bash ncat / nc / netcat connection won't close / stays open too long - bash

Question

I'm using Ubuntu 18.04 LTS with bash 4.4.20.
I'm trying to create a little daemon to schedule data transmission between threads.
On the server I am doing this:
ncat -l 2001 -k -c 'xargs -n1 ./atc-worker.sh'
On the client I am doing this:
echo "totally-legit-login-token" | nc 127.0.0.1 2001 -w 1
And it works well!
Here is the response:
LaunchCode=1589323120.957093305 = Now=1589323120.957093305 = URL=https://totally-legit-url.com/ = AuthToken=totally-legit-auth-token = LastID=167
When the server receives a request from a client, it calls my little atc-worker.sh script. The server spits out a single line of text and it is back to business, serving other clients.
Thanks to the -k option, the server listens continuously. Multiple clients can connect at the same time. The only problem, is that I cannot end the connection programmatically. I need the daemon -k functionality on the server to answer requests from the clients, but I need the clients to quit listening after receiving a response and get on to their other work.
Is there an EOF signal/character I can send from my atc-worker.sh script that would tell nc on the client side to disconnect?
On the client, I use the -w 1 option to tell the client to connect for no more than a second.
But this -w 1 option has some drawbacks.
Maybe a second is too long. The connection should just take ~150 milliseconds and waiting out the rest of the second slows each client down even if it already has its answer. And -as I said before- the client has chores to do! The client shouldn't be wasting its time after it has its answer!
Bad actors Rogue clients could connect to the server that have no intention to close out in a timely manner and I want the server to have better control and shut down bad actors.
Maybe a second is too short. atc-worker.sh has a mechanism to wait for a lock file to be removed if there is one. If that lock file is there for more than a second, the connection will close before the client can receive its response.
Possible solutions.
The atc-worker.sh script could send a magic character set to terminate the connection. Problem solved.
On the client-side set of solutions, maybe curl would be a suitable choice instead of nc? But it would not solve my concern of being able to deal with bad actors. Maybe these are two different problems? Client-side closing the connection immediately after an answer is received, and server-side dealing with bad actors who will use what ever clients they choose.
Maybe use expect? I'm investigating that now.
Thanks in advance!

Answer 1

OK. After a lot of digging, I found someone else with a similar problem. Here is a link to his answer.
Original question: Client doesn't close connection to server after receiving all responses
Original Answer: https://stackoverflow.com/a/50528286/3055756
Thanks #Unyxos
I modified my daemon to send an "ENDRESPONSE" line when it was done even though it does not drop the connection. And I modified the client to look for that "ENDRESPONSE" line. When the client gets the line, it drops the connection using the logic that #Unyxos uses below in his answer.
Here is his simple and elegant answer:
Finally found a working way (maybe not the best but at least it's perfectly doing what I want :D)
after all functions I send a "ENDRESPONSE" message, and on my client, I test if I have this message or not :
function sendMessage {
while read line; do
if [[ $line == "ENDRESPONSE" ]]; then
break
else
echo $line
fi
done < <(netcat "$ipAddress" "$port" <<< "$*")
}
Anyway thanks for your help, I'll try to implement other solutions later !