+/* Returns -1 if NBD_OPT_GO proves the export @wantname cannot be
+ * used, 0 if NBD_OPT_GO is unsupported (fall back to NBD_OPT_LIST and
+ * NBD_OPT_EXPORT_NAME in that case), and > 0 if the export is good to
+ * go (with @info populated). */
+static int nbd_opt_go(QIOChannel *ioc, const char *wantname,
+ NBDExportInfo *info, Error **errp)
+{
+ nbd_opt_reply reply;
+ uint32_t len = strlen(wantname);
+ uint16_t type;
+ int error;
+ char *buf;
+
+ /* The protocol requires that the server send NBD_INFO_EXPORT with
+ * a non-zero flags (at least NBD_FLAG_HAS_FLAGS must be set); so
+ * flags still 0 is a witness of a broken server. */
+ info->flags = 0;
+
+ trace_nbd_opt_go_start(wantname);
+ buf = g_malloc(4 + len + 2 + 1);
+ stl_be_p(buf, len);
+ memcpy(buf + 4, wantname, len);
+ /* No requests, live with whatever server sends */
+ stw_be_p(buf + 4 + len, 0);
+ if (nbd_send_option_request(ioc, NBD_OPT_GO, len + 6, buf, errp) < 0) {
+ return -1;
+ }
+
+ while (1) {
+ if (nbd_receive_option_reply(ioc, NBD_OPT_GO, &reply, errp) < 0) {
+ return -1;
+ }
+ error = nbd_handle_reply_err(ioc, &reply, errp);
+ if (error <= 0) {
+ return error;
+ }
+ len = reply.length;
+
+ if (reply.type == NBD_REP_ACK) {
+ /* Server is done sending info and moved into transmission
+ phase, but make sure it sent flags */
+ if (len) {
+ error_setg(errp, "server sent invalid NBD_REP_ACK");
+ nbd_send_opt_abort(ioc);
+ return -1;
+ }
+ trace_nbd_opt_go_success();
+ return 1;
+ }
+ if (reply.type != NBD_REP_INFO) {
+ error_setg(errp, "unexpected reply type %" PRIx32 ", expected %x",
+ reply.type, NBD_REP_INFO);
+ nbd_send_opt_abort(ioc);
+ return -1;
+ }
+ if (len < sizeof(type)) {
+ error_setg(errp, "