Detecting if a goroutine is already running - go

Question

I am writing a simple TCP proxy server that is started, stopped, and reconfigured by an HTTP handler. The proxy part works fine.
The problem I am having is starting it again each time the HTTP handler runs. i.e. it works on the first pass. But the second pass panics because the listener is already running.
My problem seems to be how to detect that the goroutine is already running, so I can stop it and start it again with new parameters. i.e. I can't figure out how to get a persistent handle on it from each execution of the web handler. It ends up crashing because the listener is already listening...
In another language, I would make it a Singleton. But what is the best way to do it in Go?
My Server has a NewServer(), Server.Stop, using a channel, etc. And they work if I keep them within the one block of code.
i.e. this works
ps = utils.NewProxyServer(listening, target[0])
time.Sleep(...)
ps.Stop()
How can I persist the handle on it between passes through the handler?
var ps *utils.Server
func MyHandler(w http.ResponseWriter, r *http.Request) {
switch r.Method {
case "GET":
...
// how can I know that ps exists so I can close it?
// and create a new one? Elegantly...?
ps.Stop() // this panics
ps = utils.NewServer(listening, target[0])
log.Print(ps.Status)
http.Redirect(w, r, redirurl, http.StatusSeeOther)
} else {
http.Redirect(w, r, "/network", http.StatusSeeOther)
}
}
}

Answer 1

I'd recommend having a goroutine manage ps, and using a channel to communicate with it:
var ps *utils.Server
// Buffer size of 1 to prevent blocking assuming it's not triggered frequently
var psControl = make(chan psConfig,1)
type psConfig struct {
// Correct the types as needed
listening string
target string
}
// This probably shouldn't be in an init(), put it wherever is appropriate
func init() {
go func() {
for cfg := range psControl {
if ps != nil {
ps.Stop()
}
ps = utils.NewServer(cfg.listening, cfg.target)
log.Print(ps.Status)
}
}()
}
func MyHandler(w http.ResponseWriter, r *http.Request) {
switch r.Method {
case "GET":
...
psControl <- psConfig{listening, target[0]}
http.Redirect(w, r, redirurl, http.StatusSeeOther)
} else {
http.Redirect(w, r, "/network", http.StatusSeeOther)
}
}
}