net.TCPConn allowing Write after FIN packet - tcp

Question

I'm trying to write unit tests for some server-side code, but I'm having trouble being deterministic with my shutdown test cases. It seems a loopback TCP connection isn't correctly handling a clean shutdown. I've reprod this in a sample app which does the following in lockstep:
Create a client & server connection.
Verify connectivity by sending a message successfully from client to server.
Use channels to tell the server to call conn.Close() and wait until that call has completed.
(Try to) verify the connection is cleanly broken by calling Write on the client connection again.
Step 4 succeeds without error. I've tried using a json.Encoder and a bare call to TCPConn.Write. I checked the traffic with WireShark. The server sent a FIN packet, but the client never does (even with a 1s sleep) The server even sent a RST packet in response to (4) and the client conn.Write still returned nil for its error.
This seems totally bonkers. Am I missing something here? Currently running Go v1.2.1/Darwin
Edit: Obligatory repro
package main
import (
"bufio"
"fmt"
"net"
)
var (
loopback = make(chan string)
shouldClose = make(chan struct{})
didClose = make(chan struct{})
)
func serve(listener *net.TCPListener) {
conn, err := listener.Accept()
if err != nil {
panic(err)
}
s := bufio.NewScanner(conn)
if !s.Scan() {
panic(fmt.Sprint("Failed to scan for line: ", s.Err()))
}
loopback <- s.Text() + "\n"
<-shouldClose
conn.Close()
close(didClose)
if s.Scan() {
panic("Expected error reading from a socket closed on this side")
}
}
func main() {
listener, err := net.ListenTCP("tcp", &net.TCPAddr{})
if err != nil {
panic(err)
}
go serve(listener)
conn, err := net.Dial("tcp", listener.Addr().String())
if err != nil {
panic(fmt.Sprint("Dialer got error ", err))
}
oracle := "Mic check\n"
if _, err = conn.Write([]byte(oracle)); err != nil {
panic(fmt.Sprint("Dialer failed to write oracle: ", err))
}
test := <-loopback
if test != oracle {
panic("Server did not receive the value sent by the client")
}
close(shouldClose)
<-didClose
// For giggles, I can also add a <-time.After(500 * time.Millisecond)
if _, err = conn.Write([]byte("This should fail after active disconnect")); err == nil {
panic("Sender 'successfully' wrote to a closed socket")
}
}

Answer 1

This is how an active close of a TCP connection works. When the client detects that the server has closed, it is then expected to close its half of the connection.
In your case, instead of closing the client you're sending more data. This causes the server to send an RST packet to force the connection closed since the message received isn't valid.
If you're still unsure, here's and equivalent python client+server which displays the same behavior. (I find using python helpful, since it closely follows the underlying BSD socket API, without using C)
Server:
import socket, time
server = socket.socket()
server.bind(("127.0.0.1", 9999))
server.listen(1)
sock, addr = server.accept()
msg = sock.recv(1024)
print msg
print "closing"
sock.close()
time.sleep(3)
print "done"
Client:
import socket, time
sock = socket.socket()
sock.connect(("127.0.0.1", 9999))
sock.send("test\n")
time.sleep(1)
print "sending again!"
sock.send("no error here")
time.sleep(1)
print "sending one last time"
sock.send("broken pipe this time")
To properly detect a remote close on the connection, you should do Read(), and look for an io.EOF error in return.
// we technically need to try and read at least one byte,
// or we will get an EOF even if the connection isn't closed.
buff := make([]byte, 1)
if _, err := conn.Read(buff); err != io.EOF {
panic("connection not closed")
}