nbd: use "" as a default export name if none provided

If the user does not provide an export name and the server
is running the new style protocol, where export names are
mandatory, use "" as the default export name if the user
has not specified any. "" is defined in the NBD protocol
as the default name to use in such scenarios.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <1455129674-17255-13-git-send-email-berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

nbd/client.c

@@ -315,8 +315,8 @@ int nbd_receive_negotiate(QIOChannel *ioc, const char *name, uint32_t *flags,
             goto fail;
         }
         if (!name) {
-            error_setg(errp, "Server requires an export name");
-            goto fail;
+            TRACE("Using default NBD export name \"\"");
+            name = "";
         }
         if (fixedNewStyle) {
             /* Check our desired export is present in the

nbd/server.c

@@ -220,6 +220,7 @@ static int nbd_negotiate_send_rep_list(QIOChannel *ioc, NBDExport *exp)
     uint64_t magic, name_len;
     uint32_t opt, type, len;
 
+    TRACE("Advertizing export name '%s'", exp->name ? exp->name : "");
     name_len = strlen(exp->name);
     magic = cpu_to_be64(NBD_REP_MAGIC);
     if (nbd_negotiate_write(ioc, &magic, sizeof(magic)) != sizeof(magic)) {