[libvirt] [PATCH 00/10] Console coruption with two or more clients series

Peter Krempa pkrempa at redhat.com
Thu Dec 1 15:35:10 UTC 2011

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.
>> Patches
>>    fdstream: Emit stream abort callback even if poll() doesnt.
>>    virnetclientstream: Propagate stream error messages to callback
>>    daemon: Subscribe the stream event callback for error events.
>> add the ability to abort a stream from the daemon side.
>>    fdstream: Add internal function to check if a fdstream is open
>> This patch adds a helper function that checks internally if a fdstream
>> is open and working.
>>    virsh: fix console stream error reporting
>>    Add flags for virDomainOpenConsole
>>    virsh: add support for VIR_DOMAIN_CONSOLE_FORCE flag
>> This patches add instrumentation to virsh that supports the new ability to
>> abort streams from the daemon side and adapts the console callback to handle
>> this. These patches also add a new flag set for virDomainOpenConsole API
>> call that allow users to control the if the existing console connection should
>> be left in place or killed in favor of the new one
>>    qemu: Add ability to abort existing console while creating new one
>>    lxc: Add ability to abort existing console when creating a new one
>>    uml: Add ability to abort existing console when creating a new one
>> These patches modify the hypervisor drivers so that they are aware of existing
>> console connections and refuse to create a new one (or kill the old).
>> The xen driver also supports console in a similar way like the previously
>> mentioned drivers, but lacks the means to store a stream reference
>> permanently. I'll look in if it's possible to modify this driver to support
>> this new functionality.
> 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.
> 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 ?
> Daniel

I've prepared an updated version of this series taking into account 
recent changes to console code and Daniel's comments on this version.

I've got some questions regarding implementation of lock files on PTY's 
suggested in this thread I'd like to discuss:

1) When using lock files, there's the risk of having them left behind 
when the daemon crashes/is killed. According to the filesystem hierarchy 
standard (http://www.pathname.com/fhs/2.2/fhs-5.9.html) lock files 
should contain PID of process holding the lock. This enables to check if 
the process holding the lock still exists. I'm not sure if we want to 
check if the process exists and remove lock files, that were left 
behind, or just warn the user that a lock exists and refuse to open console.

2) When a non-good-mannered program opens the PTY and does not check for 
lock files or create them, we will still end up with a corrupted 
console. Is there an elegant solution for this?

I'd appreciate your comments on this topic.


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