NBD_OPT_EXPORT_NAME is lousy: per the NBD protocol, any failure
requires the server to close the connection rather than report an
error to us.  Therefore, upstream NBD recently added NBD_OPT_GO as
the improved version of the option that does what we want [1]: it
reports sane errors on failures, and on success provides at least
as much info as NBD_OPT_EXPORT_NAME.

[1] https://github.com/NetworkBlockDevice/nbd/blob/extension-info/doc/proto.md

This is a first cut at use of the information types.  Note that we
do not need to use NBD_OPT_INFO, and that use of NBD_OPT_GO means
we no longer have to use NBD_OPT_LIST to learn whether a server
requires TLS (this requires servers that gracefully handle unknown
NBD_OPT, many servers prior to qemu 2.5 were buggy, but I have patched
qemu, upstream nbd, and nbdkit in the meantime, in part because of
interoperability testing with this patch).  We still fall back to
NBD_OPT_LIST when NBD_OPT_GO is not supported on the server, as it
is still one last chance for a nicer error message.  Later patches
will use further info, like NBD_INFO_BLOCK_SIZE.
Signed-off-by: NEric Blake <eblake@redhat.com>
Message-Id: <20170707203049.534-8-eblake@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

NBD_OPT_EXPORT_NAME is lousy: per the NBD protocol, any failure
requires us to close the connection rather than report an error.
Therefore, upstream NBD recently added NBD_OPT_GO as the improved
version of the option that does what we want [1], along with
NBD_OPT_INFO that returns the same information but does not
transition to transmission phase.

[1] https://github.com/NetworkBlockDevice/nbd/blob/extension-info/doc/proto.md

This is a first cut at the information types, and only passes the
same information already available through NBD_OPT_LIST and
NBD_OPT_EXPORT_NAME; items like NBD_INFO_BLOCK_SIZE (and thus any
use of NBD_REP_ERR_BLOCK_SIZE_REQD) are intentionally left for
later patches.
Signed-off-by: NEric Blake <eblake@redhat.com>
Message-Id: <20170707203049.534-7-eblake@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Simplify the tracing of client flags in the server, and return -EINVAL
instead of -EIO if we successfully read but don't like those flags.
Signed-off-by: NEric Blake <eblake@redhat.com>
Message-Id: <20170707203049.534-5-eblake@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

The NBD protocol has several constants defined in various extensions
that we are about to implement.  Expose them to the code, along with
an easy way to map various constants to strings during diagnostic
messages.
Signed-off-by: NEric Blake <eblake@redhat.com>
Message-Id: <20170707203049.534-4-eblake@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

We really don't care if our spec-compliant reply to NBD_OPT_ABORT
was received, so shave off some lines of code by not even tracing it.
Signed-off-by: NEric Blake <eblake@redhat.com>
Message-Id: <20170707203049.534-3-eblake@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Let NBD use the trace mechanisms already present in qemu. Now you can
use the -trace optino of qemu, or the -T/--trace option of qemu-img,
qemu-io, and qemu-nbd, to select nbd traces. For qemu, the QMP commands
trace-event-{get,set}-state can also toggle tracing on the fly.

Example:
   qemu-nbd --trace 'nbd_*' <image file> # enables all nbd traces

Recompilation with CFLAGS=-DDEBUG_NBD is no more needed, furthermore,
DEBUG_NBD macro is removed from the code.
Signed-off-by: NVladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20170707152918.23086-11-vsementsov@virtuozzo.com>
[eblake: minor tweaks to a couple of traces]
Signed-off-by: NEric Blake <eblake@redhat.com>