Re: [Qemu-devel] Multiple QMP socket clients

["Daniel P. Berrange" <berrange@redhat.com>]

On Tue, Oct 11, 2016 at 03:17:04PM +0100, Daniel P. Berrange wrote:
> On Tue, Oct 11, 2016 at 03:08:34PM +0100, Dr. David Alan Gilbert wrote:
> > * Daniel P. Berrange (berrange@redhat.com) wrote:
> > > On Tue, Oct 11, 2016 at 12:26:37PM +0200, Samuel Ortiz wrote:
> > > > Hi,
> > > > 
> > > > On a qemu instance started with a qmp unix socket:
> > > > 
> > > > -qmp unix:/tmp/ctrl.sock,server,nowait
> > > > 
> > > > I am trying to have multiple clients working on that socket but
> > > > although the qmp server seems to accept the connections, only the first
> > > > connected client gets his request processed. The next client requests
> > > > will just hang until the first one exits.
> > > > 
> > > > Is that an intended behaviour ?
> > > 
> > > Yes, the character device code is designed around the idea of a single
> > > endpoint.
> > > 
> > > In the case of the monitor you could work around it by adding multiple
> > > -qmp arguemnts, each with different socket. Of course you have to make
> > > sure each client doesn't trample on the other client when doing this.
> > 
> > But why does it accept the connection?
> > I thought you could say only accept a single connection on a socket.
> > (The backlog parameter to listen(2) but I can't find out listen.)
> 
> QEMU won't accept the connection, as while it is still listen()ing on
> the socket, it is not poll()ing for incoming clients, so will never
> trigger accept(). The kernel will queue the incoming connection until
> QEMU starts polling for clients again. From the client POV this is
> indistinguishable from QEMU accepting the client, but not processing
> I/O on it.

Oh, and the backlog parameter to listen() is fairly useless - it won't
stop new clients getting into an established socket state

   https://veithen.github.io/2014/01/01/how-tcp-backlog-works-in-linux.html

QEMU could just accept() all incoming connections and explicitly close
them if something is already connected, but this just burns CPU really.

Regards,
Daniel
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