On 10/18/2011 12:34 PM, Daniel P. Berrange wrote:
On Wed, Oct 12, 2011 at 03:43:10PM +0200, Peter Krempa wrote:
This series fixes anoying console corruption if two clients try to connect at same time to the console. The current state of this is, that two/more of libvirt iohelpers are spawned on the same time that compete for data from the pty. This causes that each of the consoles get scrambled and unusable.
The problem with doing the console checks by looking for an in use virStreamPtr is that it only solves it for apps using libvirt. If someone connects using 'xm console' or 'minicom' then we're not protected. I agree :(.
The traditional way to protect PTYs from concurrent usage is to place a lock file in /var/lock with a special standardized naming scheme. Should we perhaps be doing that instead ? I think, we should be doing this _along_ with alowing libvirt users to break existing connections while using libvirt. My main reason for this is that, if someone would open a console connection and successfully acquire the lock on the pty and then leave for extended period, while his connection is ok (Jiri's patches will handle crashed conections), he would effectively block other users from accessing the console. Same may happen if somebody uses an external tool, to acquire the lock, but in that case, we can't do anything (identifying the process that holds the console pty open and killing it is probably not what we want to do).
IMO, while using libvirt, the users will prefer to use the console provided by libvirt, and not to tinker with minicom/etc, so while it'd be still possible to block potential console users, it would at least handle the most common mistakes. (It will still be possible to break the console,by manually removing the lock file, etc ...). I think that addition of lockfile checking will improve this patchset, but doing it solely that way could be confusing for some and cause problems. Peter
Daniel