Accept a persistent tcp connection in Golang Server

Question

I am experimenting with Go - and would like to create a TCP server which I can telnet to, send commands and receive responses.

const (
    CONN_HOST = "localhost"
    CONN_PORT = "3333"
    CONN_TYPE = "tcp"
)

func main() {

    listener, err := net.Listen(CONN_TYPE, fmt.Sprintf("%s:%s", CONN_HOST, CONN_PORT))
    if err != nil {
        log.Panicln(err)
    }

    defer listener.Close()

    for {
        conn, err := listener.Accept()
        if err != nil {
            log.Panicln(err)
        }

        go handleRequest(conn)
    }
}

func handleRequest(conn net.Conn) {
    buffer := make([]byte, 1024)

    length, err := conn.Read(buffer)
    if err != nil {
        log.Panicln(err)
    }

    str := string(buffer[:length])

    fmt.Println(conn.RemoteAddr().String())
    fmt.Printf("Received command %d\t:%s\n", length, str)

    switch str {
        case "PING\r\n":
        sendResponse("PONG", conn)
        case "PUSH\r\n":
        sendResponse("GOT PUSH", conn)
    default:
        conn.Write([]byte(fmt.Sprintf("UNKNOWN_COMMAND: %s\n", str)))
    }

    conn.Close() // closes the connection
}

func sendResponse(res string, conn net.Conn) {
    conn.Write([]byte(res+"\n"))
}

The above snippet will close the connection every time, kicking me out of the terminal session. But what I actually want, is to be able to keep the connection open for more I/O operations. If I simply remove the conn.Close(), then the server appears to hang somewhere as it does not get any more responses.

The way I have resolved this is to have my handleRequest method endlessly loop so that it never exits till it receives a QUIT\r\n message. Is this appropriate - or is there a better way of achieving?

func handleRequest(conn net.Conn) {
    for {
        log.Println("Handling Request")
        buffer := make([]byte, 1024)

        length, err := conn.Read(buffer)
        if err != nil {
            log.Panicln(err)
        }

        str := string(buffer[:length])

        fmt.Println(conn.RemoteAddr().String())
        fmt.Printf("Received command %d\t:%s\n", length, str)

        switch str {
        case "PING\r\n":
            sendResponse("PONG", conn)
        case "PUSH\r\n":
            sendResponse("GOT PUSH", conn)
        case "QUIT\r\n":
            sendResponse("Goodbye", conn)
            conn.Close()
        default:
            conn.Write([]byte(fmt.Sprintf("UNKNOWN_COMMAND: %s\n", str)))
        }
    }
}

Answer 1

Your second example with the loop is already what you want. You simply loop and read as long as you want (or probably until some read/write timeout or an external cancellation signal).

However it still has an error in it: TCP gives you a stream of bytes, where it is not guaranteed that one write from a side will yield exactly one read on the other side with the same data length. This means if the client writes PING\r\n you could still receive only PI in the first read. You could fix that by using a bufio.Scanner and always read up to the first newline.

Answer 2

Not sure if this is what you're looking for. Taken from net/http implementation, wrapping your net.TCPListener's Accept method.

tcpKeepAliveListener{listener.(*net.TCPListener)}

type tcpKeepAliveListener struct {
    *net.TCPListener
}

func (ln tcpKeepAliveListener) Accept() (c net.Conn, err error) {
    tc, err := ln.AcceptTCP()
    if err != nil {
        return
    }
    tc.SetKeepAlive(true)
    tc.SetKeepAlivePeriod(3 * time.Minute)
    return tc, nil
}

Refer : Link 1 & Link 2