[libvirt] [PATCH] qemu: agent: Reset agentError when qemuConnectAgent success

[Nikolay Shirokovskiy]

On 27.09.2018 14:37, Wang Yechao wrote:
> qemuAgentClose and qemuAgentIO have race condition,
> as follows:
> 
> main thread:                          second thread:
> virEventPollRunOnce                processSerialChangedEvent
>   virEventPollDispatchHandles
>     virMutexUnlock(&eventLoop.lock)
>                                       qemuAgentClose
>                                         virObjectLock(mon)
>                                         virEventRemoveHandle
>                                         VIR_FORCE_CLOSE(mon->fd)
>                                         virObjectUnlock(mon)
>                                       priv->agentError = false
>     qemuAgentIO
>       virObjectLock(mon)
>         mon->fd != fd --> error = true
>         qemuProcessHandleAgentError
>           priv->agentError = true
>       virObjectUnlock(mon)
>     virMutexLock(&eventLoop.lock)
> 
> qemuAgentClose set the mon->fd to '-1', and then qemuAgentIO
> check the mon->fd not equals to fd that registered before.
> qemuProcessHandleAgentError will be called to set
> priv->agentError to 'true', then the priv->agentError is
> always 'true' except restart libvirtd or restart
> qemu-guest-agent process in guest. We can't send any
> qemu-agent-command anymore even if qemuConnectAgent return
> success later.
> 
> This is accidently occurs when hot-add-vcpu in windows2012.
>   virsh setvcpus ...
>   virsh qemu-agent-command $vm '{"execute":"guest-get-vcpus"}'
> 
> Reset the priv->agentError to 'false' when qemuConnectAgent sucess
> to fix this problem.
> 
> Signed-off-by: Wang Yechao <wang.yechao255 at zte.com.cn>
> ---
>  src/qemu/qemu_process.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c
> index 29b0ba1..4fbb955 100644
> --- a/src/qemu/qemu_process.c
> +++ b/src/qemu/qemu_process.c
> @@ -269,6 +269,7 @@ qemuConnectAgent(virQEMUDriverPtr driver, virDomainObjPtr vm)
>          virResetLastError();
>      }
>  
> +    priv->agentError = false;
>      return 0;
>  }
>  
> 

There was similar problem with qemu monitor:

commit 89563efc0209b854d2b2e554423423d7602acdbd
Author: Daniel P. Berrange <berrange at redhat.com>
Date:   Fri Sep 28 15:27:39 2012 +0100

    Avoid bogus I/O event errors when closing the QEMU monitor
    
    After calling qemuMonitorClose(), it is still possible for
    the QEMU monitor I/O event callback to get invoked. This
    will trigger an error message because mon->fd has been set
    to -1 at this point. Silently ignore the case where mon->fd
    is -1, likewise for mon->watch being zero.
    
    Signed-off-by: Daniel P. Berrange <berrange at redhat.com

This approach has advantage it does not set agentError at all. So clients
see agent state as disconnected (which is true) instead of error.

Nikolay