protocol: Diagnose extra use of NBD_OPT_STRUCTURED_REPLY

The NBD spec states that when dealing with redundant
NBD_OPT_STRUCTURED_REPLY, "the server MAY fail the repeated message
with NBD_REP_ERR_INVALID".  Although this is not mandatory, it is the
behavior qemu-nbd uses.  To date, it's mostly academic: existing
clients don't send redundant requests without one-off hacks.  But an
upcoming patch to add a new API nbd_opt_structured_reply() to libnbd
will make it easier to trigger the scenario, at which point diagnosing
the redundancy is a nicer default.

Author: ebblake

server/protocol-handshake-newstyle.c

@@ -742,6 +742,14 @@ negotiate_handshake_newstyle_options (void)
         break;
       }
 
+      if (conn->structured_replies) {
+        if (send_newstyle_option_reply (option, NBD_REP_ERR_INVALID) == -1)
+          return -1;
+        debug ("newstyle negotiation: %s: structured replies already in use",
+               name_of_nbd_opt (option));
+        break;
+      }
+
       if (send_newstyle_option_reply (option, NBD_REP_ACK) == -1)
         return -1;