Map empty string export name to default

[rwmjones]

The NBD protocol says ...

A special, "empty", name (i.e., the length field is zero and no name is specified), is reserved for a "default" export, to be used in cases where explicitly specifying an export name makes no sense.

I notice that this server doesn't seem to do this:

$ nbdinfo nbd://localhost/
nbdinfo: nbd_opt_go: server replied with error to opt_go request: No such file or directory for the default export
nbdinfo: suggestion: to list all exports on the server, use --list
protocol: newstyle-fixed without TLS, using simple packets

Using --list shows that there is one export literally called default:

$ nbdinfo --list nbd://localhost
protocol: newstyle-fixed without TLS, using simple packets
export="default":
	export-size: 1073741824 (1G)
	uri: nbd://localhost:10809/default
	is_rotational: false
	is_read_only: false
	can_block_status_payload: false
	can_cache: false
	can_df: false
	can_fast_zero: false
	can_flush: false
	can_fua: false
	can_multi_conn: true
	can_trim: false
	can_zero: false
	block_size_minimum: 1
	block_size_preferred: 4096 (4K)
	block_size_maximum: 4294967295

I would suggest mapping the empty string to default export as per the specification.

[rwmjones]

Actually the server supports multiple exports:

go-nbd/pkg/server/nbd.go

Line 41 in b9d19c2

func Handle(conn net.Conn, exports []*Export, options *Options) error {

Maybe provide a way for one of them to be flagged as the default export; or choose the first in the list as the default?

(Adding @ebblake)

[pojntfx]

Interesting; would simply using an empty string as the export name effectively make that export the default as per the specification?