Accepting a Message Before Writing One - networking

Question

I have a server that listens to 2 ports and transfers messages between the 2 ports. The only problem is that each client (or each port) has to submit a message before they can receive the previous message that the other port sent. For example, if client 1 says "hello", client 2 will not receive the message unless he sends a message first.
I want to find a way to make it so that each client will have to receive the previous message before being able to send a new one. (Since Client 1 will not have anything to receive until Client 2 says something, I am planning to send it a string that has placeholder text.)
Can anyone help me with how to go about this? I try putting my listening code and my code for writing a message for each client into their own go routines, but that did not do the job. Any help will be appreciated.
Here is my code:
Server
package main
import (
"fmt"
"log"
"net"
)
var message string = "Client 2 is receiving your message"
func main() {
fmt.Println("The server is listening on Port 3000 and 8080")
//Set up listeners for the ports each client is using
listener, err := net.Listen("tcp", "localhost:3000")
if err != nil {
log.Fatal(err)
}
listener2, err := net.Listen("tcp", "localhost:8080")
if err != nil {
log.Fatal(err)
}
go acceptLoop(listener)
acceptLoop(listener2) // run in the main goroutine
}
func acceptLoop(l net.Listener) {
defer l.Close()
for {
c, err := l.Accept()
if err != nil {
log.Fatal(err)
}
fmt.Println("New connection found!")
go listenConnection(c)
}
}
func listenConnection(conn net.Conn) {
fmt.Println("Yay")
for {
buffer := make([]byte, 1400)
dataSize, err := conn.Read(buffer)
if err != nil {
fmt.Println("Connection has closed")
return
}
//This is the message you received
data := buffer[:dataSize]
fmt.Print("Received message: ", string(data))
// Send the message back
_, err = conn.Write([]byte(message))
if err != nil {
log.Fatalln(err)
}
fmt.Print("Message sent: ", string(data))
message = string(data)
}
}
Client 1
package main
import (
"fmt"
"log"
"net"
"bufio"
"os"
)
func main() {
conn, err := net.Dial("tcp", "localhost:3000")
if err != nil {
log.Fatalln(err)
}
for {
reader := bufio.NewReader(os.Stdin)
fmt.Print("Enter text: ")
text, _ := reader.ReadString('\n')
_, err = conn.Write([]byte(text))
if err != nil {
log.Fatalln(err)
}
for {
buffer := make([]byte, 1400)
dataSize, err := conn.Read(buffer)
if err != nil {
fmt.Println("The connection has closed!")
return
}
data := buffer[:dataSize]
fmt.Println("Received message: ", string(data))
break
}
}
}
My second client is the same as my first except "localhost:3000" is replaced with "localhost:8080".
I will appreciate any help! I'm fairly new to networking and Go, so any tips will be great.

Answer 1

Right now your Server is just an echo. It responds with the message received from the client.
If you want to relay the message between two clients, the first thing you need to figure out is what should happen if 3 clients connect (2 to the first port, 1 to the other)?
If you expect to have just one client connected to each port you could do something like this (pseudo-go-code):
func main() {
// ...
# Channels must be buffered or else clients will deadlock.
relay := make(chan string, 1)
go acceptLoop(listener1, relay)
acceptLoop(listener2, relay)
}
func acceptLoop(l, relay) {
for {
conn := l.Accept()
for {
relay <- conn.Read()
conn.Write(<-relay)
}
}