[libvirt] Bug: vnc + websocket = websocket autoport not working right at live migration

[Piotr Rybicki]

Hello.

Problem description:

When i start qemu via libvirt with vnc websocket defined, it is not 
possible to live migrate to host where other qemu process is running 
with the same display id.

migration error is:

error: internal error: early end of file from monitor: possible problem:
             [1] => 2015-06-23T11:54:25.590506Z qemu-system-x86_64: -vnc 0.0.0.0:1,websocket=5700,password: Failed to start VNC server on `(null)': Failed to bind socket: Address already in use

(please note vnc display id=1 and websocket=5700 - where it should be 5701)

in libvirt's xml i have:
(...)
     <graphics type='vnc' port='-1' autoport='yes' websocket='-1' 
listen='0.0.0.0' passwd='xxx'>
       <listen type='address' address='0.0.0.0'/>
     </graphics>
(...)

for first and only qemu process on host, this creates qemu commandline:
(...) -vnc 0.0.0.0:0,websocket=5700,password (...)

for second qemu process on the same host:
(...)  -vnc 0.0.0.0:1,websocket=5701,password (...)

There is no problem with migration, when there is no websocket 
configuration.

Solution:

I believe, to solve this problem, libvirt has to omit websocket port 
definition in commandline string ('websocket=5700' => 'websocket') when 
autoport is defined in domain xml definition.

from man qemu:
      -vnc display[,option[,option[,...]]]
(...)
           websocket
                Opens an additional TCP listening port dedicated to VNC 
Websocket connections.  By definition the Websocket port is
                5700+display. If host is specified connections will only 
be allowed from this host.  As an alternative the Websocket port
                could be specified by using "websocket"=port.  TLS 
encryption for the Websocket connection is supported if the required
                certificates are specified with the VNC option x509.

So if I understand it right, not specifying websocket port means 
5700+display id, and display id is given via commandline and increments 
just fine.

Can anyone confirm this? Or perhaps there is some misconfiguration in my 
xml domain definition?

Best regards
Piotr Rybicki