Re: [Libguestfs] [libnbd PATCH] API: Add nbd_set_opt_mode to expose NEGOTIATING state

>>Should setting the opt flag cause a failure if we connect to an >>oldstyle server?  (I can see arguments both ways.) We don't know if the server is oldstyle until after the magic numbers have been parsed. As coded in this patch, state Negotiating can only be reached from a newstyle-fixed server (not even a plain newstyle server will get here, since that must go straight to NBD_OPT_EXPORT_NAME which has no error detection short of killing the connection). So our options after the user calls nbd_set_opt_mode(nbd, true) are: 1. silently proceed to nbd_aio_is_ready; the user never got a chance to reach Negotiating, and can readily deduce that this was because the server didn't support negotiation. The lack of negotiation does not stop the user from using the export. 2. error out at the magic number detection to inform the user that negotiation mode is not reachable with the given server. But the user can always create a new nbd handle, and retry again this time without setting nbd_set_opt_mode. I coded 1, but 2 also makes sense. I could go either way, if you have a preference.