[Libguestfs] [PATCH nbdkit 0/5 NOT WORKING] nbd: Implement command= and socket-fd= parameters.

The first four patches are fairly routine clean up and can be reviewed/applied on their own. The fifth patch is problematic as described below. At the moment if you want to proxy through to qemu-nbd (eg. for handling qcow2 files) it's rather complicated and you end up having to manage the sockets and clean up yourself. However the library we use for the proxying supports a perfectly good way to run qemu-nbd as a subprocess: http://libguestfs.org/nbd_connect_systemd_socket_activation.3.html#EXAMPLE Why not use that instead? Then you'd be able to write: nbdkit nbd command=' qemu-nbd -f qcow2 disk.qcow2 ' and place filters in front. nbdkit would deal with clean up of the qemu-nbd process. However I've been staring at this most of the day and for reasons I don't understand the nbd plugin deadlocks. Specifically it waits forever on the semaphore in nbdplug_reply. A second problem with this patch (maybe related to the first??) is that the command is specified as a string, but the libnbd API wants an argv[]. There's no really good way to specify an argv as parameters of an nbdkit plugin. You'd end up having to write something pretty clunky such as one of these: nbdkit nbd command='qemu-nbd,-f,qcow2,disk.qcow2' nbdkit nbd command=qemu-nbd command=-f command=qcow2 command=disk.qcow2 Rich.