Re: [PATCH] xenconsole: Allow non-interactive use

[Martin Lucina <martin@lucina.net>]

On Thursday, 23.07.2015 at 16:23, Ian Campbell wrote:
> On Thu, 2015-07-23 at 17:09 +0200, Martin Lucina wrote:
> 
> > > But maybe it would be better to set max_fd = -1 on declaration and 
> > > do
> > > the max dance here as with the following cases?
> > 
> > Declaring max_fd = -1 is indeed clearer, I can do a v2 with that 
> > change if you like.
> 
> If you are happy to then yes, please.
> 
> > One other bug that my change makes potentially easier to trigger is that
> > you can run "xl console DOMID" multiple times with the same DOMID and the
> > result is broken; each instance gets part of the data written to the
> > console.
> > 
> > Any ideas on how to address this in a simple fashion?
> 
> Perhaps the client should take some exclusive lock (fcntl based?) on an
> fd of an open file with domid in the name. Failure to get the lock
> should result in the client exiting with some message indicating the
> console is in use.

That begs the question of where to put the lock file. eg. NetBSD does not
seem to have /var/lock.

I tried using flock(pty_fd, LOCK_EX | LOCK_NB) locks directly on the pty
device and contrary to the documentation claiming support for regular files
only it worked fine. I tested using the xenconsole code on my Debian dom0,
and flock(1) on NetBSD. Would you consider this approach?

Martin