How to test TCP disconnection in GO - go

Question

I do have a process receiving data from second local process. I need to test if connection errors are handled well AND if it automatically reconnects and keep receiving data after a disconnection.
To do so I am trying to make it disconnect abruptly or put the TCP connection in an error state from a unit test.
As seen in this question to check if a connection is closed I am checking for data to come and test if it returns an error.
I am not sure how to:
close the connection ungracefully
make it be in an error state
This is the essence of my data receiver:
import (
"bufio"
"encoding/json"
"fmt"
"io"
"net"
)
type Message struct {
ID string `json:"id"`
}
func ReceiveData(listener Listener) {
var tcpConn net.Conn
var addr string = "127.0.0.1:9999"
tcpConn, err := net.Dial("tcp", addr)
socketReader := bufio.NewReader(tcpConn)
decoder := json.NewDecoder(socketReader)
for {
var msg Message
if err := decoder.Decode(&msg); err == io.EOF {
listener.ProcessUpdate(Message{}, fmt.Errorf("Received EOF"), nil)
tcpConn = nil
return
} else if err != nil {
listener.ProcessUpdate(Message{}, nil, fmt.Errorf("Error decoding message: %s", err.Error()))
tcpConn = nil
return
}
// process message
_ = msg
// Test disconnection
// This does not disconnect:
// tcpConn = nil
// This does but gracefully:
// tcpConn.Close()
}
I am not mocking the TCP connection as I'd like to try with the real data producer. If is needed I'll look at it.

Answer 1

A solution is to set a deadline to the TCP connection itself:
tcpConn.SetDeadline(time.Now())
Later this will trigger a timeout error which can be caught with:
err := decoder.Decode(&msg);
if err != nil {
if neterr, ok := err.(net.Error); ok && neterr.Timeout() {
fmt.Errorf("TCP timeout : %s", err.Error())
} else {
fmt.Errorf("Received error decoding message: %s", err.Error())
}
}