Re: [Qemu-devel] [PATCH 1/1] qemu-nbd: add the option to use pre-created server socket

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Thu, Sep 29, 2016 at 12:13:18PM +0100, Daniel P. Berrange wrote:
> On Thu, Sep 29, 2016 at 02:02:15PM +0300, Denis V. Lunev wrote:
> > From: Denis Plotnikov <dplotnikov@virtuozzo.com>
> > 
> > Originally NBD server socket was created by qemu-nbd code. This leads to
> > the race when the management layer starts qemu-nbd server and allows a
> > client to connect to the server. In this case there is a possibility that
> > qemu-nbd does not open listening server socket yet. Creating listening
> > socket before starting of qemu-ndb and passing socket fd via command line
> > solves this issue completely.
> 
> FWIW, this could be solved in qemu-nbd itself if we had a general
> ability to request "daemon" mode - currently it only daemonizes
> if attaching to a nbd block device.
> 
> The key would be that qemu-nbd would open the listening socket
> before daemonizing. Thus when the mgmt application spawned
> qemu-nbd in daemon mode, it can be sure that the listener
> socket is present when waitpid() completes.

A patch to do that was just posted:
[PATCH v4 1/3] qemu-nbd: Add --fork option

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