The first write is the one where data loss happens, and I want to avoid. if i only get an error I know i can just keep the data and try again later. I've seen https://stackoverflow.com/a/15071574/2757887 which is a very similar case and the explanation seems to apply here, but it still doesn't explain how to deal with the issue, if the tcp protocol I need to implement only does one-way communication.
I've sniffed the traffic with wireshark, and when i kill the netcat, I can see that it sends FIN to the go program, to which the go program replies with ACK. For some reason the go program doesn't immediately reply with it's own FIN - and i'm curious why that is, it might help with my problem - but there's probably a good reason for it.
Either way, from the "connection termination" section @ http://en.wikipedia.org/wiki/Transmission_Control_Protocol, I conclude that the socket is in the CLOSE_WAIT state at this point, which I also confirmed with "netstat -np", which shows the socket going from ESTABLISHED to CLOSE_WAIT after killing netstat.
Looking at wireshark, the first conn.write results in a packet with push and ack fields set, and of course my payload. this is the write that succeeds fine in go.
then the old socket that used to belong to netstat sends RST, which makes sure that as soon as i try to write in go (2nd write) it fails.
So my question is:
A) why can't I get an error on the first write? if the socket received the FIN and is in CLOSE_WAIT why does Go let me write to the socket and tell me all is fine?
B) is there any way I can check in Go whether the socket is in CLOSE_WAIT? and if so, I could for this purpose consider it closed and not do the write.
thanks, Dieter
Fundamentally, a successful write only tells you that data has been queued to be sent to the other end. If you need to make sure the other end gets that data, even if the connection closes or errors, you must store a copy of the data until the other end provides you with an application-level acknowledgment.