Go Unix Domain Socket: bind address already in use - sockets

Question

I'm having the following server code, which listens via unix domain socket
package main
import (
"log"
"net"
"os"
"os/signal"
"syscall"
)
func echoServer(c net.Conn) {
for {
buf := make([]byte, 512)
nr, err := c.Read(buf)
if err != nil {
return
}
data := buf[0:nr]
println("Server got:", string(data))
_, err = c.Write(data)
if err != nil {
log.Fatal("Writing client error: ", err)
}
}
}
func main() {
log.Println("Starting echo server")
ln, err := net.Listen("unix", "/tmp/go.sock")
if err != nil {
log.Fatal("Listen error: ", err)
}
sigc := make(chan os.Signal, 1)
signal.Notify(sigc, os.Interrupt, syscall.SIGTERM)
go func(ln net.Listener, c chan os.Signal) {
sig := <-c
log.Printf("Caught signal %s: shutting down.", sig)
ln.Close()
os.Exit(0)
}(ln, sigc)
for {
fd, err := ln.Accept()
if err != nil {
log.Fatal("Accept error: ", err)
}
go echoServer(fd)
}
}
When I close it using Ctrl+C, then the signal is captured and the socket is closed. When I rerun the program, everything works fine.
However, if the running process is abruptly killed, and if the program is restarted, the listen fails with the error Listen error: listen unix /tmp/go.sock: bind: address already in use
How to graciously handle this?
The reason why I ask this is: I know that abruptly killing is not the normal method, but my program shall be launched automatically as a daemon and if my daemon is restarted, I want to be able to listen to the socket again without this error.
It could also be because of a prior instance running, which I understand. The question here is how to programmatically identify in Go and handle this situation. As pointed in the answer here, one can use SO_REUSEADDR in C programs. Is there such a possibility in Go? Also, how do C programs handle this multiple instance problem.

Answer 1

You need to catch the signal and cleanup; some example code:
func HandleSIGINTKILL() chan os.Signal {
sig := make(chan os.Signal, 1)
signal.Notify(sig, syscall.SIGINT, syscall.SIGTERM)
return sig
}
...
go func() {
<-HandleSIGINTKILL()
log.Info("Received termination signal")
// Cleanup code here
os.Exit(0)
}()
This will of course not work if you kill -9 the process; you will need to manually remove the socket (or have your init system do it for you).