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 <dplotni...@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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