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Re: [libvirt] PATCH: 3/5: Introduce a container controller process



On Wed, Jul 16, 2008 at 11:30:53AM -0700, Dan Smith wrote:
> DB> +static int lxcControllerMoveInterfaces(int nveths,
> DB> +                                       char **veths,
> DB> +                                       pid_t container)
> DB> +{
> DB> +    int i;
> DB> +    for (i = 0 ; i < nveths ; i++)
> DB> +        if (moveInterfaceToNetNs(veths[i], container) < 0) {
> DB> +            lxcError(NULL, NULL, VIR_ERR_INTERNAL_ERROR,
> DB> +                     _("failed to move interface %s to ns %d"),
> DB> +                     veths[i], container);
> DB> +            return -1;
> DB> +        }
> DB> +
> DB> +    return 0;
> DB> +}
> 
> I'm not sure why, but the call to this function causes a failure on my
> system.  I think that it's related to luck somehow, so I don't think
> it's something that was really introduced by this set, but it prevents
> me from starting a container with a network interface.
> 
> I've tracked it down to the actual virRun() of the 'ip' command.  If I
> comment out the line that virRun()'s ip, everything works fine.
> However, if it actually gets run, the container never receives the
> continue message.

Yep, as I mentioned on IRC I've not been able to test the network
stuff myself yet, since 2.6.26 hangs on my laptop. I'll hopefully
be able to get it running on one of my development servers when
I'm back in the office next.

> I added a loop in lxcWaitForContinue() that reads the socket
> character-by-character instead of bailing out if the first character
> isn't 'c'.  The result was a bunch of control characters followed by
> 'c'.
> 
> Based on these two facts, I would tend to guess that somehow the exec
> of ip is causing some terminal stuff (yes, that's my technical term
> for it) to be written to the socket ahead of the continue message.  I
> changed 'ip' to 'true' in moveInterfaceToNetNs() and it behaves the
> same (which absolves ip itself).  I removed the virRun() and replaced
> with a fork()..exec() and it behaves the same.  I set FD_CLOEXEC on
> the socket pair, and it behaves the same.

Ok. that's useful info - I'll do some poking around this.

> DB> -    if (0 != (rc = lxcSetupInterfaces(conn, vm))) {
> 
> You changed the condition from "nonzero" here...

Opps.

> 
> <snip>
> 
> DB> +    if (lxcSetupInterfaces(conn, vm->def, &nveths, &veths) < 0)
> 
> ...to "negative" here, which makes the start process not notice some
> of the failure paths in that function, and thus will erroneously
> return success.
> 
> In general, I think this is an excellent approach, but I don't really
> like how many (dozens of) failure points there are in the controller
> and container setup procedure that result in a silent failure.  Almost
> all of them leave libvirtd thinking that the container/controller is
> running, but in fact, it _exit(1)'d long ago.

Yes, those are bugs. THe series of patches I sent missed out my change
to have the libvirtd daemon register the socket connection to the
controller with the event loop. Basically libvirtd needs to watch for
SIGHUP on its socket to the conttroller so it detects death. The
controller also doesn't seem to always exit when the container dies
so its probaly another condition i broke in the I/o forwarding loop.

The one difference i gues is that the failures are not directly fed back
as errors from the virDomainCreate() call. Ultimately I hope this isn't
too much of a problem - the really interesting errors are going to be in
the container process, and they were never fed back - the errors in the
controller process are mostly my coding bugs which should be fixed :-)
much

Btw, anything fprintf'd in the controller (eg, via lxcError() )
will end up in /var/log/libvirt/lxc/NAME.log

Daniel
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