[Libguestfs] nbdkit does not support NBD_OPT_GO?

[Nir Soffer]

When trying to connect to nbdkit with imageio nbd client, I get this error:

$ rm -f /tmp/nbd.sock && nbdkit file file=/var/tmp/fedora-27.img -e export
-U /tmp/nbd.sock

$ python -c "import logging; logging.basicConfig(level=logging.DEBUG); from
ovirt_imageio_common import nbd; c = nbd.Client('/tmp/nbd.sock', 'export')"
INFO:nbd:Connecting to '/tmp/nbd.sock' 'export'
DEBUG:nbd:Received server flags: 3
DEBUG:nbd:Sending client flags: 1:
DEBUG:nbd:Sending option: 'IHAVEOPT\x00\x00\x00\x07\x00\x00\x00\x0c' data:
bytearray(b'\x00\x00\x00\x06export\x00\x00')
DEBUG:nbd:Received reply [magic=3e889045565a9 option=7 type=80000001 len=0]
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "ovirt_imageio_common/nbd.py", line 126, in __init__
    self._newstyle_handshake(export_name)
  File "ovirt_imageio_common/nbd.py", line 181, in _newstyle_handshake
    self._receive_go_reply()
  File "ovirt_imageio_common/nbd.py", line 206, in _receive_go_reply
    .format(ERROR_REPLY[reply], message))
ovirt_imageio_common.nbd.Error: The option sent by the client is unknown by
this server implementation [message=])

According to the NBD protocol:

    For backwards compatibility, clients SHOULD be prepared to also handle
    NBD_REP_ERR_UNSUP by falling back to using NBD_OPT_EXPORT_NAME.

Should I fall back to sending NBD_OPT_EXPORT_NAME, or nbkit needs
to implement NBD_OPT_GO?

The client code is here:
https://gerrit.ovirt.org/c/93384/

Nir
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