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Date: Thu, 29 Sep 2016 12:13:18 +0100
From: "Daniel P. Berrange" <berrange@redhat.com>
Message-ID: <20160929111318.GN5312@redhat.com>
Reply-To: "Daniel P. Berrange" <berrange@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 1/1] qemu-nbd: add the option to use
 pre-created server socket
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To: "Denis V. Lunev" <den@openvz.org>
Cc: qemu-devel@nongnu.org, Denis Plotnikov <dplotnikov@virtuozzo.com>, Paolo Bonzini <pbonzini@redhat.com>

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.

Regards,
Daniel
-- 
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