LXC containers from scripts.

Please add a machine-readable mechanism to "lxc-start-ephemeral -d" and then we can modify adt-virt-lxc to use it.

[Serge Hallyn (serge-hallyn)]

Quoting Robie Basak (<email address hidden>):
> Public bug reported:
>
> A script cannot call lxc-start-ephemeral and get a named container in a
> reliable, race-free way.
>
> Having the caller specify a name directly is racy, since the name could
> have been taken in between checking that it doesn't exist already and
> calling lxc-start-ephemeral.

Practically speaking, have you actually created the container name with
mktemp (i.e. mktemp -u -t lxc-XXXXXX | sed 's@/tmp/@@') and gotten name
clashes?

> Allowing lxc-start-ephemeral allows it to take care of the mkdir (and
> retries) to generate an LXC container name in a race free manner, but
> this needs -d to return the name in a machine-readable way, so that we
> can create ephemeral LXC containers from scripts.
>
> Please add a machine-readable mechanism to "lxc-start-ephemeral -d" and
> then we can modify adt-virt-lxc to use it.

That does sound like a good idea - we could add a '--terse' suboption to
-d to only print '$name $ip'

[Robie Basak (racb)]

On Fri, Jul 05, 2013 at 12:01:08PM -0000, Serge Hallyn wrote:
> Practically speaking, have you actually created the container name with
> mktemp (i.e. mktemp -u -t lxc-XXXXXX | sed 's@/tmp/@@') and gotten name
> clashes?

No, but eyebrows were raised on my approach of generating a random name
on code review, so that's why I filed this bug.

> > Please add a machine-readable mechanism to "lxc-start-ephemeral -d" and
> > then we can modify adt-virt-lxc to use it.
>
> That does sound like a good idea - we could add a '--terse' suboption to
> -d to only print '$name $ip'

That'd be great.

[Serge Hallyn (serge-hallyn)]

 summary: lxc-start-ephemeral - provide terse -d output option
 status: Triaged
 importance: medium

[Stéphane Graber (stgraber)]

stgraber@dakara:~/data/code/lxc/lxc$ lxc-start-ephemeral -o blah -d | grep "lxc-console" | awk '{print $NF}'
blah-psrfp_3s

Not the prettiest thing ever, but definitively doable with current lxc-start-ephemeral.
I won't be spending more time on lxc-start-ephemeral though as we'll have the new lxc-copy there soon that will take-over most if not all of it (lxc-start-ephemeral will likely be gone in 2.0).

So marking this fix-released as the above does work.

[Paride Legovini (paride)]

With 0b4c5056925e7358374874c8bf4767290157e903 (in autopkgtest 5.33) lxc-start-ephemeral is not called anymore.